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[BUG]
For the following write sequence with 64K page size and 4K fs block size,
it will lead to file extent items to be inserted without any data
checksum:
mkfs.btrfs -s 4k -f $dev > /dev/null
mount $dev $mnt
xfs_io -f -c "pwrite 0 16k" -c "pwrite 32k 4k" -c pwrite "60k 64K" \
-c "truncate 16k" $mnt/foobar
umount $mnt
This will result the following 2 file extent items to be inserted (extra
trace point added to insert_ordered_extent_file_extent()):
btrfs_finish_one_ordered: root=5 ino=257 file_off=61440 num_bytes=4096 csum_bytes=0
btrfs_finish_one_ordered: root=5 ino=257 file_off=0 num_bytes=16384 csum_bytes=16384
Note for file offset 60K, we're inserting a file extent without any
data checksum.
Also note that range [32K, 36K) didn't reach
insert_ordered_extent_file_extent(), which is the correct behavior as
that OE is fully truncated, should not result any file extent.
Although file extent at 60K will be later dropped by btrfs_truncate(),
if the transaction got committed after file extent inserted but before
the file extent dropping, we will have a small window where we have a
file extent beyond EOF and without any data checksum.
That will cause "btrfs check" to report error.
[CAUSE]
The sequence happens like this:
- Buffered write dirtied the page cache and updated isize
Now the inode size is 64K, with the following page cache layout:
0 16K 32K 48K 64K
|/////////////| |//| |//|
- Truncate the inode to 16K
Which will trigger writeback through:
btrfs_setsize()
|- truncate_setsize()
| Now the inode size is set to 16K
|
|- btrfs_truncate()
|- btrfs_wait_ordered_range() for [16K, u64(-1)]
|- btrfs_fdatawrite_range() for [16K, u64(-1)}
|- extent_writepage() for folio 0
|- writepage_delalloc()
| Generated OE for [0, 16K), [32K, 36K] and [60K, 64K)
|
|- extent_writepage_io()
Then inside extent_writepage_io(), the dirty fs blocks are handled
differently:
- Submit write for range [0, 16K)
As they are still inside the inode size (16K).
- Mark OE [32K, 36K) as truncated
Since we only call btrfs_lookup_first_ordered_range() once, which
returned the first OE after file offset 16K.
- Mark all OEs inside range [16K, 64K) as finished
Which will mark OE ranges [32K, 36K) and [60K, 64K) as finished.
For OE [32K, 36K) since it's already marked as truncated, and its
truncated length is 0, no file extent will be inserted.
For OE [60K, 64K) it has never been submitted thus has no data
checksum, and we insert the file extent as usual.
This is the root cause of file extent at 60K to be inserted without
any data checksum.
- Clear dirty flags for range [16K, 64K)
It is the function btrfs_folio_clear_dirty() which searches and clears
any dirty blocks inside that range.
[FIX]
The bug itself was introduced a long time ago, way before subpage and
large folio support.
At that time, fs block size must match page size, thus the range
[cur, end) is just one fs block.
But later with subpage and large folios, the same range [cur, end)
can have multiple blocks and ordered extents.
Later commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.