Re: [PATCH v1 15/39] mm/huge_memory: batch rmap operations in __split_huge_pmd_locked()

[David Hildenbrand]

On 18.12.23 17:22, Ryan Roberts wrote:
> On 11/12/2023 15:56, David Hildenbrand wrote:
> > Let's use folio_add_anon_rmap_ptes(), batching the rmap operations.
> >
> > While at it, use more folio operations (but only in the code branch we're
> > touching), use VM_WARN_ON_FOLIO(), and pass RMAP_EXCLUSIVE instead of
> > manually setting PageAnonExclusive.
> >
> > We should never see non-anon pages on that branch: otherwise, the
> > existing page_add_anon_rmap() call would have been flawed already.
> >
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   mm/huge_memory.c | 23 +++++++++++++++--------
> >   1 file changed, 15 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 1f5634b2f374..82ad68fe0d12 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2398,6 +2398,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >   		unsigned long haddr, bool freeze)
> >   {
> >   	struct mm_struct *mm = vma->vm_mm;
> > +	struct folio *folio;
> >   	struct page *page;
> >   	pgtable_t pgtable;
> >   	pmd_t old_pmd, _pmd;
> > @@ -2493,16 +2494,18 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >   		uffd_wp = pmd_swp_uffd_wp(old_pmd);
> >   	} else {
> >   		page = pmd_page(old_pmd);
> > +		folio = page_folio(page);
> >   		if (pmd_dirty(old_pmd)) {
> >   			dirty = true;
> > -			SetPageDirty(page);
> > +			folio_set_dirty(folio);
> >   		}
> >   		write = pmd_write(old_pmd);
> >   		young = pmd_young(old_pmd);
> >   		soft_dirty = pmd_soft_dirty(old_pmd);
> >   		uffd_wp = pmd_uffd_wp(old_pmd);
> >   
> > -		VM_BUG_ON_PAGE(!page_count(page), page);
> > +		VM_WARN_ON_FOLIO(!folio_ref_count(folio), folio);
> > +		VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>
> Is this warning really correct? file-backed memory can be PMD-mapped with
> CONFIG_READ_ONLY_THP_FOR_FS, so presumably it can also have the need to be
> remapped as pte? Although I guess if we did have a file-backed folio, it
> definitely wouldn't be correct to call page_add_anon_rmap() /
> folio_add_anon_rmap_ptes()...

Yes, see the patch description where I spell that out.

PTE-remapping a file-back folio will simply zap the PMD and refault from 
the page cache after creating a page table.

So this is anon-only code.

--
Cheers,

David / dhildenb