Re: [PATCH 6/7] mm/gup: Accelerate thp gup even for "pages != NULL"

[Peter Xu]

On Sat, Jun 17, 2023 at 09:27:22PM +0100, Lorenzo Stoakes wrote:
> On Tue, Jun 13, 2023 at 05:53:45PM -0400, Peter Xu wrote:
> > The acceleration of THP was done with ctx.page_mask, however it'll be
> > ignored if **pages is non-NULL.
> >
> > The old optimization was introduced in 2013 in 240aadeedc4a ("mm:
> > accelerate mm_populate() treatment of THP pages").  It didn't explain why
> > we can't optimize the **pages non-NULL case.  It's possible that at that
> > time the major goal was for mm_populate() which should be enough back then.
> >
> > Optimize thp for all cases, by properly looping over each subpage, doing
> > cache flushes, and boost refcounts / pincounts where needed in one go.
> >
> > This can be verified using gup_test below:
> >
> >   # chrt -f 1 ./gup_test -m 512 -t -L -n 1024 -r 10
> >
> > Before:    13992.50 ( +-8.75%)
> > After:       378.50 (+-69.62%)
> >
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >  mm/gup.c | 36 +++++++++++++++++++++++++++++-------
> >  1 file changed, 29 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index a2d1b3c4b104..cdabc8ea783b 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1210,16 +1210,38 @@ static long __get_user_pages(struct mm_struct *mm,
> >  			goto out;
> >  		}
> >  next_page:
> > -		if (pages) {
> > -			pages[i] = page;
> > -			flush_anon_page(vma, page, start);
> > -			flush_dcache_page(page);
> > -			ctx.page_mask = 0;
> > -		}
> > -
> >  		page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
> >  		if (page_increm > nr_pages)
> >  			page_increm = nr_pages;
> > +
> > +		if (pages) {
> > +			struct page *subpage;
> > +			unsigned int j;
> > +
> > +			/*
> > +			 * This must be a large folio (and doesn't need to
> > +			 * be the whole folio; it can be part of it), do
> > +			 * the refcount work for all the subpages too.
> > +			 * Since we already hold refcount on the head page,
> > +			 * it should never fail.
> > +			 *
> > +			 * NOTE: here the page may not be the head page
> > +			 * e.g. when start addr is not thp-size aligned.
> > +			 */
> > +			if (page_increm > 1)
> > +				WARN_ON_ONCE(
> > +				    try_grab_folio(compound_head(page),
> > +						   page_increm - 1,
> > +						   foll_flags) == NULL);
> 
> I'm not sure this should be warning but otherwise ignoring this returning
> NULL?  This feels like a case that could come up in realtiy,
> e.g. folio_ref_try_add_rcu() fails, or !folio_is_longterm_pinnable().

Note that we hold already at least 1 refcount on the folio (also mentioned
in the comment above this chunk of code), so both folio_ref_try_add_rcu()
and folio_is_longterm_pinnable() should already have been called on the
same folio and passed.  If it will fail it should have already, afaict.

I still don't see how that would trigger if the refcount won't overflow.

Here what I can do is still guard this try_grab_folio() and fail the GUP if
for any reason it failed.  Perhaps then it means I'll also keep that one
untouched in hugetlb_follow_page_mask() too.  But I suppose keeping the
WARN_ON_ONCE() seems still proper.

Thanks,

-- 
Peter Xu