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![Ross Zwisler](/assets/img/avatar_default.png)
When servicing mmap() reads from file holes the current DAX code allocates a page cache page of all zeroes and places the struct page pointer in the mapping->page_tree radix tree. This has three major drawbacks: 1) It consumes memory unnecessarily. For every 4k page that is read via a DAX mmap() over a hole, we allocate a new page cache page. This means that if you read 1GiB worth of pages, you end up using 1GiB of zeroed memory. This is easily visible by looking at the overall memory consumption of the system or by looking at /proc/[pid]/smaps: 7f62e72b3000-7f63272b3000 rw-s 00000000 103:00 12 /root/dax/data Size: 1048576 kB Rss: 1048576 kB Pss: 1048576 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 1048576 kB Private_Dirty: 0 kB Referenced: 1048576 kB Anonymous: 0 kB LazyFree: 0 kB AnonHugePages: 0 kB ShmemPmdMapped: 0 kB Shared_Hugetlb: 0 kB Private_Hugetlb: 0 kB Swap: 0 kB SwapPss: 0 kB KernelPageSize: 4 kB MMUPageSize: 4 kB Locked: 0 kB 2) It is slower than using a common zero page because each page fault has more work to do. Instead of just inserting a common zero page we have to allocate a page cache page, zero it, and then insert it. Here are the average latencies of dax_load_hole() as measured by ftrace on a random test box: Old method, using zeroed page cache pages: 3.4 us New method, using the common 4k zero page: 0.8 us This was the average latency over 1 GiB of sequential reads done by this simple fio script: [global] size=1G filename=/root/dax/data fallocate=none [io] rw=read ioengine=mmap 3) The fact that we had to check for both DAX exceptional entries and for page cache pages in the radix tree made the DAX code more complex. Solve these issues by following the lead of the DAX PMD code and using a common 4k zero page instead. As with the PMD code we will now insert a DAX exceptional entry into the radix tree instead of a struct page pointer which allows us to remove all the special casing in the DAX code. Note that we do still pretty aggressively check for regular pages in the DAX radix tree, especially where we take action based on the bits set in the page. If we ever find a regular page in our radix tree now that most likely means that someone besides DAX is inserting pages (which has happened lots of times in the past), and we want to find that out early and fail loudly. This solution also removes the extra memory consumption. Here is that same /proc/[pid]/smaps after 1GiB of reading from a hole with the new code: 7f2054a74000-7f2094a74000 rw-s 00000000 103:00 12 /root/dax/data Size: 1048576 kB Rss: 0 kB Pss: 0 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 0 kB Referenced: 0 kB Anonymous: 0 kB LazyFree: 0 kB AnonHugePages: 0 kB ShmemPmdMapped: 0 kB Shared_Hugetlb: 0 kB Private_Hugetlb: 0 kB Swap: 0 kB SwapPss: 0 kB KernelPageSize: 4 kB MMUPageSize: 4 kB Locked: 0 kB Overall system memory consumption is similarly improved. Another major change is that we remove dax_pfn_mkwrite() from our fault flow, and instead rely on the page fault itself to make the PTE dirty and writeable. The following description from the patch adding the vm_insert_mixed_mkwrite() call explains this a little more: "To be able to use the common 4k zero page in DAX we need to have our PTE fault path look more like our PMD fault path where a PTE entry can be marked as dirty and writeable as it is first inserted rather than waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call. Right now we can rely on having a dax_pfn_mkwrite() call because we can distinguish between these two cases in do_wp_page(): case 1: 4k zero page => writable DAX storage case 2: read-only DAX storage => writeable DAX storage This distinction is made by via vm_normal_page(). vm_normal_page() returns false for the common 4k zero page, though, just as it does for DAX ptes. Instead of special casing the DAX + 4k zero page case we will simplify our DAX PTE page fault sequence so that it matches our DAX PMD sequence, and get rid of the dax_pfn_mkwrite() helper. We will instead use dax_iomap_fault() to handle write-protection faults. This means that insert_pfn() needs to follow the lead of insert_pfn_pmd() and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set insert_pfn() will do the work that was previously done by wp_page_reuse() as part of the dax_pfn_mkwrite() call path" Link: http://lkml.kernel.org/r/20170724170616.25810-4-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reviewed-by: Jan Kara <jack@suse.cz> Cc: "Darrick J. Wong" <darrick.wong@oracle.com> Cc: "Theodore Ts'o" <tytso@mit.edu> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Andreas Dilger <adilger.kernel@dilger.ca> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Matthew Wilcox <mawilcox@microsoft.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
206 lines
5.1 KiB
C
206 lines
5.1 KiB
C
/*
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* linux/fs/ext2/file.c
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*
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* Copyright (C) 1992, 1993, 1994, 1995
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* Remy Card (card@masi.ibp.fr)
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* Laboratoire MASI - Institut Blaise Pascal
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* Universite Pierre et Marie Curie (Paris VI)
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*
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* from
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*
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* linux/fs/minix/file.c
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*
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* Copyright (C) 1991, 1992 Linus Torvalds
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*
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* ext2 fs regular file handling primitives
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*
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* 64-bit file support on 64-bit platforms by Jakub Jelinek
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* (jj@sunsite.ms.mff.cuni.cz)
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*/
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#include <linux/time.h>
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#include <linux/pagemap.h>
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#include <linux/dax.h>
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#include <linux/quotaops.h>
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#include <linux/iomap.h>
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#include <linux/uio.h>
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#include "ext2.h"
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#include "xattr.h"
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#include "acl.h"
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#ifdef CONFIG_FS_DAX
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static ssize_t ext2_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
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{
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struct inode *inode = iocb->ki_filp->f_mapping->host;
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ssize_t ret;
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if (!iov_iter_count(to))
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return 0; /* skip atime */
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inode_lock_shared(inode);
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ret = dax_iomap_rw(iocb, to, &ext2_iomap_ops);
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inode_unlock_shared(inode);
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file_accessed(iocb->ki_filp);
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return ret;
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}
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static ssize_t ext2_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
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{
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struct file *file = iocb->ki_filp;
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struct inode *inode = file->f_mapping->host;
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ssize_t ret;
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inode_lock(inode);
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ret = generic_write_checks(iocb, from);
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if (ret <= 0)
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goto out_unlock;
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ret = file_remove_privs(file);
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if (ret)
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goto out_unlock;
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ret = file_update_time(file);
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if (ret)
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goto out_unlock;
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ret = dax_iomap_rw(iocb, from, &ext2_iomap_ops);
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if (ret > 0 && iocb->ki_pos > i_size_read(inode)) {
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i_size_write(inode, iocb->ki_pos);
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mark_inode_dirty(inode);
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}
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out_unlock:
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inode_unlock(inode);
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if (ret > 0)
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ret = generic_write_sync(iocb, ret);
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return ret;
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}
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/*
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* The lock ordering for ext2 DAX fault paths is:
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*
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* mmap_sem (MM)
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* sb_start_pagefault (vfs, freeze)
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* ext2_inode_info->dax_sem
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* address_space->i_mmap_rwsem or page_lock (mutually exclusive in DAX)
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* ext2_inode_info->truncate_mutex
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*
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* The default page_lock and i_size verification done by non-DAX fault paths
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* is sufficient because ext2 doesn't support hole punching.
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*/
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static int ext2_dax_fault(struct vm_fault *vmf)
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{
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struct inode *inode = file_inode(vmf->vma->vm_file);
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struct ext2_inode_info *ei = EXT2_I(inode);
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int ret;
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if (vmf->flags & FAULT_FLAG_WRITE) {
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sb_start_pagefault(inode->i_sb);
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file_update_time(vmf->vma->vm_file);
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}
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down_read(&ei->dax_sem);
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ret = dax_iomap_fault(vmf, PE_SIZE_PTE, &ext2_iomap_ops);
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up_read(&ei->dax_sem);
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if (vmf->flags & FAULT_FLAG_WRITE)
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sb_end_pagefault(inode->i_sb);
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return ret;
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}
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static const struct vm_operations_struct ext2_dax_vm_ops = {
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.fault = ext2_dax_fault,
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/*
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* .huge_fault is not supported for DAX because allocation in ext2
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* cannot be reliably aligned to huge page sizes and so pmd faults
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* will always fail and fail back to regular faults.
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*/
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.page_mkwrite = ext2_dax_fault,
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.pfn_mkwrite = ext2_dax_fault,
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};
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static int ext2_file_mmap(struct file *file, struct vm_area_struct *vma)
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{
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if (!IS_DAX(file_inode(file)))
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return generic_file_mmap(file, vma);
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file_accessed(file);
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vma->vm_ops = &ext2_dax_vm_ops;
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vma->vm_flags |= VM_MIXEDMAP;
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return 0;
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}
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#else
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#define ext2_file_mmap generic_file_mmap
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#endif
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/*
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* Called when filp is released. This happens when all file descriptors
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* for a single struct file are closed. Note that different open() calls
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* for the same file yield different struct file structures.
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*/
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static int ext2_release_file (struct inode * inode, struct file * filp)
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{
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if (filp->f_mode & FMODE_WRITE) {
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mutex_lock(&EXT2_I(inode)->truncate_mutex);
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ext2_discard_reservation(inode);
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mutex_unlock(&EXT2_I(inode)->truncate_mutex);
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}
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return 0;
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}
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int ext2_fsync(struct file *file, loff_t start, loff_t end, int datasync)
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{
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int ret;
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struct super_block *sb = file->f_mapping->host->i_sb;
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ret = generic_file_fsync(file, start, end, datasync);
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if (ret == -EIO)
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/* We don't really know where the IO error happened... */
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ext2_error(sb, __func__,
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"detected IO error when writing metadata buffers");
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return ret;
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}
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static ssize_t ext2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
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{
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#ifdef CONFIG_FS_DAX
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if (IS_DAX(iocb->ki_filp->f_mapping->host))
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return ext2_dax_read_iter(iocb, to);
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#endif
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return generic_file_read_iter(iocb, to);
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}
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static ssize_t ext2_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
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{
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#ifdef CONFIG_FS_DAX
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if (IS_DAX(iocb->ki_filp->f_mapping->host))
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return ext2_dax_write_iter(iocb, from);
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#endif
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return generic_file_write_iter(iocb, from);
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}
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const struct file_operations ext2_file_operations = {
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.llseek = generic_file_llseek,
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.read_iter = ext2_file_read_iter,
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.write_iter = ext2_file_write_iter,
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.unlocked_ioctl = ext2_ioctl,
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#ifdef CONFIG_COMPAT
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.compat_ioctl = ext2_compat_ioctl,
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#endif
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.mmap = ext2_file_mmap,
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.open = dquot_file_open,
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.release = ext2_release_file,
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.fsync = ext2_fsync,
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.get_unmapped_area = thp_get_unmapped_area,
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.splice_read = generic_file_splice_read,
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.splice_write = iter_file_splice_write,
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};
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const struct inode_operations ext2_file_inode_operations = {
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#ifdef CONFIG_EXT2_FS_XATTR
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.listxattr = ext2_listxattr,
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#endif
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.setattr = ext2_setattr,
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.get_acl = ext2_get_acl,
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.set_acl = ext2_set_acl,
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.fiemap = ext2_fiemap,
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};
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