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Date: Fri, 21 Jun 2024 11:18:24 +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 11:59, Barry Song wrote:
> On Thu, Jun 20, 2024 at 8:49 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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.
>>
>
> Since this is primarily a documentation update, I'll wait for two or
> three days to see if
> there are any more Linux-next reports before sending v3 combining these fixes
> together(I've already fixed another doc warn reported by lkp) and seek Andrew's
> assistance to drop v2 and apply v3.
Feel free to send fixup patches for such small stuff (for example, as
reply to this mail). Usually, no need for a new series. Andrew will
squash all fixups before merging it to mm-stable.
--
Cheers,
David / dhildenb
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