On Fri, Mar 02, 2018 at 09:36:32AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> When looking at an event trace recently, I noticed that non-blocking
> buffer lookup attempts would fail on cached locked buffers and then
> run the slow cache-miss path. This means we are doing an xfs_buf
> allocation, lookup and free unnecessarily every time we avoid
> blocking on a locked buffer.
>
> Fix this by changing _xfs_buf_find() to return an error status
> encoded via ERR_PTR() to the caller to indicate that we failed the
> lock attempt rather than just returning a NULL. This allows the
> higher level code to discriminate between a cache miss and an cache
> hit that we failed to lock.
>
> This also allows us to return a -EFSCORRUPTED state if we are asked
> to look up a block number outside the range of the filesystem in
> _xfs_buf_find(), which moves us one step closer to being able to
> handle such errors in a more graceful manner at the higher levels.
>
> Finally, to ensure code outside the buffer cache does not see any
> change, convert external callers to use xfs_incore() and change that
> to an inline function that maintains the old "buffer or NULL" return
> values so the external code doesn't need to care about this internal
> change to _xfs_buf_find() semantics.
>
> Signed-Off-By: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_buf.c | 56 +++++++++++++++++++++++++++++++++++++++++---------------
> fs/xfs/xfs_buf.h | 8 +++++++-
> fs/xfs/xfs_qm.c | 5 ++---
> 3 files changed, 50 insertions(+), 19 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 88587b33dd15..fa1e62ac5e8c 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -549,20 +549,32 @@ xfs_buf_hash_destroy(
> }
>
> /*
> - * Look up, and creates if absent, a lockable buffer for
> - * a given range of an inode. The buffer is returned
> - * locked. No I/O is implied by this call.
> + * Look up a buffer in the buffer cache and return it referenced and locked.
> + *
> + * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
> + * cache.
> + *
> + * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
> + * -EAGAIN if we fail to lock it.
> + *
> + * Return values are:
> + * -EFSCORRUPTED if have been supplied with an invalid address
> + * -EAGAIN on trylock failure
> + * NULL if we fail to find a match and @new_bp was NULL
> + * @new_bp if we inserted it into the cache
> + * the buffer we found and locked.
> */
> -xfs_buf_t *
> +
> +struct xfs_buf *
> _xfs_buf_find(
> struct xfs_buftarg *btp,
> struct xfs_buf_map *map,
> int nmaps,
> xfs_buf_flags_t flags,
> - xfs_buf_t *new_bp)
> + struct xfs_buf *new_bp)
> {
> struct xfs_perag *pag;
> - xfs_buf_t *bp;
> + struct xfs_buf *bp;
> struct xfs_buf_map cmap = { .bm_bn = map[0].bm_bn };
> xfs_daddr_t eofs;
> int i;
> @@ -580,16 +592,11 @@ _xfs_buf_find(
> */
> eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
> if (cmap.bm_bn < 0 || cmap.bm_bn >= eofs) {
> - /*
> - * XXX (dgc): we should really be returning -EFSCORRUPTED here,
> - * but none of the higher level infrastructure supports
> - * returning a specific error on buffer lookup failures.
> - */
> xfs_alert(btp->bt_mount,
> "%s: daddr 0x%llx out of range, EOFS 0x%llx",
> __func__, cmap.bm_bn, eofs);
> WARN_ON(1);
> - return NULL;
> + return ERR_PTR(-EFSCORRUPTED);
> }
>
> pag = xfs_perag_get(btp->bt_mount,
> @@ -626,7 +633,7 @@ _xfs_buf_find(
> if (flags & XBF_TRYLOCK) {
> xfs_buf_rele(bp);
> XFS_STATS_INC(btp->bt_mount, xb_busy_locked);
> - return NULL;
> + return ERR_PTR(-EAGAIN);
> }
> xfs_buf_lock(bp);
> XFS_STATS_INC(btp->bt_mount, xb_get_locked_waited);
> @@ -666,9 +673,28 @@ xfs_buf_get_map(
> int error = 0;
>
> bp = _xfs_buf_find(target, map, nmaps, flags, NULL);
> - if (likely(bp))
> + if (!IS_ERR_OR_NULL(bp))
> goto found;
>
> + switch (PTR_ERR(bp)) {
> + case 0:
> + /* cache miss, need to insert new buffer */
> + break;
> +
> + case -EAGAIN:
> + /* cache hit, trylock failure, caller handles failure */
> + ASSERT(flags & XBF_TRYLOCK);
> + return NULL;
> +
> + case -EFSCORRUPTED:
> + default:
> + /*
> + * None of the higher layers understand failure types
> + * yet, so return NULL to signal a fatal lookup error.
> + */
> + return NULL;
Should I expect a follow-on patch to fix the higher layers?
> + }
> +
> new_bp = _xfs_buf_alloc(target, map, nmaps, flags);
> if (unlikely(!new_bp))
> return NULL;
> @@ -680,7 +706,7 @@ xfs_buf_get_map(
> }
>
> bp = _xfs_buf_find(target, map, nmaps, flags, new_bp);
> - if (!bp) {
> + if (IS_ERR_OR_NULL(bp)) {
> xfs_buf_free(new_bp);
> return NULL;
> }
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index 2f4c91452861..db87ad0b9b79 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -229,8 +229,14 @@ xfs_incore(
> size_t numblks,
> xfs_buf_flags_t flags)
> {
> + struct xfs_buf *bp;
> +
> DEFINE_SINGLE_BUF_MAP(map, blkno, numblks);
> - return _xfs_buf_find(target, &map, 1, flags, NULL);
> +
> + bp = _xfs_buf_find(target, &map, 1, flags, NULL);
> + if (IS_ERR_OR_NULL(bp))
> + return NULL;
> + return bp;
> }
>
> struct xfs_buf *_xfs_buf_alloc(struct xfs_buftarg *target,
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index 5b848f4b637f..8d90d19684a7 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -1249,9 +1249,8 @@ xfs_qm_flush_one(
> */
> if (!xfs_dqflock_nowait(dqp)) {
> /* buf is pinned in-core by delwri list */
> - DEFINE_SINGLE_BUF_MAP(map, dqp->q_blkno,
> - mp->m_quotainfo->qi_dqchunklen);
> - bp = _xfs_buf_find(mp->m_ddev_targp, &map, 1, 0, NULL);
> + bp = xfs_incore(mp->m_ddev_targp, dqp->q_blkno,
> + mp->m_quotainfo->qi_dqchunklen, 0);
Looks ok otherwise.
--D
> if (!bp) {
> error = -EINVAL;
> goto out_unlock;
> --
> 2.16.1
>
> --
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