[Linux-cachefs] [PATCH v10 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

[Dave Chinner]

On Tue, Dec 08, 2020 at 10:15:43AM +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 at oracle.com>
> Cc: Matthew Wilcox (Oracle) <willy at infradead.org>
> Cc: Christoph Hellwig <hch at lst.de>
> Cc: Dave Chinner <david at fromorbit.com>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: David Howells <dhowells at redhat.com>
> Cc: Jeff Layton <jlayton at redhat.com>
> Signed-off-by: Yafang Shao <laoar.shao at 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     | 25 +++++++++++++++++++++++++
>  4 files changed, 45 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..28db93d0da97 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 (xfs_trans_context_active()) {
> +		redirty_page_for_writepage(wbc, page);
> +		unlock_page(page);
> +		return 0;
> +	}

hmmm. Missing warning....

> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 44b11c64a15e..82c6735e40fc 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -268,16 +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 */
> +	if (WARN_ON_ONCE(current->journal_info != NULL))
> +		return true;
> +
> +	return false;
> +}

Ah, this is wrong. The call sites should be:

	if (WARN_ON_ONCE(xfs_trans_context_active())) {
		/* do error handling */
		return error_value;
	}

because we might want to use xfs_trans_context_active() to check if
we are in a transaction context or not and that should not generate
a warning. Also, placing the warning at the call site gives a more
accurate indication of which IO path generated the warning....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com