Re: [PATCH v2 4/5] mm: swap: entirely map large folios found in swapcache

[Huang, Ying]

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.

> +	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.

> +
> +	/* 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.

> +		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.

>  	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