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![Maxim Patlasov](/assets/img/avatar_default.png)
The feature prevents mistrusted filesystems (ie: FUSE mounts created by unprivileged users) to grow a large number of dirty pages before throttling. For such filesystems balance_dirty_pages always check bdi counters against bdi limits. I.e. even if global "nr_dirty" is under "freerun", it's not allowed to skip bdi checks. The only use case for now is fuse: it sets bdi max_ratio to 1% by default and system administrators are supposed to expect that this limit won't be exceeded. The feature is on if a BDI is marked by BDI_CAP_STRICTLIMIT flag. A filesystem may set the flag when it initializes its BDI. The problematic scenario comes from the fact that nobody pays attention to the NR_WRITEBACK_TEMP counter (i.e. number of pages under fuse writeback). The implementation of fuse writeback releases original page (by calling end_page_writeback) almost immediately. A fuse request queued for real processing bears a copy of original page. Hence, if userspace fuse daemon doesn't finalize write requests in timely manner, an aggressive mmap writer can pollute virtually all memory by those temporary fuse page copies. They are carefully accounted in NR_WRITEBACK_TEMP, but nobody cares. To make further explanations shorter, let me use "NR_WRITEBACK_TEMP problem" as a shortcut for "a possibility of uncontrolled grow of amount of RAM consumed by temporary pages allocated by kernel fuse to process writeback". The problem was very easy to reproduce. There is a trivial example filesystem implementation in fuse userspace distribution: fusexmp_fh.c. I added "sleep(1);" to the write methods, then recompiled and mounted it. Then created a huge file on the mount point and run a simple program which mmap-ed the file to a memory region, then wrote a data to the region. An hour later I observed almost all RAM consumed by fuse writeback. Since then some unrelated changes in kernel fuse made it more difficult to reproduce, but it is still possible now. Putting this theoretical happens-in-the-lab thing aside, there is another thing that really hurts real world (FUSE) users. This is write-through page cache policy FUSE currently uses. I.e. handling write(2), kernel fuse populates page cache and flushes user data to the server synchronously. This is excessively suboptimal. Pavel Emelyanov's patches ("writeback cache policy") solve the problem, but they also make resolving NR_WRITEBACK_TEMP problem absolutely necessary. Otherwise, simply copying a huge file to a fuse mount would result in memory starvation. Miklos, the maintainer of FUSE, believes strictlimit feature the way to go. And eventually putting FUSE topics aside, there is one more use-case for strictlimit feature. Using a slow USB stick (mass storage) in a machine with huge amount of RAM installed is a well-known pain. Let's make simple computations. Assuming 64GB of RAM installed, existing implementation of balance_dirty_pages will start throttling only after 9.6GB of RAM becomes dirty (freerun == 15% of total RAM). So, the command "cp 9GB_file /media/my-usb-storage/" may return in a few seconds, but subsequent "umount /media/my-usb-storage/" will take more than two hours if effective throughput of the storage is, to say, 1MB/sec. After inclusion of strictlimit feature, it will be trivial to add a knob (e.g. /sys/devices/virtual/bdi/x:y/strictlimit) to enable it on demand. Manually or via udev rule. May be I'm wrong, but it seems to be quite a natural desire to limit the amount of dirty memory for some devices we are not fully trust (in the sense of sustainable throughput). [akpm@linux-foundation.org: fix warning in page-writeback.c] Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com> Cc: Jan Kara <jack@suse.cz> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: James Bottomley <James.Bottomley@HansenPartnership.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
362 lines
10 KiB
C
362 lines
10 KiB
C
/*
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* include/linux/backing-dev.h
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*
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* low-level device information and state which is propagated up through
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* to high-level code.
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*/
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#ifndef _LINUX_BACKING_DEV_H
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#define _LINUX_BACKING_DEV_H
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#include <linux/percpu_counter.h>
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#include <linux/log2.h>
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#include <linux/flex_proportions.h>
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#include <linux/kernel.h>
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#include <linux/fs.h>
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#include <linux/sched.h>
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#include <linux/timer.h>
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#include <linux/writeback.h>
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#include <linux/atomic.h>
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#include <linux/sysctl.h>
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#include <linux/workqueue.h>
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struct page;
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struct device;
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struct dentry;
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/*
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* Bits in backing_dev_info.state
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*/
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enum bdi_state {
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BDI_wb_alloc, /* Default embedded wb allocated */
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BDI_async_congested, /* The async (write) queue is getting full */
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BDI_sync_congested, /* The sync queue is getting full */
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BDI_registered, /* bdi_register() was done */
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BDI_writeback_running, /* Writeback is in progress */
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BDI_unused, /* Available bits start here */
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};
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typedef int (congested_fn)(void *, int);
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enum bdi_stat_item {
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BDI_RECLAIMABLE,
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BDI_WRITEBACK,
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BDI_DIRTIED,
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BDI_WRITTEN,
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NR_BDI_STAT_ITEMS
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};
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#define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
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struct bdi_writeback {
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struct backing_dev_info *bdi; /* our parent bdi */
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unsigned int nr;
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unsigned long last_old_flush; /* last old data flush */
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struct delayed_work dwork; /* work item used for writeback */
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struct list_head b_dirty; /* dirty inodes */
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struct list_head b_io; /* parked for writeback */
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struct list_head b_more_io; /* parked for more writeback */
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spinlock_t list_lock; /* protects the b_* lists */
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};
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struct backing_dev_info {
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struct list_head bdi_list;
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unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
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unsigned long state; /* Always use atomic bitops on this */
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unsigned int capabilities; /* Device capabilities */
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congested_fn *congested_fn; /* Function pointer if device is md/dm */
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void *congested_data; /* Pointer to aux data for congested func */
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char *name;
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struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];
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unsigned long bw_time_stamp; /* last time write bw is updated */
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unsigned long dirtied_stamp;
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unsigned long written_stamp; /* pages written at bw_time_stamp */
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unsigned long write_bandwidth; /* the estimated write bandwidth */
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unsigned long avg_write_bandwidth; /* further smoothed write bw */
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/*
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* The base dirty throttle rate, re-calculated on every 200ms.
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* All the bdi tasks' dirty rate will be curbed under it.
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* @dirty_ratelimit tracks the estimated @balanced_dirty_ratelimit
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* in small steps and is much more smooth/stable than the latter.
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*/
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unsigned long dirty_ratelimit;
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unsigned long balanced_dirty_ratelimit;
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struct fprop_local_percpu completions;
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int dirty_exceeded;
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unsigned int min_ratio;
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unsigned int max_ratio, max_prop_frac;
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struct bdi_writeback wb; /* default writeback info for this bdi */
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spinlock_t wb_lock; /* protects work_list */
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struct list_head work_list;
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struct device *dev;
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struct timer_list laptop_mode_wb_timer;
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#ifdef CONFIG_DEBUG_FS
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struct dentry *debug_dir;
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struct dentry *debug_stats;
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#endif
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};
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int bdi_init(struct backing_dev_info *bdi);
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void bdi_destroy(struct backing_dev_info *bdi);
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__printf(3, 4)
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int bdi_register(struct backing_dev_info *bdi, struct device *parent,
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const char *fmt, ...);
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int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev);
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void bdi_unregister(struct backing_dev_info *bdi);
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int bdi_setup_and_register(struct backing_dev_info *, char *, unsigned int);
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void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
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enum wb_reason reason);
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void bdi_start_background_writeback(struct backing_dev_info *bdi);
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void bdi_writeback_workfn(struct work_struct *work);
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int bdi_has_dirty_io(struct backing_dev_info *bdi);
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void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);
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void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2);
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extern spinlock_t bdi_lock;
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extern struct list_head bdi_list;
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extern struct workqueue_struct *bdi_wq;
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static inline int wb_has_dirty_io(struct bdi_writeback *wb)
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{
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return !list_empty(&wb->b_dirty) ||
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!list_empty(&wb->b_io) ||
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!list_empty(&wb->b_more_io);
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}
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static inline void __add_bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item, s64 amount)
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{
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__percpu_counter_add(&bdi->bdi_stat[item], amount, BDI_STAT_BATCH);
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}
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static inline void __inc_bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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__add_bdi_stat(bdi, item, 1);
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}
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static inline void inc_bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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unsigned long flags;
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local_irq_save(flags);
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__inc_bdi_stat(bdi, item);
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local_irq_restore(flags);
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}
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static inline void __dec_bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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__add_bdi_stat(bdi, item, -1);
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}
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static inline void dec_bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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unsigned long flags;
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local_irq_save(flags);
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__dec_bdi_stat(bdi, item);
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local_irq_restore(flags);
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}
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static inline s64 bdi_stat(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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return percpu_counter_read_positive(&bdi->bdi_stat[item]);
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}
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static inline s64 __bdi_stat_sum(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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return percpu_counter_sum_positive(&bdi->bdi_stat[item]);
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}
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static inline s64 bdi_stat_sum(struct backing_dev_info *bdi,
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enum bdi_stat_item item)
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{
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s64 sum;
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unsigned long flags;
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local_irq_save(flags);
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sum = __bdi_stat_sum(bdi, item);
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local_irq_restore(flags);
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return sum;
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}
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extern void bdi_writeout_inc(struct backing_dev_info *bdi);
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/*
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* maximal error of a stat counter.
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*/
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static inline unsigned long bdi_stat_error(struct backing_dev_info *bdi)
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{
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#ifdef CONFIG_SMP
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return nr_cpu_ids * BDI_STAT_BATCH;
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#else
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return 1;
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#endif
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}
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int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
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int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
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/*
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* Flags in backing_dev_info::capability
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*
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* The first three flags control whether dirty pages will contribute to the
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* VM's accounting and whether writepages() should be called for dirty pages
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* (something that would not, for example, be appropriate for ramfs)
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*
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* WARNING: these flags are closely related and should not normally be
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* used separately. The BDI_CAP_NO_ACCT_AND_WRITEBACK combines these
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* three flags into a single convenience macro.
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*
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* BDI_CAP_NO_ACCT_DIRTY: Dirty pages shouldn't contribute to accounting
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* BDI_CAP_NO_WRITEBACK: Don't write pages back
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* BDI_CAP_NO_ACCT_WB: Don't automatically account writeback pages
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*
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* These flags let !MMU mmap() govern direct device mapping vs immediate
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* copying more easily for MAP_PRIVATE, especially for ROM filesystems.
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*
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* BDI_CAP_MAP_COPY: Copy can be mapped (MAP_PRIVATE)
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* BDI_CAP_MAP_DIRECT: Can be mapped directly (MAP_SHARED)
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* BDI_CAP_READ_MAP: Can be mapped for reading
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* BDI_CAP_WRITE_MAP: Can be mapped for writing
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* BDI_CAP_EXEC_MAP: Can be mapped for execution
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*
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* BDI_CAP_SWAP_BACKED: Count shmem/tmpfs objects as swap-backed.
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*
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* BDI_CAP_STRICTLIMIT: Keep number of dirty pages below bdi threshold.
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*/
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#define BDI_CAP_NO_ACCT_DIRTY 0x00000001
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#define BDI_CAP_NO_WRITEBACK 0x00000002
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#define BDI_CAP_MAP_COPY 0x00000004
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#define BDI_CAP_MAP_DIRECT 0x00000008
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#define BDI_CAP_READ_MAP 0x00000010
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#define BDI_CAP_WRITE_MAP 0x00000020
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#define BDI_CAP_EXEC_MAP 0x00000040
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#define BDI_CAP_NO_ACCT_WB 0x00000080
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#define BDI_CAP_SWAP_BACKED 0x00000100
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#define BDI_CAP_STABLE_WRITES 0x00000200
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#define BDI_CAP_STRICTLIMIT 0x00000400
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#define BDI_CAP_VMFLAGS \
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(BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP)
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#define BDI_CAP_NO_ACCT_AND_WRITEBACK \
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(BDI_CAP_NO_WRITEBACK | BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_ACCT_WB)
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#if defined(VM_MAYREAD) && \
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(BDI_CAP_READ_MAP != VM_MAYREAD || \
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BDI_CAP_WRITE_MAP != VM_MAYWRITE || \
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BDI_CAP_EXEC_MAP != VM_MAYEXEC)
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#error please change backing_dev_info::capabilities flags
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#endif
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extern struct backing_dev_info default_backing_dev_info;
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extern struct backing_dev_info noop_backing_dev_info;
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int writeback_in_progress(struct backing_dev_info *bdi);
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static inline int bdi_congested(struct backing_dev_info *bdi, int bdi_bits)
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{
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if (bdi->congested_fn)
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return bdi->congested_fn(bdi->congested_data, bdi_bits);
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return (bdi->state & bdi_bits);
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}
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static inline int bdi_read_congested(struct backing_dev_info *bdi)
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{
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return bdi_congested(bdi, 1 << BDI_sync_congested);
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}
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static inline int bdi_write_congested(struct backing_dev_info *bdi)
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{
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return bdi_congested(bdi, 1 << BDI_async_congested);
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}
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static inline int bdi_rw_congested(struct backing_dev_info *bdi)
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{
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return bdi_congested(bdi, (1 << BDI_sync_congested) |
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(1 << BDI_async_congested));
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}
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enum {
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BLK_RW_ASYNC = 0,
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BLK_RW_SYNC = 1,
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};
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void clear_bdi_congested(struct backing_dev_info *bdi, int sync);
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void set_bdi_congested(struct backing_dev_info *bdi, int sync);
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long congestion_wait(int sync, long timeout);
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long wait_iff_congested(struct zone *zone, int sync, long timeout);
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int pdflush_proc_obsolete(struct ctl_table *table, int write,
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void __user *buffer, size_t *lenp, loff_t *ppos);
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static inline bool bdi_cap_stable_pages_required(struct backing_dev_info *bdi)
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{
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return bdi->capabilities & BDI_CAP_STABLE_WRITES;
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}
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static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
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{
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return !(bdi->capabilities & BDI_CAP_NO_WRITEBACK);
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}
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static inline bool bdi_cap_account_dirty(struct backing_dev_info *bdi)
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{
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return !(bdi->capabilities & BDI_CAP_NO_ACCT_DIRTY);
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}
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static inline bool bdi_cap_account_writeback(struct backing_dev_info *bdi)
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{
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/* Paranoia: BDI_CAP_NO_WRITEBACK implies BDI_CAP_NO_ACCT_WB */
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return !(bdi->capabilities & (BDI_CAP_NO_ACCT_WB |
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BDI_CAP_NO_WRITEBACK));
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}
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static inline bool bdi_cap_swap_backed(struct backing_dev_info *bdi)
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{
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return bdi->capabilities & BDI_CAP_SWAP_BACKED;
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}
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static inline bool mapping_cap_writeback_dirty(struct address_space *mapping)
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{
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return bdi_cap_writeback_dirty(mapping->backing_dev_info);
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}
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static inline bool mapping_cap_account_dirty(struct address_space *mapping)
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{
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return bdi_cap_account_dirty(mapping->backing_dev_info);
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}
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static inline bool mapping_cap_swap_backed(struct address_space *mapping)
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{
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return bdi_cap_swap_backed(mapping->backing_dev_info);
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}
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static inline int bdi_sched_wait(void *word)
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{
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schedule();
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return 0;
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}
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#endif /* _LINUX_BACKING_DEV_H */
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