On Tue, Jun 02, 2020 at 07:42:37AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> When we dirty an inode, we are going to have to write it disk at
> some point in the near future. This requires the inode cluster
> backing buffer to be present in memory. Unfortunately, under severe
> memory pressure we can reclaim the inode backing buffer while the
> inode is dirty in memory, resulting in stalling the AIL pushing
> because it has to do a read-modify-write cycle on the cluster
> buffer.
>
> When we have no memory available, the read of the cluster buffer
> blocks the AIL pushing process, and this causes all sorts of issues
> for memory reclaim as it requires inode writeback to make forwards
> progress. Allocating a cluster buffer causes more memory pressure,
> and results in more cluster buffers to be reclaimed, resulting in
> more RMW cycles to be done in the AIL context and everything then
> backs up on AIL progress. Only the synchronous inode cluster
> writeback in the the inode reclaim code provides some level of
> forwards progress guarantees that prevent OOM-killer rampages in
> this situation.
>
> Fix this by pinning the inode backing buffer to the inode log item
> when the inode is first dirtied (i.e. in xfs_trans_log_inode()).
> This may mean the first modification of an inode that has been held
> in cache for a long time may block on a cluster buffer read, but
> we can do that in transaction context and block safely until the
> buffer has been allocated and read.
>
> Once we have the cluster buffer, the inode log item takes a
> reference to it, pinning it in memory, and attaches it to the log
> item for future reference. This means we can always grab the cluster
> buffer from the inode log item when we need it.
>
> When the inode is finally cleaned and removed from the AIL, we can
> drop the reference the inode log item holds on the cluster buffer.
> Once all inodes on the cluster buffer are clean, the cluster buffer
> will be unpinned and it will be available for memory reclaim to
> reclaim again.
>
> This avoids the issues with needing to do RMW cycles in the AIL
> pushing context, and hence allows complete non-blocking inode
> flushing to be performed by the AIL pushing context.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_inode_buf.c | 3 +-
> fs/xfs/libxfs/xfs_trans_inode.c | 53 +++++++++++++++++++++---
> fs/xfs/xfs_buf_item.c | 4 +-
> fs/xfs/xfs_inode_item.c | 73 +++++++++++++++++++++++++++------
> fs/xfs/xfs_trans_ail.c | 8 +++-
> 5 files changed, 117 insertions(+), 24 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
> index 6f84ea85fdd83..1af97235785c8 100644
> --- a/fs/xfs/libxfs/xfs_inode_buf.c
> +++ b/fs/xfs/libxfs/xfs_inode_buf.c
> @@ -176,7 +176,8 @@ xfs_imap_to_bp(
> }
>
> *bpp = bp;
> - *dipp = xfs_buf_offset(bp, imap->im_boffset);
> + if (dipp)
> + *dipp = xfs_buf_offset(bp, imap->im_boffset);
> return 0;
> }
>
> diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
> index fe6c2e39be85d..1e7147b90725e 100644
> --- a/fs/xfs/libxfs/xfs_trans_inode.c
> +++ b/fs/xfs/libxfs/xfs_trans_inode.c
> @@ -8,6 +8,8 @@
> #include "xfs_shared.h"
> #include "xfs_format.h"
> #include "xfs_log_format.h"
> +#include "xfs_trans_resv.h"
> +#include "xfs_mount.h"
> #include "xfs_inode.h"
> #include "xfs_trans.h"
> #include "xfs_trans_priv.h"
> @@ -72,13 +74,19 @@ xfs_trans_ichgtime(
> }
>
> /*
> - * This is called to mark the fields indicated in fieldmask as needing
> - * to be logged when the transaction is committed. The inode must
> - * already be associated with the given transaction.
> + * This is called to mark the fields indicated in fieldmask as needing to be
> + * logged when the transaction is committed. The inode must already be
> + * associated with the given transaction.
> *
> - * The values for fieldmask are defined in xfs_inode_item.h. We always
> - * log all of the core inode if any of it has changed, and we always log
> - * all of the inline data/extents/b-tree root if any of them has changed.
> + * The values for fieldmask are defined in xfs_inode_item.h. We always log all
> + * of the core inode if any of it has changed, and we always log all of the
> + * inline data/extents/b-tree root if any of them has changed.
> + *
> + * Grab and pin the cluster buffer associated with this inode to avoid RMW
> + * cycles at inode writeback time. Avoid the need to add error handling to every
> + * xfs_trans_log_inode() call by shutting down on read error. This will cause
> + * transactions to fail and everything to error out, just like if we return a
> + * read error in a dirty transaction and cancel it.
> */
> void
> xfs_trans_log_inode(
> @@ -132,6 +140,39 @@ xfs_trans_log_inode(
> spin_lock(&iip->ili_lock);
> iip->ili_fsync_fields |= flags;
>
> + if (!iip->ili_item.li_buf) {
> + struct xfs_buf *bp;
> + int error;
> +
> + /*
> + * We hold the ILOCK here, so this inode is not going to be
> + * flushed while we are here. Further, because there is no
> + * buffer attached to the item, we know that there is no IO in
> + * progress, so nothing will clear the ili_fields while we read
> + * in the buffer. Hence we can safely drop the spin lock and
> + * read the buffer knowing that the state will not change from
> + * here.
> + */
> + spin_unlock(&iip->ili_lock);
> + error = xfs_imap_to_bp(ip->i_mount, tp, &ip->i_imap, NULL,
> + &bp, 0);
> + if (error) {
> + xfs_force_shutdown(ip->i_mount, SHUTDOWN_META_IO_ERROR);
> + return;
> + }
> +
> + /*
> + * We need an explicit buffer reference for the log item but
> + * don't want the buffer to remain attached to the transaction.
> + * Hold the buffer but release the transaction reference.
> + */
> + xfs_buf_hold(bp);
> + xfs_trans_brelse(tp, bp);
> +
> + spin_lock(&iip->ili_lock);
> + iip->ili_item.li_buf = bp;
> + }
> +
> /*
> * Always OR in the bits from the ili_last_fields field. This is to
> * coordinate with the xfs_iflush() and xfs_iflush_done() routines in
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 2364a9aa2d71a..9739d64a46443 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -1131,11 +1131,9 @@ xfs_buf_inode_iodone(
> if (ret == 1)
> return;
> ASSERT(ret == 2);
> - spin_lock(&bp->b_mount->m_ail->ail_lock);
> list_for_each_entry(lip, &bp->b_li_list, li_bio_list) {
> - xfs_set_li_failed(lip, bp);
> + set_bit(XFS_LI_FAILED, &lip->li_flags);
Hm. So if I read this right, for inode buffers we set/clear LI_FAILED
directly (i.e. without messing with li_buf) because for inodes we want
to manage the pointer directly without LI_FAILED messing with it. That
way we can attach the buffer to the item when we dirty the inode, and
release it when iflush is finished (or aborts). Dquots retain the old
behavior (grab the buffer only while we're checkpointing a dquot item)
which is why the v1 series crashed in xfs/438, so we have to leave
xfs_set/clear_li_failed alone for now. Right?
If so,
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
--D
> }
> - spin_unlock(&bp->b_mount->m_ail->ail_lock);
> xfs_buf_relse(bp);
> return;
> }
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index 0ba75764a8dc5..0a7720b7a821a 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -130,6 +130,8 @@ xfs_inode_item_size(
> xfs_inode_item_data_fork_size(iip, nvecs, nbytes);
> if (XFS_IFORK_Q(ip))
> xfs_inode_item_attr_fork_size(iip, nvecs, nbytes);
> +
> + ASSERT(iip->ili_item.li_buf);
> }
>
> STATIC void
> @@ -439,6 +441,7 @@ xfs_inode_item_pin(
> struct xfs_inode *ip = INODE_ITEM(lip)->ili_inode;
>
> ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
> + ASSERT(lip->li_buf);
>
> trace_xfs_inode_pin(ip, _RET_IP_);
> atomic_inc(&ip->i_pincount);
> @@ -450,6 +453,12 @@ xfs_inode_item_pin(
> * item which was previously pinned with a call to xfs_inode_item_pin().
> *
> * Also wake up anyone in xfs_iunpin_wait() if the count goes to 0.
> + *
> + * Note that unpin can race with inode cluster buffer freeing marking the buffer
> + * stale. In that case, flush completions are run from the buffer unpin call,
> + * which may happen before the inode is unpinned. If we lose the race, there
> + * will be no buffer attached to the log item, but the inode will be marked
> + * XFS_ISTALE.
> */
> STATIC void
> xfs_inode_item_unpin(
> @@ -459,6 +468,7 @@ xfs_inode_item_unpin(
> struct xfs_inode *ip = INODE_ITEM(lip)->ili_inode;
>
> trace_xfs_inode_unpin(ip, _RET_IP_);
> + ASSERT(lip->li_buf || xfs_iflags_test(ip, XFS_ISTALE));
> ASSERT(atomic_read(&ip->i_pincount) > 0);
> if (atomic_dec_and_test(&ip->i_pincount))
> wake_up_bit(&ip->i_flags, __XFS_IPINNED_BIT);
> @@ -629,10 +639,15 @@ xfs_inode_item_init(
> */
> void
> xfs_inode_item_destroy(
> - xfs_inode_t *ip)
> + struct xfs_inode *ip)
> {
> - kmem_free(ip->i_itemp->ili_item.li_lv_shadow);
> - kmem_cache_free(xfs_ili_zone, ip->i_itemp);
> + struct xfs_inode_log_item *iip = ip->i_itemp;
> +
> + ASSERT(iip->ili_item.li_buf == NULL);
> +
> + ip->i_itemp = NULL;
> + kmem_free(iip->ili_item.li_lv_shadow);
> + kmem_cache_free(xfs_ili_zone, iip);
> }
>
>
> @@ -647,6 +662,13 @@ xfs_inode_item_destroy(
> * list for other inodes that will run this function. We remove them from the
> * buffer list so we can process all the inode IO completions in one AIL lock
> * traversal.
> + *
> + * Note: Now that we attach the log item to the buffer when we first log the
> + * inode in memory, we can have unflushed inodes on the buffer list here. These
> + * inodes will have a zero ili_last_fields, so skip over them here. We do
> + * this check -after- we've checked for stale inodes, because we're guaranteed
> + * to have XFS_ISTALE set in the case that dirty inodes are in the CIL and have
> + * not yet had their dirtying transactions committed to disk.
> */
> void
> xfs_iflush_done(
> @@ -670,14 +692,16 @@ xfs_iflush_done(
> continue;
> }
>
> + if (!iip->ili_last_fields)
> + continue;
> +
> list_move_tail(&lip->li_bio_list, &tmp);
>
> /* Do an unlocked check for needing the AIL lock. */
> - if (lip->li_lsn == iip->ili_flush_lsn ||
> + if (iip->ili_flush_lsn == lip->li_lsn ||
> test_bit(XFS_LI_FAILED, &lip->li_flags))
> need_ail++;
> }
> - ASSERT(list_empty(&bp->b_li_list));
>
> /*
> * We only want to pull the item from the AIL if it is actually there
> @@ -690,7 +714,7 @@ xfs_iflush_done(
> /* this is an opencoded batch version of xfs_trans_ail_delete */
> spin_lock(&ailp->ail_lock);
> list_for_each_entry(lip, &tmp, li_bio_list) {
> - xfs_clear_li_failed(lip);
> + clear_bit(XFS_LI_FAILED, &lip->li_flags);
> if (lip->li_lsn == INODE_ITEM(lip)->ili_flush_lsn) {
> xfs_lsn_t lsn = xfs_ail_delete_one(ailp, lip);
> if (!tail_lsn && lsn)
> @@ -706,14 +730,29 @@ xfs_iflush_done(
> * them is safely on disk.
> */
> list_for_each_entry_safe(lip, n, &tmp, li_bio_list) {
> + bool drop_buffer = false;
> +
> list_del_init(&lip->li_bio_list);
> iip = INODE_ITEM(lip);
>
> spin_lock(&iip->ili_lock);
> +
> + /*
> + * Remove the reference to the cluster buffer if the inode is
> + * clean in memory. Drop the buffer reference once we've dropped
> + * the locks we hold.
> + */
> + ASSERT(iip->ili_item.li_buf == bp);
> + if (!iip->ili_fields) {
> + iip->ili_item.li_buf = NULL;
> + drop_buffer = true;
> + }
> iip->ili_last_fields = 0;
> + iip->ili_flush_lsn = 0;
> spin_unlock(&iip->ili_lock);
> -
> xfs_ifunlock(iip->ili_inode);
> + if (drop_buffer)
> + xfs_buf_rele(bp);
> }
> }
>
> @@ -725,12 +764,20 @@ xfs_iflush_done(
> */
> void
> xfs_iflush_abort(
> - struct xfs_inode *ip)
> + struct xfs_inode *ip)
> {
> - struct xfs_inode_log_item *iip = ip->i_itemp;
> + struct xfs_inode_log_item *iip = ip->i_itemp;
> + struct xfs_buf *bp = NULL;
>
> if (iip) {
> + /*
> + * Clear the failed bit before removing the item from the AIL so
> + * xfs_trans_ail_delete() doesn't try to clear and release the
> + * buffer attached to the log item before we are done with it.
> + */
> + clear_bit(XFS_LI_FAILED, &iip->ili_item.li_flags);
> xfs_trans_ail_delete(&iip->ili_item, 0);
> +
> /*
> * Clear the inode logging fields so no more flushes are
> * attempted.
> @@ -739,12 +786,14 @@ xfs_iflush_abort(
> iip->ili_last_fields = 0;
> iip->ili_fields = 0;
> iip->ili_fsync_fields = 0;
> + iip->ili_flush_lsn = 0;
> + bp = iip->ili_item.li_buf;
> + iip->ili_item.li_buf = NULL;
> spin_unlock(&iip->ili_lock);
> }
> - /*
> - * Release the inode's flush lock since we're done with it.
> - */
> xfs_ifunlock(ip);
> + if (bp)
> + xfs_buf_rele(bp);
> }
>
> /*
> diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> index ac33f6393f99c..c3be6e4401343 100644
> --- a/fs/xfs/xfs_trans_ail.c
> +++ b/fs/xfs/xfs_trans_ail.c
> @@ -377,8 +377,12 @@ xfsaild_resubmit_item(
> }
>
> /* protected by ail_lock */
> - list_for_each_entry(lip, &bp->b_li_list, li_bio_list)
> - xfs_clear_li_failed(lip);
> + list_for_each_entry(lip, &bp->b_li_list, li_bio_list) {
> + if (bp->b_flags & _XBF_INODES)
> + clear_bit(XFS_LI_FAILED, &lip->li_flags);
> + else
> + xfs_clear_li_failed(lip);
> + }
>
> xfs_buf_unlock(bp);
> return XFS_ITEM_SUCCESS;
> --
> 2.26.2.761.g0e0b3e54be
>
