Re: [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages()
From: Dave Chinner <david@fromorbit.com>
Date: 2021-09-14 02:08:47
Also in:
linux-ext4, linux-fsdevel, linux-mm, linux-nfs, lkml
On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote:
quoted hunk ↗ jump to hunk
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.
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