Re: [PATCH v2 4/5] mm: swap: entirely map large folios found in swapcache
From: Barry Song
Date: Mon Apr 15 2024 - 04:54:15 EST
On Mon, Apr 15, 2024 at 8:39 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > From: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >
> > When a large folio is found in the swapcache, the current implementation
> > requires calling do_swap_page() nr_pages times, resulting in nr_pages
> > page faults. This patch opts to map the entire large folio at once to
> > minimize page faults. Additionally, redundant checks and early exits
> > for ARM64 MTE restoring are removed.
> >
> > Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> > Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> > mm/memory.c | 64 +++++++++++++++++++++++++++++++++++++++++++----------
> > 1 file changed, 52 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c4a52e8d740a..9818dc1893c8 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3947,6 +3947,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > pte_t pte;
> > vm_fault_t ret = 0;
> > void *shadow = NULL;
> > + int nr_pages = 1;
> > + unsigned long start_address = vmf->address;
> > + pte_t *start_pte = vmf->pte;
>
> IMHO, it's better to rename the above 2 local variables to "address" and
> "ptep". Just my personal opinion. Feel free to ignore the comments.
fine.
>
> > + bool any_swap_shared = false;
> >
> > if (!pte_unmap_same(vmf))
> > goto out;
> > @@ -4137,6 +4141,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > */
> > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> > &vmf->ptl);
>
> We should move pte check here. That is,
>
> if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> goto out_nomap;
>
> This will simplify the situation for large folio.
the plan is moving the whole code block
if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio))
after
if (unlikely(!folio_test_uptodate(folio))) {
ret = VM_FAULT_SIGBUS;
goto out_nomap;
}
though we couldn't be !folio_test_uptodate(folio)) for hitting
swapcache but it seems
logically better for future use.
>
> > +
> > + /* We hit large folios in swapcache */
>
> The comments seems unnecessary because the code tells that already.
>
> > + if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio)) {
> > + int nr = folio_nr_pages(folio);
> > + int idx = folio_page_idx(folio, page);
> > + unsigned long folio_start = vmf->address - idx * PAGE_SIZE;
> > + unsigned long folio_end = folio_start + nr * PAGE_SIZE;
> > + pte_t *folio_ptep;
> > + pte_t folio_pte;
> > +
> > + if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start)))
> > + goto check_pte;
> > + if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> > + goto check_pte;
> > +
> > + folio_ptep = vmf->pte - idx;
> > + folio_pte = ptep_get(folio_ptep);
>
> It's better to construct pte based on fault PTE via generalizing
> pte_next_swp_offset() (may be pte_move_swp_offset()). Then we can find
> inconsistent PTEs quicker.
it seems your point is getting the pte of page0 by pte_next_swp_offset()
unfortunately pte_next_swp_offset can't go back. on the other hand,
we have to check the real pte value of the 0nd entry right now because
swap_pte_batch() only really reads pte from the 1st entry. it assumes
pte argument is the real value for the 0nd pte entry.
static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
{
pte_t expected_pte = pte_next_swp_offset(pte);
const pte_t *end_ptep = start_ptep + max_nr;
pte_t *ptep = start_ptep + 1;
VM_WARN_ON(max_nr < 1);
VM_WARN_ON(!is_swap_pte(pte));
VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
while (ptep < end_ptep) {
pte = ptep_get(ptep);
if (!pte_same(pte, expected_pte))
break;
expected_pte = pte_next_swp_offset(expected_pte);
ptep++;
}
return ptep - start_ptep;
}
>
> > + if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
> > + swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
> > + goto check_pte;
> > +
> > + start_address = folio_start;
> > + start_pte = folio_ptep;
> > + nr_pages = nr;
> > + entry = folio->swap;
> > + page = &folio->page;
> > + }
> > +
> > +check_pte:
> > if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> > goto out_nomap;
> >
> > @@ -4190,6 +4223,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > */
> > exclusive = false;
> > }
> > +
> > + /* Reuse the whole large folio iff all entries are exclusive */
> > + if (nr_pages > 1 && any_swap_shared)
> > + exclusive = false;
> > }
> >
> > /*
> > @@ -4204,12 +4241,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > * We're already holding a reference on the page but haven't mapped it
> > * yet.
> > */
> > - swap_free(entry);
> > + swap_free_nr(entry, nr_pages);
> > if (should_try_to_free_swap(folio, vma, vmf->flags))
> > folio_free_swap(folio);
> >
> > - inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > - dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> > + folio_ref_add(folio, nr_pages - 1);
> > + add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> > + add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
> > +
> > pte = mk_pte(page, vma->vm_page_prot);
> >
> > /*
> > @@ -4219,33 +4258,34 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > * exclusivity.
> > */
> > if (!folio_test_ksm(folio) &&
> > - (exclusive || folio_ref_count(folio) == 1)) {
> > + (exclusive || (folio_ref_count(folio) == nr_pages &&
> > + folio_nr_pages(folio) == nr_pages))) {
> > if (vmf->flags & FAULT_FLAG_WRITE) {
> > pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> > vmf->flags &= ~FAULT_FLAG_WRITE;
> > }
> > rmap_flags |= RMAP_EXCLUSIVE;
> > }
> > - flush_icache_page(vma, page);
> > + flush_icache_pages(vma, page, nr_pages);
> > if (pte_swp_soft_dirty(vmf->orig_pte))
> > pte = pte_mksoft_dirty(pte);
> > if (pte_swp_uffd_wp(vmf->orig_pte))
> > pte = pte_mkuffd_wp(pte);
> > - vmf->orig_pte = pte;
> >
> > /* ksm created a completely new copy */
> > if (unlikely(folio != swapcache && swapcache)) {
> > - folio_add_new_anon_rmap(folio, vma, vmf->address);
> > + folio_add_new_anon_rmap(folio, vma, start_address);
> > folio_add_lru_vma(folio, vma);
> > } else {
> > - folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> > - rmap_flags);
> > + folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, start_address,
> > + rmap_flags);
> > }
> >
> > VM_BUG_ON(!folio_test_anon(folio) ||
> > (pte_write(pte) && !PageAnonExclusive(page)));
> > - set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> > - arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > + set_ptes(vma->vm_mm, start_address, start_pte, pte, nr_pages);
> > + vmf->orig_pte = ptep_get(vmf->pte);
> > + arch_do_swap_page(vma->vm_mm, vma, start_address, pte, pte);
>
> Do we need to call arch_do_swap_page() for each subpage? IIUC, the
> corresponding arch_unmap_one() will be called for each subpage.
i actually thought about this very carefully, right now, the only one who
needs this is sparc and it doesn't support THP_SWAPOUT at all. and
there is no proof doing restoration one by one won't really break sparc.
so i'd like to defer this to when sparc really needs THP_SWAPOUT.
on the other hand, it seems really bad we have both
arch_swap_restore - for this, arm64 has moved to using folio
and
arch_do_swap_page
we should somehow unify them later if sparc wants THP_SWPOUT.
>
> > folio_unlock(folio);
> > if (folio != swapcache && swapcache) {
> > @@ -4269,7 +4309,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > }
> >
> > /* No need to invalidate - it was non-present before */
> > - update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> > + update_mmu_cache_range(vmf, vma, start_address, start_pte, nr_pages);
> > unlock:
> > if (vmf->pte)
> > pte_unmap_unlock(vmf->pte, vmf->ptl);
>
> --
> Best Regards,
> Huang, Ying