Re: [PATCH v13 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

Dave Chinner
2020-12-18T00:14:49Z

On Thu, Dec 17, 2020 at 09:11:57AM +0800, Yafang Shao wrote:
> PF_FSTRANS which is used to avoid transaction reservation recursion, is
> dropped since commit 9070733b4efa ("xfs: abstract PF_FSTRANS to
> PF_MEMALLOC_NOFS") and replaced by PF_MEMALLOC_NOFS which means to avoid
> filesystem reclaim recursion.
>
> As these two flags have different meanings, we'd better reintroduce
> PF_FSTRANS back. To avoid wasting the space of PF_* flags in task_struct,
> we can reuse the current->journal_info to do that, per Willy. As the
> check of transaction reservation recursion is used by XFS only, we can
> move the check into xfs_vm_writepage(s), per Dave.
>
> Cc: Darrick J. Wong <darrick.wong@oracle.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Jeff Layton <jlayton@redhat.com>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
> fs/iomap/buffered-io.c | 7 -------
> fs/xfs/xfs_aops.c | 17 +++++++++++++++++
> fs/xfs/xfs_trans.h | 26 +++++++++++++++++++-------
> 3 files changed, 36 insertions(+), 14 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 10cc7979ce38..3c53fa6ce64d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1458,13 +1458,6 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
> PF_MEMALLOC))
> goto redirty;
>
> - /*
> - * Given that we do not allow direct reclaim to call us, we should
> - * never be called in a recursive filesystem reclaim context.
> - */
> - if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> - goto redirty;
> -
> /*
> * Is this page beyond the end of the file?
> *
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 2371187b7615..0da0242d42c3 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -568,6 +568,16 @@ xfs_vm_writepage(
> {
> struct xfs_writepage_ctx wpc = { };
>
> + /*
> + * Given that we do not allow direct reclaim to call us, we should
> + * never be called while in a filesystem transaction.
> + */

Comment is wrong. This is not protecting against direct reclaim
recursion, this is protecting against writeback from within a
transaction context.

Best to remove the comment altogether, because it is largely
redundant.

> + if (WARN_ON_ONCE(xfs_trans_context_active())) {
> + redirty_page_for_writepage(wbc, page);
> + unlock_page(page);
> + return 0;
> + }
> +
> return iomap_writepage(page, wbc, &wpc.ctx, &xfs_writeback_ops);
> }
>
> @@ -579,6 +589,13 @@ xfs_vm_writepages(
> struct xfs_writepage_ctx wpc = { };
>
> xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
> + /*
> + * Given that we do not allow direct reclaim to call us, we should
> + * never be called while in a filesystem transaction.
> + */

same here.

> + if (WARN_ON_ONCE(xfs_trans_context_active()))
> + return 0;
> +
> return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
> }
>
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 12380eaaf7ce..0c8140147b9b 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -268,29 +268,41 @@ xfs_trans_item_relog(
> return lip->li_ops->iop_relog(lip, tp);
> }
>
> +static inline bool
> +xfs_trans_context_active(void)
> +{
> + /* Use journal_info to indicate current is in a transaction */
> + return current->journal_info != NULL;
> +}

Comment is not necessary.

> +
> static inline void
> xfs_trans_context_set(struct xfs_trans *tp)
> {
> + ASSERT(!current->journal_info);
> + current->journal_info = tp;
> tp->t_pflags = memalloc_nofs_save();
> }
>
> static inline void
> xfs_trans_context_clear(struct xfs_trans *tp)
> {
> + /*
> + * If xfs_trans_context_swap() handed the NOFS context to a
> + * new transaction we do not clear the context here.
> + */

It's a transaction context, not a "NOFS context". Setting NOFS is
just something we implement inside the transaction context. More
correct would be:

/*
* If we handed over the context via xfs_trans_context_swap() then
* the context is no longer ours to clear.
*/

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com