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zram: add a huge_idle writeback mode

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Today it's only possible to write back as a page, idle, or huge.  A user
might want to writeback pages which are huge and idle first as these idle
pages do not require decompression and make a good first pass for
writeback.

Idle writeback specifically has the advantage that a refault is unlikely
given that the page has been swapped for some amount of time without being
refaulted.

Huge writeback has the advantage that you're guaranteed to get the maximum
benefit from a single page writeback, that is, you're reclaiming one full
page of memory.  Pages which are compressed in zram being written back
result in some benefit which is always less than a page size because of
the fact that it was compressed.

The primary use of this is for minimizing refaults in situations where the
device has to be sensitive to storage endurance.  On ChromeOS we have
devices with slow eMMC and repeated writes and refaults can negatively
affect performance and endurance.

Link: https://lkml.kernel.org/r/20220322215821.1196994-1-bgeffon@google.com
Signed-off-by: Brian Geffon <bgeffon@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Brian Geffon 2022-04-29 14:36:59 -07:00 committed by akpm
parent d137a7cb9b
commit 30226b69f8
2 changed files with 11 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
Documentation/admin-guide/blockdev/zram.rst
View file

@ -343,6 +343,11 @@ Admin can request writeback of those idle pages at right timing via::
With the command, zram will writeback idle pages from memory to the storage.
Additionally, if a user choose to writeback only huge and idle pages
this can be accomplished with::
echo huge_idle > /sys/block/zramX/writeback
If an admin wants to write a specific page in zram device to the backing device,
they could write a page index into the interface.

10
drivers/block/zram/zram_drv.c
View file

@ -639,8 +639,8 @@ static int read_from_bdev_async(struct zram *zram, struct bio_vec *bvec,
#define PAGE_WB_SIG "page_index="
#define PAGE_WRITEBACK 0
#define HUGE_WRITEBACK 1
#define IDLE_WRITEBACK 2
#define HUGE_WRITEBACK (1<<0)
#define IDLE_WRITEBACK (1<<1)
static ssize_t writeback_store(struct device *dev,
@ -660,6 +660,8 @@ static ssize_t writeback_store(struct device *dev,
mode = IDLE_WRITEBACK;
else if (sysfs_streq(buf, "huge"))
mode = HUGE_WRITEBACK;
else if (sysfs_streq(buf, "huge_idle"))
mode = IDLE_WRITEBACK | HUGE_WRITEBACK;
else {
if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1))
return -EINVAL;
@ -721,10 +723,10 @@ static ssize_t writeback_store(struct device *dev,
zram_test_flag(zram, index, ZRAM_UNDER_WB))
goto next;
if (mode == IDLE_WRITEBACK &&
if (mode & IDLE_WRITEBACK &&
!zram_test_flag(zram, index, ZRAM_IDLE))
goto next;
if (mode == HUGE_WRITEBACK &&
if (mode & HUGE_WRITEBACK &&
!zram_test_flag(zram, index, ZRAM_HUGE))
goto next;
/*