Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache

[Ryan Roberts]

On 03/05/2024 01:50, Barry Song wrote:
> 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>

With the suggested changes below:

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

> ---
>  mm/memory.c | 60 ++++++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 48 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 22e7c33cc747..940fdbe69fa1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3968,6 +3968,10 @@ 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 page_idx = 0;
> +	unsigned long address = vmf->address;
> +	pte_t *ptep;

nit: Personally I'd prefer all these to get initialised just before the "if
(folio_test_large()..." block below. That way it is clear they are fresh (incase
any logic between here and there makes an adjustment) and its clear that they
are only to be used after that block (the compiler will warn if using an
uninitialized value).

>  
>  	if (!pte_unmap_same(vmf))
>  		goto out;
> @@ -4166,6 +4170,36 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		goto out_nomap;
>  	}
>  
> +	ptep = vmf->pte;
> +	if (folio_test_large(folio) && folio_test_swapcache(folio)) {
> +		int nr = folio_nr_pages(folio);
> +		unsigned long 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_folio;
> +		if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> +			goto check_folio;
> +
> +		folio_ptep = vmf->pte - idx;
> +		folio_pte = ptep_get(folio_ptep);
> +		if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) ||
> +		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> +			goto check_folio;
> +
> +		page_idx = idx;
> +		address = folio_start;
> +		ptep = folio_ptep;
> +		nr_pages = nr;
> +		entry = folio->swap;
> +		page = &folio->page;
> +	}
> +
> +check_folio:

Is this still the correct label name, given the checks are now above the new
block? Perhaps "one_page" or something like that?

> +
>  	/*
>  	 * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
>  	 * must never point at an anonymous page in the swapcache that is
> @@ -4225,12 +4259,13 @@ 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_nr(entry, 1);
> +	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);
>  
>  	/*
> @@ -4240,34 +4275,35 @@ 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))) {

I think in practice there is no change here? If nr_pages > 1 then the folio is
in the swapcache, so there is an extra ref on it? I agree with the change for
robustness sake. Just checking my understanding.

>  		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;
> +	vmf->orig_pte = pte_advance_pfn(pte, page_idx);
>  
>  	/* 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, address);
>  		folio_add_lru_vma(folio, vma);
>  	} else {
> -		folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> +		folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, 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_nr(vma->vm_mm, vma, vmf->address,
> -			pte, vmf->orig_pte, 1);
> +	set_ptes(vma->vm_mm, address, ptep, pte, nr_pages);
> +	arch_do_swap_page_nr(vma->vm_mm, vma, address,
> +			pte, pte, nr_pages);
>  
>  	folio_unlock(folio);
>  	if (folio != swapcache && swapcache) {
> @@ -4291,7 +4327,7 @@ 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, address, ptep, nr_pages);
>  unlock:
>  	if (vmf->pte)
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);