linux-ext4 - [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc

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Date:   Tue, 14 Sep 2021 10:13:04 +1000
From:   NeilBrown <neilb@...e.de>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.com>
Cc:     linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 6/6] XFS: remove congestion_wait() loop from
 xfs_buf_alloc_pages()

Documentation commment in gfp.h discourages indefinite retry loops on
ENOMEM and says of __GFP_NOFAIL that it

    is definitely preferable to use the flag rather than opencode
    endless loop around allocator.

congestion_wait() is indistinguishable from
schedule_timeout_uninterruptible() in practice and it is not a good way
to wait for memory to become available.

So instead of waiting, allocate a single page using __GFP_NOFAIL, then
loop around and try to get any more pages that might be needed with a
bulk allocation.  This single-page allocation will wait in the most
appropriate way.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/xfs/xfs_buf.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 5fa6cd947dd4..1ae3768f6504 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -372,8 +372,8 @@ xfs_buf_alloc_pages(
 
 	/*
 	 * Bulk filling of pages can take multiple calls. Not filling the entire
-	 * array is not an allocation failure, so don't back off if we get at
-	 * least one extra page.
+	 * array is not an allocation failure, so don't fail or fall back on
+	 * __GFP_NOFAIL if we get at least one extra page.
 	 */
 	for (;;) {
 		long	last = filled;
@@ -394,7 +394,7 @@ xfs_buf_alloc_pages(
 		}
 
 		XFS_STATS_INC(bp->b_mount, xb_page_retries);
-		congestion_wait(BLK_RW_ASYNC, HZ / 50);
+		bp->b_pages[filled++] = alloc_page(gfp_mask | __GFP_NOFAIL);
 	}
 	return 0;
 }