Re: [PATCH v11 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Dec 08, 2020 at 08:28:24PM +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 commit 7dea19f9ee63 ("mm: introduce
> memalloc_nofs_{save,restore} API") 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.
> 
> To better abstract that behavoir, two new helpers are introduced, as
> follows,
> - xfs_trans_context_active
>   To check whehter current is in fs transcation or not
> - xfs_trans_context_swap
>   Transfer the transaction context when rolling a permanent transaction
> 
> These two new helpers are instroduced in xfs_trans.h.
> 
> 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.c     |  3 +++
>  fs/xfs/xfs_trans.h     | 22 ++++++++++++++++++++++
>  4 files changed, 42 insertions(+), 7 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.
> +	 */
> +	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.
> +	 */
> +	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.c b/fs/xfs/xfs_trans.c
> index fe20398a214e..08d4916ffb13 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -124,6 +124,9 @@ xfs_trans_dup(
>  	tp->t_rtx_res = tp->t_rtx_res_used;
>  	ntp->t_pflags = tp->t_pflags;

This one line (ntp->t_pflags = tp->t_pflags) should move to
xfs_trans_context_swap.

--D

>  
> +	/* Associate the new transaction with this thread. */
> +	xfs_trans_context_swap(tp, ntp);
> +
>  	/* move deferred ops over to the new tp */
>  	xfs_defer_move(ntp, tp);
>  
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 44b11c64a15e..d596a375e3bf 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -268,16 +268,38 @@ 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;
> +}
> +
>  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)
>  {
> +	ASSERT(current->journal_info == tp);
> +	current->journal_info = NULL;
>  	memalloc_nofs_restore(tp->t_pflags);
>  }
>  
> +/*
> + * Transfer the transaction context when rolling a permanent
> + * transaction.
> + */
> +static inline void
> +xfs_trans_context_swap(struct xfs_trans *tp, struct xfs_trans *ntp)
> +{
> +	ASSERT(current->journal_info == tp);
> +	current->journal_info = ntp;
> +}
> +
>  #endif	/* __XFS_TRANS_H__ */
> -- 
> 2.18.4
>