Re: [PATCH v2] ceph: force a cap message when a deferred revoke can't be acked immediately

On Thu, Aug 13, 2026 at 1:49 AM Max Kellermann <[email protected]> wrote:
>
> When the MDS revokes capabilities, handle_cap_grant() normally
> guarantees a response by setting `CHECK_CAPS_FLUSH_FORCE` (see
> commit 31634d7597d8 ("ceph: force sending a cap update msg back to MDS
> for revoke op")), so ceph_check_caps() sends a cap message even if the
> client would otherwise decide it has nothing to do.  That guarantee is
> skipped whenever the revoke has to be deferred (via revoke_wait):
> revoking Fb while dirty data is still buffered (writeback is queued
> first) or revoking Fc while pages are cached (async invalidation is
> queued first).
>
> In those cases, the ack is left to the deferred completion
> (ceph_put_wrbuffer_cap_refs() after writeback, or the invalidate
> worker after invalidation); both of which call ceph_check_caps(ci,0)
> i.e.  without `CHECK_CAPS_FLUSH_FORCE`.  Nothing gets sent under one
> of the following conditions:
>
> - the inode is retaining caps because the file was used recently
>   (file_wanted != 0; retain |= CEPH_CAP_ANY)
>
> - the revoked cap is still used because the page was re-cached (e.g. a
>   file being re-read)
>
> - the MDS has meanwhile re-granted, so `issued==implemented` and the
>   client sees nothing being revoked
>
> The client then never emits the cap message which the MDS is waiting
> for.  The MDS blocks on the revoke indefinitely and logs, for minutes
> or hours:
>
>   client.NNN isn't responding to mclientcaps(revoke), ino 0x... pending
>   pAsxLsXsxFsxcrwb issued pAsxLsXsxFsxcrwb, sent 964.899182 seconds ago
>
> The client-side state at that point shows the full cap set still
> issued, nothing in the revoking/flushing sets.  Thus nothing gets
> sent.
>
> This patch fixes it by remembering that a forced response is expected.
> When a revoke is deferred, set `CEPH_I_FLUSH_FORCE` on the inode.
> ceph_check_caps() replays it as `CHECK_CAPS_FLUSH_FORCE`, so whichever
> path re-checks the inode next (the writeback/invalidate completion,
> the delayed worker, or any other caller) is guaranteed to send a cap
> message to the MDS.  __prep_cap() clears the flag once a message is
> actually built.

Thanks for the patch, Max. This probably fixes
https://tracker.ceph.com/issues/75735

>
> This is the deferred-path counterpart of the existing
> `CHECK_CAPS_FLUSH_FORCE` handling; a normal (non-deferred) revoke
> still forces the response inline as before.
>
> Cc: [email protected]
> Fixes: 31634d7597d8 ("ceph: force sending a cap update msg back to MDS for revoke op")
> Fixes: 257e6172ab36 ("ceph: don't let check_caps skip sending responses for revoke msgs")
> Signed-off-by: Max Kellermann <[email protected]>
> ---
> v1->v2: fix CEPH_I_FLUSH_FORCE/CEPH_I_FLUSH_FORCE_BIT mixup
> ---
>  fs/ceph/caps.c  | 40 +++++++++++++++++++++++++++++++++-------
>  fs/ceph/super.h |  5 +++++
>  2 files changed, 38 insertions(+), 7 deletions(-)
>
> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> index d7283fb54cec..8ad17787b172 100644
> --- a/fs/ceph/caps.c
> +++ b/fs/ceph/caps.c
> @@ -1410,6 +1410,7 @@ static void __prep_cap(struct cap_msg_args *arg, struct ceph_cap *cap,
>         BUG_ON((retain & CEPH_CAP_PIN) == 0);
>
>         clear_bit(CEPH_I_FLUSH_BIT, &ci->i_ceph_flags);
> +       clear_bit(CEPH_I_FLUSH_FORCE_BIT, &ci->i_ceph_flags);
>
>         cap->issued &= retain;  /* drop bits we don't want */
>         /*
> @@ -2038,6 +2039,14 @@ void ceph_check_caps(struct ceph_inode_info *ci, int flags)
>
>         if (ci->i_ceph_flags & CEPH_I_FLUSH)
>                 flags |= CHECK_CAPS_FLUSH;
> +       /*
> +        * A revoke whose response was deferred (see handle_cap_grant()) must
> +        * still be acknowledged.  Replay the forced flush here so that even a
> +        * check triggered by writeback/invalidation completion sends a cap
> +        * message to the MDS.
> +        */
> +       if (ci->i_ceph_flags & CEPH_I_FLUSH_FORCE)
> +               flags |= CHECK_CAPS_FLUSH_FORCE;
>  retry:
>         /* Caps wanted by virtue of active open files. */
>         file_wanted = __ceph_caps_file_wanted(ci);
> @@ -3757,13 +3766,30 @@ static void handle_cap_grant(struct inode *inode,
>         BUG_ON(cap->issued & ~cap->implemented);
>
>         /* don't let check_caps skip sending a response to MDS for revoke msgs */
> -       if (!revoke_wait && le32_to_cpu(grant->op) == CEPH_CAP_OP_REVOKE) {
> -               cap->mds_wanted = 0;
> -               flags |= CHECK_CAPS_FLUSH_FORCE;
> -               if (cap == ci->i_auth_cap)
> -                       check_caps = 1; /* check auth cap only */
> -               else
> -                       check_caps = 2; /* check all caps */
> +       if (le32_to_cpu(grant->op) == CEPH_CAP_OP_REVOKE) {
> +               if (revoke_wait) {
> +                       /*
> +                        * We can't ack the revoke yet: the response is deferred
> +                        * until the writeback or cache invalidation queued above
> +                        * completes.  Set the CEPH_I_FLUSH_FORCE flag to remember
> +                        * that a forced cap message is owed so that deferred
> +                        * completion (ceph_put_wrbuffer_cap_refs() or the
> +                        * invalidate worker, both of which call ceph_check_caps())
> +                        * actually sends one, even if by then the revoked caps look
> +                        * unused, the inode is retaining caps, or the MDS has
> +                        * re-granted them.  Without this, the cap message is never
> +                        * sent and the MDS hangs ("isn't responding to
> +                        * mclientcaps(revoke)").
> +                        */
> +                       ci->i_ceph_flags |= CEPH_I_FLUSH_FORCE;
> +               } else {
> +                       cap->mds_wanted = 0;
> +                       flags |= CHECK_CAPS_FLUSH_FORCE;
> +                       if (cap == ci->i_auth_cap)
> +                               check_caps = 1; /* check auth cap only */
> +                       else
> +                               check_caps = 2; /* check all caps */
> +               }
>         }
>
>         if (extra_info->inline_version > 0 &&
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index 1d6aab060780..878440a2eb3b 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -687,6 +687,10 @@ static inline struct inode *ceph_find_inode(struct super_block *sb,
>  #define CEPH_I_ASYNC_CREATE_BIT                (12) /* async create in flight for this */
>  #define CEPH_I_SHUTDOWN_BIT            (13) /* inode is no longer usable */
>  #define CEPH_I_ASYNC_CHECK_CAPS_BIT    (14) /* check caps after async creating finishes */
> +#define CEPH_I_FLUSH_FORCE_BIT         (15) /* a revoke's response was deferred;
> +                                             * force a cap message to the MDS once
> +                                             * the deferred work completes
> +                                             */
>
>  #define CEPH_I_DIR_ORDERED             (1 << CEPH_I_DIR_ORDERED_BIT)
>  #define CEPH_I_FLUSH                   (1 << CEPH_I_FLUSH_BIT)
> @@ -699,6 +703,7 @@ static inline struct inode *ceph_find_inode(struct super_block *sb,
>  #define CEPH_I_ODIRECT                 (1 << CEPH_I_ODIRECT_BIT)
>  #define CEPH_I_ASYNC_CREATE            (1 << CEPH_I_ASYNC_CREATE_BIT)
>  #define CEPH_I_SHUTDOWN                        (1 << CEPH_I_SHUTDOWN_BIT)
> +#define CEPH_I_FLUSH_FORCE             (1 << CEPH_I_FLUSH_FORCE_BIT)
>
>  /*
>   * Masks of ceph inode work.
> --
> 2.47.3
>
>


-- 
Cheers,
Venky