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*. 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. -- Peter Xu