On Mon, Dec 16, 2024 at 09:39:06AM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>
> When zeroing a range of folios on the filesystem which block size is
> less than the page size, the file's mapped blocks within one page will
> be marked as unwritten, we should remove writable userspace mappings to
> ensure that ext4_page_mkwrite() can be called during subsequent write
> access to these partial folios. Otherwise, data written by subsequent
> mmap writes may not be saved to disk.
>
> $mkfs.ext4 -b 1024 /dev/vdb
> $mount /dev/vdb /mnt
> $xfs_io -t -f -c "pwrite -S 0x58 0 4096" -c "mmap -rw 0 4096" \
> -c "mwrite -S 0x5a 2048 2048" -c "fzero 2048 2048" \
> -c "mwrite -S 0x59 2048 2048" -c "close" /mnt/foo
>
> $od -Ax -t x1z /mnt/foo
> 000000 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58
> *
> 000800 59 59 59 59 59 59 59 59 59 59 59 59 59 59 59 59
> *
> 001000
>
> $umount /mnt && mount /dev/vdb /mnt
> $od -Ax -t x1z /mnt/foo
> 000000 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58
> *
> 000800 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> *
> 001000
>
> Fix this by introducing ext4_truncate_page_cache_block_range() to remove
> writable userspace mappings when truncating a partial folio range.
> Additionally, move the journal data mode-specific handlers and
> truncate_pagecache_range() into this function, allowing it to serve as a
> common helper that correctly manages the page cache in preparation for
> block range manipulations.
Hi Zhang,
Thanks for the fix, just to confirm my understanding, the issue arises
because of the following flow:
1. page_mkwrite() makes folio dirty when we write to the mmap'd region
2. ext4_zero_range (2kb to 4kb)
truncate_pagecache_range
truncate_inode_pages_range
truncate_inode_partial_folio
folio_zero_range (2kb to 4kb)
folio_invalidate
ext4_invalidate_folio
block_invalidate_folio -> clear the bh dirty bit
3. mwrite (2kb to 4kb): Again we write in pagecache but the bh is not
dirty hence after a remount the data is not seen on disk
Also, we won't see this issue if we are zeroing a page aligned range
since we end up unmapping the pages from the proccess address space in
that case. Correct?
I have also tested the patch in PowerPC with 64k pagesize and 4k blocks
size and can confirm that it fixes the data loss issue. That being said,
I have a few minor comments on the patch below:
>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> ---
> fs/ext4/ext4.h | 2 ++
> fs/ext4/extents.c | 19 ++++-----------
> fs/ext4/inode.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 69 insertions(+), 14 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 74f2071189b2..8843929b46ce 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3016,6 +3016,8 @@ extern int ext4_inode_attach_jinode(struct inode *inode);
> extern int ext4_can_truncate(struct inode *inode);
> extern int ext4_truncate(struct inode *);
> extern int ext4_break_layouts(struct inode *);
> +extern int ext4_truncate_page_cache_block_range(struct inode *inode,
> + loff_t start, loff_t end);
> extern int ext4_punch_hole(struct file *file, loff_t offset, loff_t length);
> extern void ext4_set_inode_flags(struct inode *, bool init);
> extern int ext4_alloc_da_blocks(struct inode *inode);
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index a07a98a4b97a..8dc6b4271b15 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4667,22 +4667,13 @@ static long ext4_zero_range(struct file *file, loff_t offset,
> goto out_mutex;
> }
>
> - /*
> - * For journalled data we need to write (and checkpoint) pages
> - * before discarding page cache to avoid inconsitent data on
> - * disk in case of crash before zeroing trans is committed.
> - */
> - if (ext4_should_journal_data(inode)) {
> - ret = filemap_write_and_wait_range(mapping, start,
> - end - 1);
> - if (ret) {
> - filemap_invalidate_unlock(mapping);
> - goto out_mutex;
> - }
> + /* Now release the pages and zero block aligned part of pages */
> + ret = ext4_truncate_page_cache_block_range(inode, start, end);
> + if (ret) {
> + filemap_invalidate_unlock(mapping);
> + goto out_mutex;
> }
>
> - /* Now release the pages and zero block aligned part of pages */
> - truncate_pagecache_range(inode, start, end - 1);
> inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
>
> ret = ext4_alloc_file_blocks(file, lblk, max_blocks, new_size,
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 89aade6f45f6..c68a8b841148 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -31,6 +31,7 @@
> #include <linux/writeback.h>
> #include <linux/pagevec.h>
> #include <linux/mpage.h>
> +#include <linux/rmap.h>
> #include <linux/namei.h>
> #include <linux/uio.h>
> #include <linux/bio.h>
> @@ -3902,6 +3903,67 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
> return ret;
> }
>
> +static inline void ext4_truncate_folio(struct inode *inode,
> + loff_t start, loff_t end)
> +{
> + unsigned long blocksize = i_blocksize(inode);
> + struct folio *folio;
> +
> + /* Nothing to be done if no complete block needs to be truncated. */
> + if (round_up(start, blocksize) >= round_down(end, blocksize))
> + return;
> +
> + folio = filemap_lock_folio(inode->i_mapping, start >> PAGE_SHIFT);
> + if (IS_ERR(folio))
> + return;
> +
> + if (folio_mkclean(folio))
> + folio_mark_dirty(folio);
> + folio_unlock(folio);
> + folio_put(folio);
> +}
> +
> +int ext4_truncate_page_cache_block_range(struct inode *inode,
> + loff_t start, loff_t end)
> +{
> + unsigned long blocksize = i_blocksize(inode);
> + int ret;
> +
> + /*
> + * For journalled data we need to write (and checkpoint) pages
> + * before discarding page cache to avoid inconsitent data on disk
> + * in case of crash before freeing or unwritten converting trans
> + * is committed.
> + */
> + if (ext4_should_journal_data(inode)) {
> + ret = filemap_write_and_wait_range(inode->i_mapping, start,
> + end - 1);
> + if (ret)
> + return ret;
> + goto truncate_pagecache;
> + }
> +
> + /*
> + * If the block size is less than the page size, the file's mapped
> + * blocks within one page could be freed or converted to unwritten.
> + * So it's necessary to remove writable userspace mappings, and then
> + * ext4_page_mkwrite() can be called during subsequent write access
> + * to these partial folios.
> + */
> + if (blocksize < PAGE_SIZE && start < inode->i_size) {
Maybe we should only call ext4_truncate_folio() if the range is not page
aligned, rather than calling it everytime for bs < ps?
> + loff_t start_boundary = round_up(start, PAGE_SIZE);
I think page_boundary seems like a more suitable name for the variable.
Regards,
ojaswin
> +
> + ext4_truncate_folio(inode, start, min(start_boundary, end));
> + if (end > start_boundary)
> + ext4_truncate_folio(inode,
> + round_down(end, PAGE_SIZE), end);
> + }
> +
> +truncate_pagecache:
> + truncate_pagecache_range(inode, start, end - 1);
> + return 0;
> +}
> +
> static void ext4_wait_dax_page(struct inode *inode)
> {
> filemap_invalidate_unlock(inode->i_mapping);
> --
> 2.46.1
>
