Date: Thu, 20 Jun 2024 10:49:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 baolin.wang@...ux.alibaba.com, chrisl@...nel.org,
 linux-kernel@...r.kernel.org, mhocko@...e.com, ryan.roberts@....com,
 shy828301@...il.com, surenb@...gle.com, v-songbaohua@...o.com,
 willy@...radead.org, ying.huang@...el.com, yosryahmed@...gle.com,
 yuzhao@...gle.com, Shuai Yuan <yuanshuai@...o.com>
Subject: Re: [PATCH v2 2/3] mm: use folio_add_new_anon_rmap() if
 folio_test_anon(folio)==false

On 20.06.24 10:33, Barry Song wrote:
> On Thu, Jun 20, 2024 at 7:46 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 18.06.24 01:11, 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 improves the code’s readability. Currently, all new anonymous
>>> folios calling folio_add_anon_rmap_ptes() are order-0. This ensures
>>> that new folios cannot be partially exclusive; they are either entirely
>>> exclusive or entirely shared.
>>>
>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>> Tested-by: Shuai Yuan <yuanshuai@...o.com>
>>> ---
>>>    mm/memory.c   |  8 ++++++++
>>>    mm/swapfile.c | 13 +++++++++++--
>>>    2 files changed, 19 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 1f24ecdafe05..620654c13b2f 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -4339,6 +4339,14 @@ 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)) {
>>> +             /*
>>> +              * We currently only expect small !anon folios, for which we now
>>> +              * that they are either fully exclusive or fully shared. If we
>>> +              * ever get large folios here, we have to be careful.
>>> +              */
>>> +             VM_WARN_ON_ONCE(folio_test_large(folio));
>>> +             folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
>>>        } else {
>>>                folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
>>>                                        rmap_flags);
>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>> index ae1d2700f6a3..69efa1a57087 100644
>>> --- a/mm/swapfile.c
>>> +++ b/mm/swapfile.c
>>> @@ -1908,8 +1908,17 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>>>                VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
>>>                if (pte_swp_exclusive(old_pte))
>>>                        rmap_flags |= RMAP_EXCLUSIVE;
>>> -
>>> -             folio_add_anon_rmap_pte(folio, page, vma, addr, rmap_flags);
>>> +             /*
>>> +              * We currently only expect small !anon folios, for which we now that
>>> +              * they are either fully exclusive or fully shared. If we ever get
>>> +              * large folios here, we have to be careful.
>>> +              */
>>> +             if (!folio_test_anon(folio)) {
>>> +                     VM_WARN_ON_ONCE(folio_test_large(folio));
>>
>> (comment applies to both cases)
>>
>> Thinking about Hugh's comment, we should likely add here:
>>
>> VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>>
>> [the check we are removing from __folio_add_anon_rmap()]
>>
>> and document for folio_add_new_anon_rmap() in patch #1, that when
>> dealing with folios that might be mapped concurrently by others, the
>> folio lock must be held.
> 
> I assume you mean something like the following for patch#1?
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index df1a43295c85..20986b25f1b2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1394,7 +1394,8 @@ void folio_add_anon_rmap_pmd(struct folio
> *folio, struct page *page,
>    *
>    * Like folio_add_anon_rmap_*() but must only be called on *new* folios.
>    * This means the inc-and-test can be bypassed.
> - * The folio does not have to be locked.
> + * The folio doesn't necessarily need to be locked while it's
> exclusive unless two threads
> + * map it concurrently. However, the folio must be locked if it's shared.
>    *
>    * If the folio is pmd-mappable, it is accounted as a THP.
>    */
> @@ -1406,6 +1407,7 @@ void folio_add_new_anon_rmap(struct folio
> *folio, struct vm_area_struct *vma,
>          int nr_pmdmapped = 0;
> 
>          VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> +       VM_WARN_ON_FOLIO(!exclusive && !folio_test_locked(folio), folio);

For now this would likely do. I was concerned about a concurrent 
scenario in the exclusive case, but that shouldn't really happen I guess.

-- 
Cheers,

David / dhildenb

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