On Tue, 14 Sep 2021, Dave Chinner wrote:

On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote:

quoted

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 <redacted>
---
 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);

This smells wrong - the whole point of using the bulk page allocator
in this loop is to avoid the costly individual calls to
alloc_page().

What we are implementing here fail-fast semantics for readahead and
fail-never for everything else.  If the bulk allocator fails to get
a page from the fast path free lists, it already falls back to
__alloc_pages(gfp, 0, ...) to allocate a single page. So AFAICT
there's no need to add another call to alloc_page() because we can
just do this instead:

	if (flags & XBF_READ_AHEAD)
		gfp_mask |= __GFP_NORETRY;
	else
-		gfp_mask |= GFP_NOFS;
+		gfp_mask |= GFP_NOFS | __GFP_NOFAIL;

Which should make the __alloc_pages() call in
alloc_pages_bulk_array() do a __GFP_NOFAIL allocation and hence
provide the necessary never-fail guarantee that is needed here.

That is a nice simplification.
Mel Gorman told me
  https://lore.kernel.org/linux-nfs/20210907153116.GJ3828@suse.com/ (local)
that alloc_pages_bulk ignores GFP_NOFAIL.  I added that to the
documentation comment in an earlier patch.

I had a look at the code and cannot see how it would fail to allocate at
least one page.  Maybe Mel can help....

NeilBrown
 

At which point, the bulk allocation loop can be simplified because
we can only fail bulk allocation for readahead, so something like:

		if (filled == bp->b_page_count) {
			XFS_STATS_INC(bp->b_mount, xb_page_found);
			break;
		}

-		if (filled != last)
+		if (filled == last) {
-			continue;
-
-		if (flags & XBF_READ_AHEAD) {
			ASSERT(flags & XBF_READ_AHEAD);
			xfs_buf_free_pages(bp);
			return -ENOMEM;
		}

		XFS_STATS_INC(bp->b_mount, xb_page_retries);
-		congestion_wait(BLK_RW_ASYNC, HZ / 50);
	}
	return 0;
}

would do the right thing and still record that we are doing
blocking allocations (via the xb_page_retries stat) in this loop.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com