linux-kernel - Re: [PATCH RFC 2/3] mm: do_swap_page: use folio_add_new_anon_rmap() if folio_test

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Date: Thu, 13 Jun 2024 11:23:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: chrisl@...nel.org, linux-kernel@...r.kernel.org, mhocko@...e.com,
 ryan.roberts@....com, baolin.wang@...ux.alibaba.com, yosryahmed@...gle.com,
 shy828301@...il.com, surenb@...gle.com, v-songbaohua@...o.com,
 willy@...radead.org, ying.huang@...el.com, yuzhao@...gle.com
Subject: Re: [PATCH RFC 2/3] mm: do_swap_page: use folio_add_new_anon_rmap()
 if folio_test_anon(folio)==false

On 13.06.24 02:07, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> For the !folio_test_anon(folio) case, we can now invoke folio_add_new_anon_rmap()
> with the rmap flags set to either EXCLUSIVE or non-EXCLUSIVE. This action will
> suppress the VM_WARN_ON_FOLIO check within __folio_add_anon_rmap() while initiating
> the process of bringing up mTHP swapin.
> 
>   static __always_inline void __folio_add_anon_rmap(struct folio *folio,
>                   struct page *page, int nr_pages, struct vm_area_struct *vma,
>                   unsigned long address, rmap_t flags, enum rmap_level level)
>   {
>           ...
>           if (unlikely(!folio_test_anon(folio))) {
>                   VM_WARN_ON_FOLIO(folio_test_large(folio) &&
>                                    level != RMAP_LEVEL_PMD, folio);
>           }
>           ...
>   }
> 
> It also enhances the code’s readability.
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>   mm/memory.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2f94921091fb..9c962f62f928 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4339,6 +4339,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>   	if (unlikely(folio != swapcache && swapcache)) {
>   		folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
>   		folio_add_lru_vma(folio, vma);
> +	} else if (!folio_test_anon(folio)) {
> +		folio_add_new_anon_rmap(folio, vma, address, rmap_flags);

So, with large folio swapin, we would never end up here if any swp PTE 
is !exclusive, because we would make sure to allocate a large folio only 
for suitable regions, correct?

It can certainly happen during swapout + refault with large folios. But 
there, we will always have folio_test_anon() still set and wouldn't run 
into this code path.

(it will all be better with per-folio PAE bit, but that will take some 
more time until I actually get to implement it properly, handling all 
the nasty corner cases)

Better add a comment regarding why we are sure that the complete thing 
is exclusive (e.g., currently only small folios).

>   	} else {
>   		folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
>   					rmap_flags);

-- 
Cheers,

David / dhildenb