Re: [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages()

[Dave Chinner]

On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote:
> 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@xxxxxxx>
> ---
>  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.

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@xxxxxxxxxxxxx