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

[NeilBrown]

On Wed, 15 Sep 2021, Mel Gorman wrote:
> On Tue, Sep 14, 2021 at 12:35:59PM +1000, NeilBrown wrote:
> > 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....
> > 
> 
> If there are already at least one page an the array and the first attempt
> at bulk allocation fails, it'll simply return. It's an odd corner case
> that may never apply but it's possible.  That said, I'm of the opinion that
> __GFP_NOFAIL should not be expanded and instead congestion_wait should be
> deleted and replaced with something triggered by reclaim making progress.

Ahh.... that was (I think) fixed by
https://patchwork.kernel.org/project/linux-mm/patch/163027609524.7591.4987241695872857175@xxxxxxxxxxxxxxxxxxxxx/
(which I cannot find on lore.kernel.org - strange)
which you acked - and which I meant to include in this series but
somehow missed.

NeilBrown