Re: [PATCH 26/30 V2] xfs: xfs_iflush() is no longer necessary

[Brian Foster <bfoster@redhat.com>]

On Tue, Jun 09, 2020 at 08:26:45AM +1000, Dave Chinner wrote:
> 
> From: Dave Chinner <dchinner@redhat.com>
> 
> Now we have a cached buffer on inode log items, we don't need
> to do buffer lookups when flushing inodes anymore - all we need
> to do is lock the buffer and we are ready to go.
> 
> This largely gets rid of the need for xfs_iflush(), which is
> essentially just a mechanism to look up the buffer and flush the
> inode to it. Instead, we can just call xfs_iflush_cluster() with a
> few modifications to ensure it also flushes the inode we already
> hold locked.
> 
> This allows the AIL inode item pushing to be almost entirely
> non-blocking in XFS - we won't block unless memory allocation
> for the cluster inode lookup blocks or the block device queues are
> full.
> 
> Writeback during inode reclaim becomes a little more complex because
> we now have to lock the buffer ourselves, but otherwise this change
> is largely a functional no-op that removes a whole lot of code.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> Version 2
> - removed unnecessary asserts
> - removed buffer delwri queue check from xfs_inode_item_push()
> 

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_inode.c      | 107 +++++++-----------------------------------------
>  fs/xfs/xfs_inode.h      |   2 +-
>  fs/xfs/xfs_inode_item.c |  51 ++++++++---------------
>  3 files changed, 33 insertions(+), 127 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index af65acd24ec4e..70e2197aad382 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -3450,7 +3450,18 @@ xfs_rename(
>  	return error;
>  }
>  
> -STATIC int
> +/*
> + * Non-blocking flush of dirty inode metadata into the backing buffer.
> + *
> + * The caller must have a reference to the inode and hold the cluster buffer
> + * locked. The function will walk across all the inodes on the cluster buffer it
> + * can find and lock without blocking, and flush them to the cluster buffer.
> + *
> + * On success, the caller must write out the buffer returned in *bp and
> + * release it. On failure, the filesystem will be shut down, the buffer will
> + * have been unlocked and released, and EFSCORRUPTED will be returned.
> + */
> +int
>  xfs_iflush_cluster(
>  	struct xfs_inode	*ip,
>  	struct xfs_buf		*bp)
> @@ -3485,8 +3496,6 @@ xfs_iflush_cluster(
>  
>  	for (i = 0; i < nr_found; i++) {
>  		cip = cilist[i];
> -		if (cip == ip)
> -			continue;
>  
>  		/*
>  		 * because this is an RCU protected lookup, we could find a
> @@ -3577,99 +3586,11 @@ xfs_iflush_cluster(
>  	kmem_free(cilist);
>  out_put:
>  	xfs_perag_put(pag);
> -	return error;
> -}
> -
> -/*
> - * Flush dirty inode metadata into the backing buffer.
> - *
> - * The caller must have the inode lock and the inode flush lock held.  The
> - * inode lock will still be held upon return to the caller, and the inode
> - * flush lock will be released after the inode has reached the disk.
> - *
> - * The caller must write out the buffer returned in *bpp and release it.
> - */
> -int
> -xfs_iflush(
> -	struct xfs_inode	*ip,
> -	struct xfs_buf		**bpp)
> -{
> -	struct xfs_mount	*mp = ip->i_mount;
> -	struct xfs_buf		*bp = NULL;
> -	struct xfs_dinode	*dip;
> -	int			error;
> -
> -	XFS_STATS_INC(mp, xs_iflush_count);
> -
> -	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL|XFS_ILOCK_SHARED));
> -	ASSERT(xfs_isiflocked(ip));
> -	ASSERT(ip->i_df.if_format != XFS_DINODE_FMT_BTREE ||
> -	       ip->i_df.if_nextents > XFS_IFORK_MAXEXT(ip, XFS_DATA_FORK));
> -
> -	*bpp = NULL;
> -
> -	xfs_iunpin_wait(ip);
> -
> -	/*
> -	 * For stale inodes we cannot rely on the backing buffer remaining
> -	 * stale in cache for the remaining life of the stale inode and so
> -	 * xfs_imap_to_bp() below may give us a buffer that no longer contains
> -	 * inodes below. We have to check this after ensuring the inode is
> -	 * unpinned so that it is safe to reclaim the stale inode after the
> -	 * flush call.
> -	 */
> -	if (xfs_iflags_test(ip, XFS_ISTALE)) {
> -		xfs_ifunlock(ip);
> -		return 0;
> -	}
> -
> -	/*
> -	 * Get the buffer containing the on-disk inode. We are doing a try-lock
> -	 * operation here, so we may get an EAGAIN error. In that case, return
> -	 * leaving the inode dirty.
> -	 *
> -	 * If we get any other error, we effectively have a corruption situation
> -	 * and we cannot flush the inode. Abort the flush and shut down.
> -	 */
> -	error = xfs_imap_to_bp(mp, NULL, &ip->i_imap, &dip, &bp, XBF_TRYLOCK);
> -	if (error == -EAGAIN) {
> -		xfs_ifunlock(ip);
> -		return error;
> -	}
> -	if (error)
> -		goto abort;
> -
> -	/*
> -	 * If the buffer is pinned then push on the log now so we won't
> -	 * get stuck waiting in the write for too long.
> -	 */
> -	if (xfs_buf_ispinned(bp))
> -		xfs_log_force(mp, 0);
> -
> -	/*
> -	 * Flush the provided inode then attempt to gather others from the
> -	 * cluster into the write.
> -	 *
> -	 * Note: Once we attempt to flush an inode, we must run buffer
> -	 * completion callbacks on any failure. If this fails, simulate an I/O
> -	 * failure on the buffer and shut down.
> -	 */
> -	error = xfs_iflush_int(ip, bp);
> -	if (!error)
> -		error = xfs_iflush_cluster(ip, bp);
>  	if (error) {
>  		bp->b_flags |= XBF_ASYNC;
>  		xfs_buf_ioend_fail(bp);
> -		goto shutdown;
> +		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
>  	}
> -
> -	*bpp = bp;
> -	return 0;
> -
> -abort:
> -	xfs_iflush_abort(ip);
> -shutdown:
> -	xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
>  	return error;
>  }
>  
> @@ -3687,7 +3608,7 @@ xfs_iflush_int(
>  	ASSERT(xfs_isiflocked(ip));
>  	ASSERT(ip->i_df.if_format != XFS_DINODE_FMT_BTREE ||
>  	       ip->i_df.if_nextents > XFS_IFORK_MAXEXT(ip, XFS_DATA_FORK));
> -	ASSERT(iip != NULL && iip->ili_fields != 0);
> +	ASSERT(iip->ili_item.li_buf == bp);
>  
>  	dip = xfs_buf_offset(bp, ip->i_imap.im_boffset);
>  
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index dadcf19458960..d1109eb13ba2e 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -427,7 +427,7 @@ int		xfs_log_force_inode(struct xfs_inode *ip);
>  void		xfs_iunpin_wait(xfs_inode_t *);
>  #define xfs_ipincount(ip)	((unsigned int) atomic_read(&ip->i_pincount))
>  
> -int		xfs_iflush(struct xfs_inode *, struct xfs_buf **);
> +int		xfs_iflush_cluster(struct xfs_inode *, struct xfs_buf *);
>  void		xfs_lock_two_inodes(struct xfs_inode *ip0, uint ip0_mode,
>  				struct xfs_inode *ip1, uint ip1_mode);
>  
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index 697248b7eb2be..e8eda2ac25fb4 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -485,53 +485,38 @@ xfs_inode_item_push(
>  	uint			rval = XFS_ITEM_SUCCESS;
>  	int			error;
>  
> -	if (xfs_ipincount(ip) > 0)
> +	ASSERT(iip->ili_item.li_buf);
> +
> +	if (xfs_ipincount(ip) > 0 || xfs_buf_ispinned(bp) ||
> +	    (ip->i_flags & XFS_ISTALE))
>  		return XFS_ITEM_PINNED;
>  
> -	if (!xfs_ilock_nowait(ip, XFS_ILOCK_SHARED))
> -		return XFS_ITEM_LOCKED;
> +	/* If the inode is already flush locked, we're already flushing. */
> +	if (xfs_isiflocked(ip))
> +		return XFS_ITEM_FLUSHING;
>  
> -	/*
> -	 * Re-check the pincount now that we stabilized the value by
> -	 * taking the ilock.
> -	 */
> -	if (xfs_ipincount(ip) > 0) {
> -		rval = XFS_ITEM_PINNED;
> -		goto out_unlock;
> -	}
> +	if (!xfs_buf_trylock(bp))
> +		return XFS_ITEM_LOCKED;
>  
> -	/*
> -	 * Stale inode items should force out the iclog.
> -	 */
> -	if (ip->i_flags & XFS_ISTALE) {
> -		rval = XFS_ITEM_PINNED;
> -		goto out_unlock;
> -	}
> +	spin_unlock(&lip->li_ailp->ail_lock);
>  
>  	/*
> -	 * Someone else is already flushing the inode.  Nothing we can do
> -	 * here but wait for the flush to finish and remove the item from
> -	 * the AIL.
> +	 * We need to hold a reference for flushing the cluster buffer as it may
> +	 * fail the buffer without IO submission. In which case, we better get a
> +	 * reference for that completion because otherwise we don't get a
> +	 * reference for IO until we queue the buffer for delwri submission.
>  	 */
> -	if (!xfs_iflock_nowait(ip)) {
> -		rval = XFS_ITEM_FLUSHING;
> -		goto out_unlock;
> -	}
> -
> -	ASSERT(iip->ili_fields != 0 || XFS_FORCED_SHUTDOWN(ip->i_mount));
> -	spin_unlock(&lip->li_ailp->ail_lock);
> -
> -	error = xfs_iflush(ip, &bp);
> +	xfs_buf_hold(bp);
> +	error = xfs_iflush_cluster(ip, bp);
>  	if (!error) {
>  		if (!xfs_buf_delwri_queue(bp, buffer_list))
>  			rval = XFS_ITEM_FLUSHING;
>  		xfs_buf_relse(bp);
> -	} else if (error == -EAGAIN)
> +	} else {
>  		rval = XFS_ITEM_LOCKED;
> +	}
>  
>  	spin_lock(&lip->li_ailp->ail_lock);
> -out_unlock:
> -	xfs_iunlock(ip, XFS_ILOCK_SHARED);
>  	return rval;
>  }
>  
>