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ceph: revalidate ki_pos for O_APPEND writes after cap acquisition

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For O_APPEND writes, ki_pos is set to the current EOF via
generic_write_checks() after fetching i_size from the MDS.  However,
ceph_get_caps() may need to wait for Fwx exclusive caps if the write
extends the file (endoff > i_max_size).  While waiting for Fwx, the
previous Fwx holder (another client) may have already extended the
file.  When the MDS grants us Fwx, the cap grant message updates the
local i_size, but ki_pos remains at the old EOF, causing the append
write to land at a stale offset and overwrite data from the other
client.

Fix by re-reading i_size_read(inode) after ceph_get_caps() returns.
At this point we hold Fwx exclusive caps, no other client can modify
the file, and i_size reflects the true EOF from the MDS cap grant.
No extra MDS round-trip is needed.  Only adjust ki_pos when the EOF
has actually changed.

After adjusting ki_pos forward, the write range [pos, pos+count) may
now exceed the i_max_size that was validated by ceph_get_caps() for
the old range.  Re-check against i_max_size and truncate the write
if necessary to stay within the MDS-granted limit.

Link: https://tracker.ceph.com/issues/7333
Fixes: 8e4473bb50 ("ceph: do not execute direct write in parallel if O_APPEND is specified")
Signed-off-by: Xiubo Li <xiubo.li@clyso.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
This commit is contained in:
Xiubo Li
2026-07-21 13:06:53 +08:00
committed by Ilya Dryomov
parent a354d7eaa1
commit d2a8d446a0
1 changed files with 48 additions and 0 deletions
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48
fs/ceph/file.c
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@@ -2477,6 +2477,54 @@ static ssize_t ceph_write_iter(struct kiocb *iocb, struct iov_iter *from)
if (err < 0)
goto out;
/*
* For O_APPEND writes we may have waited for Fwx exclusive caps
* while the previous Fwx holder (another client) extended the
* file. i_size has been updated via the cap grant message from
* the MDS, but ki_pos is still the old EOF. Re-read i_size here
* (no extra MDS round-trip needed) and adjust ki_pos to the true
* EOF. Since we hold Fwx, no other client can change the file.
*/
if (iocb->ki_flags & IOCB_APPEND) {
loff_t cur_eof = i_size_read(inode);
if (cur_eof != pos) {
doutc(cl,
"%p %llx.%llx O_APPEND: pos adjusted %lld -> %lld\n",
inode, ceph_vinop(inode), pos, cur_eof);
iocb->ki_pos = cur_eof;
pos = cur_eof;
if (pos >= limit) {
err = -EFBIG;
goto out_caps;
}
iov_iter_truncate(from, limit - pos);
count = iov_iter_count(from);
/*
* ceph_get_caps() validated the old endoff
* against i_max_size; adjusting ki_pos forward
* may have shifted the write range beyond the
* granted max_size. Re-check and truncate if
* necessary.
*/
spin_lock(&ci->i_ceph_lock);
if (pos + count > (loff_t)ci->i_max_size) {
loff_t max_size = ci->i_max_size;
spin_unlock(&ci->i_ceph_lock);
if (pos >= max_size) {
err = -EFBIG;
goto out_caps;
}
iov_iter_truncate(from, max_size - pos);
count = iov_iter_count(from);
} else {
spin_unlock(&ci->i_ceph_lock);
}
}
}
err = file_update_time(file);
if (err)
goto out_caps;