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

[NeilBrown]

On Tue, 14 Sep 2021, Dave Chinner wrote:
> 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.

That is a nice simplification.
Mel Gorman told me
  https://lore.kernel.org/linux-nfs/20210907153116.GJ3828@xxxxxxxx/
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@xxxxxxxxxxxxx
> 
>