Re: [PATCH v2 5/8] mm/gup: Accelerate thp gup even for "pages != NULL"

[David Hildenbrand]

On 20.06.23 18:23, Peter Xu wrote:
> On Tue, Jun 20, 2023 at 05:43:35PM +0200, David Hildenbrand wrote:
> > On 20.06.23 01:10, 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.
> >
> > In the past we had these sub-page refcounts for THP. My best guess (and I
> > didn't check if that was still the case in 2013) would be that it was
> > simpler regarding refcount handling to to do it one-subpage at a time.
> >
> > But I might be just wrong.
> >
> > > 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 | 51 ++++++++++++++++++++++++++++++++++++++++++++-------
> > >    1 file changed, 44 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/mm/gup.c b/mm/gup.c
> > > index 4a00d609033e..b50272012e49 100644
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -1199,16 +1199,53 @@ 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.
> > > +			 *
> > > +			 * NOTE: here the page may not be the head page
> > > +			 * e.g. when start addr is not thp-size aligned.
> > > +			 * try_grab_folio() should have taken care of tail
> > > +			 * pages.
> > > +			 */
> > > +			if (page_increm > 1) {
> > > +				struct folio *folio;
> > > +
> > > +				/*
> > > +				 * Since we already hold refcount on the
> > > +				 * large folio, this should never fail.
> > > +				 */
> > > +				folio = try_grab_folio(page, page_increm - 1,
> > > +						       foll_flags);
> > > +				if (WARN_ON_ONCE(!folio)) {
> > > +					/*
> > > +					 * Release the 1st page ref if the
> > > +					 * folio is problematic, fail hard.
> > > +					 */
> > > +					gup_put_folio(page_folio(page), 1,
> > > +						      foll_flags);
> > > +					ret = -EFAULT;
> > > +					goto out;
> > > +				}
> > > +			}
> > > +
> > > +			for (j = 0; j < page_increm; j++) {
> > > +				subpage = nth_page(page, j);
> > > +				pages[i+j] = subpage;
> >
> > Doe checkpatch like pages[i+j]? I'd have used spaces around the +.
>
> Can do.
>
> > > +				flush_anon_page(vma, subpage, start + j * PAGE_SIZE);
> > > +				flush_dcache_page(subpage);
> > > +			}
> > > +		}
> > > +
> > >    		i += page_increm;
> > >    		start += page_increm * PAGE_SIZE;
> > >    		nr_pages -= page_increm;
> >
> >
> > So, we did the first try_grab_folio() while our page was PMD-mapped udner
> > the PT lock and we had sufficient permissions (e.g., mapped writable, no
> > unsharing required). With FOLL_PIN, we incremented the pincount.
> >
> >
> > I was wondering if something could have happened ever since we unlocked the
> > PT table lock and possibly PTE-mapped the THP. ... but as it's already
> > pinned, it cannot get shared during fork() [will stay exclusive].
> >
> > So we can just take additional pins on that folio.
> >
> >
> > LGTM, although I do like the GUP-fast way of recording+ref'ing it at a
> > central place (see gup_huge_pmd() with record_subpages() and friends), not
> > after the effects.
>
> My read on this is follow_page_mask() is also used in follow page, which
> does not need page*.

Right ... maybe one day we can do that "better".

> No strong opinion here. Maybe we leave this as a follow up even if it can
> be justified?  This patch is probably still the smallest (and still clean)
> change to speed this whole thing up over either thp or hugetlb.

Sure, we can leave that as a follow-up.


Thinking about why we have the flush_anon_page/flush_dcache_page stuff 
here and not in GUP-fast ... I suspect that all GUP-fast archs don't 
need that stuff.

I was wondering if there are some possible races with the 
flush_anon_page() / flush_dcache_page() on a page that might have been 
unmapped in the meantime (as we dropped the PT lock ...).

Some flush_dcache_page() implementations do some IMHO confusing 
page_mapcount() things (like in arch/arc/mm/cache.c). But maybe the 
unmap code handles that as well ... and most likely these archs don't 
support THP.

Anyhow, just a note that the flush_anon_page/flush_dcache_page left me 
confused.

--
Cheers,

David / dhildenb