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Date: Tue, 7 Mar 2023 16:36:51 -0800
From: James Houghton <jthoughton@...gle.com>
To: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Hugh Dickins <hughd@...gle.com>,
Muchun Song <songmuchun@...edance.com>,
Peter Xu <peterx@...hat.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
David Hildenbrand <david@...hat.com>,
David Rientjes <rientjes@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: rmap: make hugetlb pages participate in _nr_pages_mapped
On Tue, Mar 7, 2023 at 1:54 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 03/06/23 23:00, James Houghton wrote:
> > For compound mappings (compound=true), _nr_pages_mapped will now be
> > incremented by COMPOUND_MAPPED when the first compound mapping is
> > created.
>
> This sentence makes it sound like incrementing by COMPOUND_MAPPED for
> compound pages is introduced by this patch. Rather, it is just for
> hugetlb (now always) compound mappings. Perhaps change that to read:
> For hugetlb mappings ...
Yes this is kind of confusing. I'll fix it like you suggest.
>
> > For small mappings, _nr_pages_mapped is incremented by 1 when the
> > particular small page is mapped for the first time. This is incompatible
> > with HPageVmemmapOptimize()ed folios, as most of the tail page structs
> > will be mapped read-only.
> >
> > Currently HugeTLB always passes compound=true, but in the future,
> > HugeTLB pages may be mapped with small mappings.
> >
> > To implement this change:
> > 1. Replace most of HugeTLB's calls to page_dup_file_rmap() with
> > page_add_file_rmap(). The call in copy_hugetlb_page_range() is kept.
> > 2. Update page_add_file_rmap() and page_remove_rmap() to support
> > HugeTLB folios.
> > 3. Update hugepage_add_anon_rmap() and hugepage_add_new_anon_rmap() to
> > also increment _nr_pages_mapped properly.
> >
> > With these changes, folio_large_is_mapped() no longer needs to check
> > _entire_mapcount.
> >
> > HugeTLB doesn't use LRU or mlock, so page_add_file_rmap() and
> > page_remove_rmap() excludes those pieces. It is also important that
> > the folio_test_pmd_mappable() check is removed (or changed), as it's
> > possible to have a HugeTLB page whose order is not >= HPAGE_PMD_ORDER,
> > like arm64's CONT_PTE_SIZE HugeTLB pages.
> >
> > This patch limits HugeTLB pages to 16G in size. That limit can be
> > increased if COMPOUND_MAPPED is raised.
> >
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
> >
>
> Thanks!
>
> This is a step in the direction of having hugetlb use the same mapcount
> scheme as elsewhere. As you mention, with this in place future mapcount
> changes should mostly 'just work' for hugetlb.
>
> Because of this,
> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
Thanks!
>
> I have a few nits below, and I'm sure others will chime in later.
>
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index ba901c416785..4a975429b91a 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1316,19 +1316,21 @@ 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)) {
> > + if (unlikely(folio_test_hugetlb(folio)))
> > + VM_BUG_ON_PAGE(HPageVmemmapOptimized(&folio->page),
> > + page);
> > first = atomic_inc_and_test(&page->_mapcount);
> > nr = first;
> > if (first && folio_test_large(folio)) {
> > 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 */
>
> I 'think' removing this check is OK. It would seem that the caller
> knows if the folio is mappable. If we want a similar test, we might be
> able to use something like:
>
> arch_hugetlb_valid_size(folio_size(folio))
>
Ack. I think leaving the check(s) removed is fine.
> > -
> > + } else {
> > first = atomic_inc_and_test(&folio->_entire_mapcount);
> > if (first) {
> > nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
> > @@ -1345,6 +1347,9 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> > }
> > }
> >
> > + if (folio_test_hugetlb(folio))
> > + return;
>
> IMO, a comment saying hugetlb is special and does not participate in lru
> would be appropriate here.
Will do.
>
> > +
> > if (nr_pmdmapped)
> > __lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
> > NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
> > @@ -1373,24 +1378,18 @@ 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 */
> > - atomic_dec(&folio->_entire_mapcount);
> > - return;
> > - }
> > -
> > /* Is page being unmapped by PTE? Is this its last map to be removed? */
> > if (likely(!compound)) {
> > + if (unlikely(folio_test_hugetlb(folio)))
> > + VM_BUG_ON_PAGE(HPageVmemmapOptimized(&folio->page),
> > + page);
> > last = atomic_add_negative(-1, &page->_mapcount);
> > nr = last;
> > if (last && folio_test_large(folio)) {
> > 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) {
> > nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped);
> > @@ -1407,6 +1406,9 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> > }
> > }
> >
> > + if (folio_test_hugetlb(folio))
> > + return;
>
> Same as above in page_add_file_rmap.
>
> > +
> > if (nr_pmdmapped) {
> > if (folio_test_anon(folio))
> > idx = NR_ANON_THPS;
> > @@ -2541,9 +2543,11 @@ void hugepage_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
> > first = atomic_inc_and_test(&folio->_entire_mapcount);
> > VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
> > VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
> > - if (first)
> > + if (first) {
> > + atomic_add(COMPOUND_MAPPED, &folio->_nr_pages_mapped);
> > __page_set_anon_rmap(folio, page, vma, address,
> > !!(flags & RMAP_EXCLUSIVE));
> > + }
> > }
> >
> > void hugepage_add_new_anon_rmap(struct folio *folio,
> > @@ -2552,6 +2556,7 @@ void hugepage_add_new_anon_rmap(struct folio *folio,
> > BUG_ON(address < vma->vm_start || address >= vma->vm_end);
> > /* increment count (starts at -1) */
> > atomic_set(&folio->_entire_mapcount, 0);
> > + atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
> > folio_clear_hugetlb_restore_reserve(folio);
> > __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
> > }
>
> Should we look at perhaps modifying page_add_anon_rmap and
> folio_add_new_anon_rmap as well?
I think I can merge hugepage_add_anon_rmap with page_add_anon_rmap and
hugepage_add_new_anon_rmap with folio_add_new_anon_rmap. With them
merged, it's pretty easy to see what HugeTLB does differently from
generic mm, which is nice. :)
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