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Message-ID: <60a075da-7c7e-4d99-ac52-059e5a17b72e@redhat.com>
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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