From: Dave Chinner <david@fromorbit.com>
To: NeilBrown <neilb@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Theodore Ts'o <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
"Darrick J. Wong" <djwong@kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Mel Gorman <mgorman@suse.com>,
linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages()
Date: Tue, 14 Sep 2021 12:08:37 +1000 [thread overview]
Message-ID: <20210914020837.GH2361455@dread.disaster.area> (raw)
In-Reply-To: <163157838440.13293.12568710689057349786.stgit@noble.brown>
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@suse.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);
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@fromorbit.com
next prev parent reply other threads:[~2021-09-14 2:08 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-14 0:13 [PATCH 0/6] congestion_wait() and GFP_NOFAIL NeilBrown
2021-09-14 0:13 ` [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective NeilBrown
2021-09-15 11:56 ` Michal Hocko
2021-09-16 22:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc() NeilBrown
2021-09-14 1:31 ` Dave Chinner
2021-09-14 3:27 ` NeilBrown
2021-09-14 6:05 ` Dave Chinner
2021-09-14 0:13 ` [PATCH 3/6] EXT4: Remove ENOMEM/congestion_wait() loops NeilBrown
2021-09-14 16:34 ` Mel Gorman
2021-09-14 21:48 ` NeilBrown
2021-09-15 12:06 ` Michal Hocko
2021-09-15 22:35 ` NeilBrown
2021-09-16 0:37 ` Dave Chinner
2021-09-16 6:52 ` Michal Hocko
2021-09-14 23:55 ` Dave Chinner
2021-09-15 8:59 ` Mel Gorman
2021-09-15 12:20 ` Michal Hocko
2021-09-15 14:35 ` Mel Gorman
2021-09-15 22:38 ` Dave Chinner
2021-09-16 9:00 ` Mel Gorman
2021-09-15 0:28 ` Theodore Ts'o
2021-09-15 5:25 ` NeilBrown
2021-09-15 17:02 ` Theodore Ts'o
2021-09-14 0:13 ` [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL NeilBrown
2021-09-15 11:51 ` Michal Hocko
2021-09-14 0:13 ` [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() NeilBrown
2021-09-14 2:08 ` Dave Chinner [this message]
2021-09-14 2:35 ` NeilBrown
2021-09-14 5:33 ` Dave Chinner
2021-09-14 16:45 ` Mel Gorman
2021-09-14 21:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 4/6] EXT4: remove congestion_wait from ext4_bio_write_page, and simplify NeilBrown
2021-09-17 2:56 [PATCH 0/6 v2] congestion_wait() and GFP_NOFAIL NeilBrown
2021-09-17 2:56 ` [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() NeilBrown
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