Re: [PATCH 18/24] xfs: clean up inode reclaim comments

Darrick J. Wong
2020-05-22T23:19:54Z

On Fri, May 22, 2020 at 01:50:23PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Inode reclaim is quite different now to the way described in various
> comments, so update all the comments explaining what it does and how
> it works.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_icache.c | 128 ++++++++++++--------------------------------
> 1 file changed, 35 insertions(+), 93 deletions(-)
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 8b366bc7b53c9..81c23384d3310 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -141,11 +141,8 @@ xfs_inode_free(
> }
>
> /*
> - * Queue a new inode reclaim pass if there are reclaimable inodes and there
> - * isn't a reclaim pass already in progress. By default it runs every 5s based
> - * on the xfs periodic sync default of 30s. Perhaps this should have it's own
> - * tunable, but that can be done if this method proves to be ineffective or too
> - * aggressive.
> + * Queue background inode reclaim work if there are reclaimable inodes and there
> + * isn't reclaim work already scheduled or in progress.
> */
> static void
> xfs_reclaim_work_queue(
> @@ -164,8 +161,7 @@ xfs_reclaim_work_queue(
> * This is a fast pass over the inode cache to try to get reclaim moving on as
> * many inodes as possible in a short period of time. It kicks itself every few
> * seconds, as well as being kicked by the inode cache shrinker when memory
> - * goes low. It scans as quickly as possible avoiding locked inodes or those
> - * already being flushed, and once done schedules a future pass.
> + * goes low.
> */
> void
> xfs_reclaim_worker(
> @@ -618,48 +614,31 @@ xfs_iget_cache_miss(
> }
>
> /*
> - * Look up an inode by number in the given file system.
> - * The inode is looked up in the cache held in each AG.
> - * If the inode is found in the cache, initialise the vfs inode
> - * if necessary.
> + * Look up an inode by number in the given file system. The inode is looked up
> + * in the cache held in each AG. If the inode is found in the cache, initialise
> + * the vfs inode if necessary.
> *
> - * If it is not in core, read it in from the file system's device,
> - * add it to the cache and initialise the vfs inode.
> + * If it is not in core, read it in from the file system's device, add it to the
> + * cache and initialise the vfs inode.
> *
> * The inode is locked according to the value of the lock_flags parameter.
> - * This flag parameter indicates how and if the inode's IO lock and inode lock
> - * should be taken.
> - *
> - * mp -- the mount point structure for the current file system. It points
> - * to the inode hash table.
> - * tp -- a pointer to the current transaction if there is one. This is
> - * simply passed through to the xfs_iread() call.
> - * ino -- the number of the inode desired. This is the unique identifier
> - * within the file system for the inode being requested.
> - * lock_flags -- flags indicating how to lock the inode. See the comment
> - * for xfs_ilock() for a list of valid values.
> + * Inode lookup is only done during metadata operations and not as part of the
> + * data IO path. Hence we only allow locking of the XFS_ILOCK during lookup.
> */
> int
> xfs_iget(
> - xfs_mount_t *mp,
> - xfs_trans_t *tp,
> - xfs_ino_t ino,
> - uint flags,
> - uint lock_flags,
> - xfs_inode_t **ipp)
> + struct xfs_mount *mp,
> + struct xfs_trans *tp,
> + xfs_ino_t ino,
> + uint flags,
> + uint lock_flags,
> + struct xfs_inode **ipp)
> {
> - xfs_inode_t *ip;
> - int error;
> - xfs_perag_t *pag;
> - xfs_agino_t agino;
> + struct xfs_inode *ip;
> + struct xfs_perag *pag;
> + xfs_agino_t agino;
> + int error;
>
> - /*
> - * xfs_reclaim_inode() uses the ILOCK to ensure an inode
> - * doesn't get freed while it's being referenced during a
> - * radix tree traversal here. It assumes this function
> - * aqcuires only the ILOCK (and therefore it has no need to
> - * involve the IOLOCK in this synchronization).
> - */
> ASSERT((lock_flags & (XFS_IOLOCK_EXCL | XFS_IOLOCK_SHARED)) == 0);
>
> /* reject inode numbers outside existing AGs */
> @@ -771,15 +750,7 @@ xfs_inode_ag_walk_grab(
>
> ASSERT(rcu_read_lock_held());
>
> - /*
> - * check for stale RCU freed inode
> - *
> - * If the inode has been reallocated, it doesn't matter if it's not in
> - * the AG we are walking - we are walking for writeback, so if it
> - * passes all the "valid inode" checks and is dirty, then we'll write
> - * it back anyway. If it has been reallocated and still being
> - * initialised, the XFS_INEW check below will catch it.
> - */
> + /* Check for stale RCU freed inode */
> spin_lock(&ip->i_flags_lock);
> if (!ip->i_ino)
> goto out_unlock_noent;
> @@ -1089,43 +1060,16 @@ xfs_reclaim_inode_grab(
> }
>
> /*
> - * Inodes in different states need to be treated differently. The following
> - * table lists the inode states and the reclaim actions necessary:
> - *
> - * inode state iflush ret required action
> - * --------------- ---------- ---------------
> - * bad - reclaim
> - * shutdown EIO unpin and reclaim
> - * clean, unpinned 0 reclaim
> - * stale, unpinned 0 reclaim
> - * clean, pinned(*) 0 requeue
> - * stale, pinned EAGAIN requeue
> - * dirty, async - requeue
> - * dirty, sync 0 reclaim
> + * Inode reclaim is non-blocking, so the default action if progress cannot be
> + * made is to "requeue" the inode for reclaim by unlocking it and clearing the
> + * XFS_IRECLAIM flag. If we are in a shutdown state, we don't care about
> + * blocking anymore and hence we can wait for the inode to be able to reclaim
> + * it.
> *
> - * (*) dgc: I don't think the clean, pinned state is possible but it gets
> - * handled anyway given the order of checks implemented.
> - *
> - * Also, because we get the flush lock first, we know that any inode that has
> - * been flushed delwri has had the flush completed by the time we check that
> - * the inode is clean.
> - *
> - * Note that because the inode is flushed delayed write by AIL pushing, the
> - * flush lock may already be held here and waiting on it can result in very
> - * long latencies. Hence for sync reclaims, where we wait on the flush lock,
> - * the caller should push the AIL first before trying to reclaim inodes to
> - * minimise the amount of time spent waiting. For background relaim, we only
> - * bother to reclaim clean inodes anyway.
> - *
> - * Hence the order of actions after gaining the locks should be:
> - * bad => reclaim
> - * shutdown => unpin and reclaim
> - * pinned, async => requeue
> - * pinned, sync => unpin
> - * stale => reclaim
> - * clean => reclaim
> - * dirty, async => requeue
> - * dirty, sync => flush, wait and reclaim
> + * We do no IO here - if callers require indoes to be cleaned they must push the

"...require inodes to be cleaned..."

> + * AIL first to trigger writeback of dirty inodes. The enbales both the

"This enables writeback to be done in the background in a non-blocking
manner, and enables memory reclaim to make progress without blocking."

--D

> + * writeback to be done in the background in a non-blocking manner, as well as
> + * allowing reclaim to make progress without blocking.
> */
> static bool
> xfs_reclaim_inode(
> @@ -1327,13 +1271,11 @@ xfs_reclaim_inodes(
> }
>
> /*
> - * Scan a certain number of inodes for reclaim.
> - *
> - * When called we make sure that there is a background (fast) inode reclaim in
> - * progress, while we will throttle the speed of reclaim via doing synchronous
> - * reclaim of inodes. That means if we come across dirty inodes, we wait for
> - * them to be cleaned, which we hope will not be very long due to the
> - * background walker having already kicked the IO off on those dirty inodes.
> + * The shrinker infrastructure determines how many inodes we should scan for
> + * reclaim. We want as many clean inodes ready to reclaim as possible, so we
> + * push the AIL here. We also want to proactively free up memory if we can to
> + * minimise the amount of work memory reclaim has to do so we kick the
> + * background reclaim if it isn't already scheduled.
> */
> long
> xfs_reclaim_inodes_nr(
> --
> 2.26.2.761.g0e0b3e54be
>