Messages in this thread | | | Date | Tue, 20 Jun 2023 20:02:41 +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 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@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 +. > > 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
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