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charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (by Flathub.org) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-07-17 at 17:33 +0800, Xiubo Li via B4 Relay wrote: > From: Xiubo Li >=20 > For O_APPEND writes, ki_pos is set to the current EOF via > generic_write_checks() after fetching i_size from the MDS.=C2=A0 However, > ceph_get_caps() may need to wait for Fwx exclusive caps if the write > extends the file (endoff > i_max_size).=C2=A0 While waiting for Fwx, the > previous Fwx holder (another client) may have already extended the > file.=C2=A0 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. >=20 > 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.=C2=A0 Only adjust ki_pos when the EOF > has actually changed. >=20 > Link: https://tracker.ceph.com/issues/7333 > Signed-off-by: Xiubo Li > --- > For O_APPEND writes, ki_pos is set to EOF via generic_write_checks() > after fetching i_size from the MDS via ceph_do_getattr().=C2=A0 However, > ceph_get_caps() may wait for Fwx exclusive caps (because endoff > > i_max_size).=C2=A0 During that wait, the previous Fwx holder may have > already extended the file.=C2=A0 When the MDS grants us Fwx, the local > i_size is updated via the cap grant message, but ki_pos is still > the old EOF, resulting in the append write overwriting data. >=20 > Fix by revalidating ki_pos against i_size_read(inode) after > ceph_get_caps() returns.=C2=A0 We now hold Fwx =E2=80=94 no other client = can > change the file, so i_size is stable and correct. >=20 > Fixes: 8e4473bb50a1 ("ceph: do not execute direct write in parallel > if O_APPEND is specified") > Link: https://tracker.ceph.com/issues/7333 The patch has two --- separators. The Fixes and Link will be finally dropped. > --- > =C2=A0fs/ceph/file.c | 26 ++++++++++++++++++++++++++ > =C2=A01 file changed, 26 insertions(+) >=20 > diff --git a/fs/ceph/file.c b/fs/ceph/file.c > index 9d89d7fc1095..b35c9aee48dd 100644 > --- a/fs/ceph/file.c > +++ b/fs/ceph/file.c > @@ -2491,6 +2491,32 @@ static ssize_t ceph_write_iter(struct kiocb > *iocb, struct iov_iter *from) > =C2=A0 if (err < 0) > =C2=A0 goto out; ceph_get_caps() validated caps against the old endoff =3D pos + count. After adjusting pos forward, the real write range is [cur_eof, cur_eof + count), which isn't re-validated against i_max_size. Could it be a potential issue? Thanks, Slava. > =C2=A0 > + /* > + * For O_APPEND writes we may have waited for Fwx exclusive > caps > + * while the previous Fwx holder (another client) extended > the > + * file.=C2=A0 i_size has been updated via the cap grant message > from > + * the MDS, but ki_pos is still the old EOF.=C2=A0 Re-read i_size > here > + * (no extra MDS round-trip needed) and adjust ki_pos to the > true > + * EOF.=C2=A0 Since we hold Fwx, no other client can change the > file. > + */ > + if (iocb->ki_flags & IOCB_APPEND) { > + loff_t cur_eof =3D i_size_read(inode); > + > + if (cur_eof !=3D pos) { > + doutc(cl, > + =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "%p %llx.%llx O_APPEND: pos adjusted > %lld -> %lld\n", > + =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 inode, ceph_vinop(inode), pos, > cur_eof); > + iocb->ki_pos =3D cur_eof; > + pos =3D cur_eof; > + if (pos >=3D limit) { > + err =3D -EFBIG; > + goto out_caps; > + } > + iov_iter_truncate(from, limit - pos); > + count =3D iov_iter_count(from); > + } > + } > + > =C2=A0 err =3D file_update_time(file); > =C2=A0 if (err) > =C2=A0 goto out_caps; >=20 > --- > base-commit: df47791ab9f12cd9144aca477ddae195f68072a5 > change-id: 20260717-ceph-append-fix-9820e3b1a78c >=20 > Best regards, > --=C2=A0=20 > Xiubo Li >=20