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iomap: fix handling of dirty folios over unwritten extents 

JIRA: https://issues.redhat.com/browse/RHEL-7901

commit c5c810b94cfd818fc2f58c96feee58a9e5ead96d
Author: Brian Foster <bfoster@redhat.com>
Date:   Fri Aug 30 10:56:33 2024 -0400

    iomap: fix handling of dirty folios over unwritten extents

    The iomap zero range implementation doesn't properly handle dirty
    pagecache over unwritten mappings. It skips such mappings as if they
    were pre-zeroed. If some part of an unwritten mapping is dirty in
    pagecache from a previous write, the data in cache should be zeroed
    as well. Instead, the data is left in cache and creates a stale data
    exposure problem if writeback occurs sometime after the zero range.

    Most callers are unaffected by this because the higher level
    filesystem contexts that call zero range typically perform a filemap
    flush of the target range for other reasons. A couple contexts that
    don't otherwise need to flush are write file size extension and
    truncate in XFS. The former path is currently susceptible to the
    stale data exposure problem and the latter performs a flush
    specifically to work around it.

    This is clearly inconsistent and incomplete. As a first step toward
    correcting behavior, lift the XFS workaround to iomap_zero_range()
    and unconditionally flush the range before the zero range operation
    proceeds. While this appears to be a bit of a big hammer, most all
    users already do this from calling context save for the couple of
    exceptions noted above. Future patches will optimize or elide this
    flush while maintaining functional correctness.

    Fixes: ae259a9c85 ("fs: introduce iomap infrastructure")
    Signed-off-by: Brian Foster <bfoster@redhat.com>
    Link: https://lore.kernel.org/r/20240830145634.138439-2-bfoster@redhat.com
    Reviewed-by: Darrick J. Wong <djwong@kernel.org>
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Brian Foster <bfoster@redhat.com>
This commit is contained in:
Brian Foster 2024-12-18 07:51:21 -05:00
parent 5150d3a815
commit cc305c4271
2 changed files with 10 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
fs/iomap/buffered-io.c
View File

@ -1369,6 +1369,16 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
};
int ret;
/*
* Zero range wants to skip pre-zeroed (i.e. unwritten) mappings, but
* pagecache must be flushed to ensure stale data from previous
* buffered writes is not exposed.
*/
ret = filemap_write_and_wait_range(inode->i_mapping,
pos, pos + len - 1);
if (ret)
return ret;
while ((ret = iomap_iter(&iter, ops)) > 0)
iter.processed = iomap_zero_iter(&iter, did_zero);
return ret;

10
fs/xfs/xfs_iops.c
View File

@ -841,16 +841,6 @@ xfs_setattr_size(
error = xfs_zero_range(ip, oldsize, newsize - oldsize,
&did_zeroing);
} else {
/*
* iomap won't detect a dirty page over an unwritten block (or a
* cow block over a hole) and subsequently skips zeroing the
* newly post-EOF portion of the page. Flush the new EOF to
* convert the block before the pagecache truncate.
*/
error = filemap_write_and_wait_range(inode->i_mapping, newsize,
newsize);
if (error)
return error;
error = xfs_truncate_page(ip, newsize, &did_zeroing);
}