Re: [PATCH v2 05/46] rmap: hugetlb: switch from page_dup_file_rmap to page_add_file_rmap

From: James Houghton
Date: Thu Mar 02 2023 - 10:45:35 EST

On Wed, Mar 1, 2023 at 5:06 PM Jiaqi Yan <jiaqiyan@xxxxxxxxxx> wrote:
>
> On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
> >
> > This only applies to file-backed HugeTLB, and it should be a no-op until
> > high-granularity mapping is possible. Also update page_remove_rmap to
> > support the eventual case where !compound && folio_test_hugetlb().
> >
> > HugeTLB doesn't use LRU or mlock, so we avoid those bits. This also
> > means we don't need to use subpage_mapcount; if we did, it would
> > overflow with only a few mappings.

This is wrong; I guess I misunderstood the code when I wrote this
commit. subpages_mapcount (now called _nr_pages_mapped) won't overflow
(unless HugeTLB pages could be greater than 16G). It is indeed a bug
not to update _nr_pages_mapped the same way THPs do.

>
> >
> > There is still one caller of page_dup_file_rmap left: copy_present_pte,
> > and it is always called with compound=false in this case.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 08004371cfed..6c008c9de80e 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5077,7 +5077,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> > * sleep during the process.
> > */
> > if (!PageAnon(ptepage)) {
> > - page_dup_file_rmap(ptepage, true);
> > + page_add_file_rmap(ptepage, src_vma, true);
> > } else if (page_try_dup_anon_rmap(ptepage, true,
> > src_vma)) {
> > pte_t src_pte_old = entry;
> > @@ -5910,7 +5910,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> > if (anon_rmap)
> > hugepage_add_new_anon_rmap(folio, vma, haddr);
> > else
> > - page_dup_file_rmap(&folio->page, true);
> > + page_add_file_rmap(&folio->page, vma, true);
> > new_pte = make_huge_pte(vma, &folio->page, ((vma->vm_flags & VM_WRITE)
> > && (vma->vm_flags & VM_SHARED)));
> > /*
> > @@ -6301,7 +6301,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > goto out_release_unlock;
> >
> > if (folio_in_pagecache)
> > - page_dup_file_rmap(&folio->page, true);
> > + page_add_file_rmap(&folio->page, dst_vma, true);
> > else
> > hugepage_add_new_anon_rmap(folio, dst_vma, dst_addr);
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index d3964c414010..b0f87f19b536 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -254,7 +254,7 @@ static bool remove_migration_pte(struct folio *folio,
> > hugepage_add_anon_rmap(new, vma, pvmw.address,
> > rmap_flags);
> > else
> > - page_dup_file_rmap(new, true);
> > + page_add_file_rmap(new, vma, true);
> > set_huge_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
> > } else
> > #endif
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 15ae24585fc4..c010d0af3a82 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
>
> Given you are making hugetlb's ref/mapcount mechanism to be consistent
> with THP, I think the special folio_test_hugetlb checks you added in
> this commit will break page_mapped() and folio_mapped() if page/folio
> is HGMed. With these checks, folio->_nr_pages_mapped are not properly
> increased/decreased.

Thank you, Jiaqi! I didn't realize I broke
folio_mapped()/page_mapped(). The end result is that page_mapped() may
report that an HGMed page isn't mapped when it is. Not good!

>
> > @@ -1318,21 +1318,21 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> > int nr = 0, nr_pmdmapped = 0;
> > bool first;
> >
> > - VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
> > + VM_BUG_ON_PAGE(compound && !PageTransHuge(page)
> > + && !folio_test_hugetlb(folio), page);
> >
> > /* Is page being mapped by PTE? Is this its first map to be added? */
> > if (likely(!compound)) {
> > first = atomic_inc_and_test(&page->_mapcount);
> > nr = first;
> > - if (first && folio_test_large(folio)) {
> > + if (first && folio_test_large(folio)
> > + && !folio_test_hugetlb(folio)) {
>
> So we should still increment _nr_pages_mapped for hugetlb case here,
> and decrement in the corresponding place in page_remove_rmap.
>
> > nr = atomic_inc_return_relaxed(mapped);
> > nr = (nr < COMPOUND_MAPPED);
> > }
> > - } else if (folio_test_pmd_mappable(folio)) {
> > - /* That test is redundant: it's for safety or to optimize out */
> > -
> > + } else {
> > first = atomic_inc_and_test(&folio->_entire_mapcount);
> > - if (first) {
> > + if (first && !folio_test_hugetlb(folio)) {
>
> Same here: we should still increase _nr_pages_mapped by
> COMPOUND_MAPPED and decrease by COMPOUND_MAPPED in the corresponding
> place in page_remove_rmap.
>
> > nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
> > if (likely(nr < COMPOUND_MAPPED + COMPOUND_MAPPED)) {
> > nr_pmdmapped = folio_nr_pages(folio);
> > @@ -1347,6 +1347,9 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> > }
> > }
> >
> > + if (folio_test_hugetlb(folio))
> > + return;
> > +
> > if (nr_pmdmapped)
> > __lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
> > NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
> > @@ -1376,8 +1379,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> > VM_BUG_ON_PAGE(compound && !PageHead(page), page);
> >
> > /* Hugetlb pages are not counted in NR_*MAPPED */
> > - if (unlikely(folio_test_hugetlb(folio))) {
> > - /* hugetlb pages are always mapped with pmds */
> > + if (unlikely(folio_test_hugetlb(folio)) && compound) {
> > atomic_dec(&folio->_entire_mapcount);
> > return;
> > }
>
> This entire if-block should be removed after you remove the
> !folio_test_hugetlb checks in page_add_file_rmap.

This is the not-so-obvious change that is needed. Thank you!

>
> > @@ -1386,15 +1388,14 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> > if (likely(!compound)) {
> > last = atomic_add_negative(-1, &page->_mapcount);
> > nr = last;
> > - if (last && folio_test_large(folio)) {
> > + if (last && folio_test_large(folio)
> > + && !folio_test_hugetlb(folio)) {
>
> ditto.
>
> > nr = atomic_dec_return_relaxed(mapped);
> > nr = (nr < COMPOUND_MAPPED);
> > }
> > - } else if (folio_test_pmd_mappable(folio)) {
> > - /* That test is redundant: it's for safety or to optimize out */
> > -
> > + } else {
> > last = atomic_add_negative(-1, &folio->_entire_mapcount);
> > - if (last) {
> > + if (last && !folio_test_hugetlb(folio)) {
>
> ditto.

I agree with all of your suggestions. Testing with the hugetlb-hgm
selftest, nothing seems to break. :)

Given that this is at least the third or fourth major bug in this
version of the series, I'll go ahead and send a v3 sooner rather than
later.

>
> > nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped);
> > if (likely(nr < COMPOUND_MAPPED)) {
> > nr_pmdmapped = folio_nr_pages(folio);
> > @@ -1409,6 +1410,9 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> > }
> > }
> >
> > + if (folio_test_hugetlb(folio))
> > + return;
> > +
> > if (nr_pmdmapped) {
> > if (folio_test_anon(folio))
> > idx = NR_ANON_THPS;
> > --
> > 2.39.2.637.g21b0678d19-goog
> >