Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 20 Jun 2023 17:43:35 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v2 5/8] mm/gup: Accelerate thp gup even for "pages != NULL" |
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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@redhat.com> > --- > 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 +.
> + 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.
-- Cheers,
David / dhildenb
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