From: Zi Yan <ziy@nvidia.com>
To: David Hildenbrand <david@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
Ryan Roberts <ryan.roberts@arm.com>,
Barry Song <21cnbao@gmail.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list
Date: Thu, 25 Apr 2024 10:50:58 -0400 [thread overview]
Message-ID: <935E19A6-752C-49E7-8D25-8091195E0104@nvidia.com> (raw)
In-Reply-To: <815e2114-f739-4f2f-b09f-a23a2fc3214b@redhat.com>
[-- Attachment #1: Type: text/plain, Size: 3257 bytes --]
On 25 Apr 2024, at 3:15, David Hildenbrand wrote:
> On 25.04.24 00:39, Zi Yan wrote:
>> On 24 Apr 2024, at 18:32, Yang Shi wrote:
>>
>>> On Wed, Apr 24, 2024 at 2:10 PM Zi Yan <zi.yan@sent.com> wrote:
>>>>
>>>> From: Zi Yan <ziy@nvidia.com>
>>>>
>>>> In __folio_remove_rmap(), a large folio is added to deferred split list
>>>> if any page in a folio loses its final mapping. It is possible that
>>>> the folio is unmapped fully, but it is unnecessary to add the folio
>>>> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
>>>> before adding a folio to deferred split list. If the folio is already
>>>> on the deferred split list, it will be skipped.
>>>>
>>>> Commit 98046944a159 ("mm: huge_memory: add the missing
>>>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
>>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
>>>> fix everything. A fully unmapped PTE-mapped order-9 THP was also added to
>>>> deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, since nr is
>>>> 512 (non zero), level is RMAP_LEVEL_PTE, and inside deferred_split_folio()
>>>> the order-9 folio is folio_test_pmd_mappable(). However, this miscount
>>>> was present even earlier due to implementation, since PTEs are unmapped
>>>> individually and first PTE unmapping adds the THP into the deferred split
>>>> list.
>>>
>>> Shall you mention the miscounting for mTHP too? There is another patch
>>> series adding the counter support for mTHP.
>>
>> OK, will add it.
>
> I thought I made it clear: this patch won't "fix" it. Misaccounting will still happen. Just less frequently.
>
> Please spell that out.
Sure. Sorry I did not make that clear.
>
>>>
>>>>
>>>> With commit b06dc281aa99 ("mm/rmap: introduce
>>>> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
>>>> folios in one shot without causing the miscount, hence this patch.
>>>>
>>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>>> ---
>>>> mm/rmap.c | 8 +++++---
>>>> 1 file changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index a7913a454028..220ad8a83589 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>> * page of the folio is unmapped and at least one page
>>>> * is still mapped.
>>>> */
>>>> - if (folio_test_large(folio) && folio_test_anon(folio))
>>>> - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
>>>> - deferred_split_folio(folio);
>>>> + if (folio_test_large(folio) && folio_test_anon(folio) &&
>>>> + list_empty(&folio->_deferred_list) &&
>>>
>>> Do we really need this check? deferred_split_folio() does the same
>>> check too. Bailing out earlier sounds ok too, but there may not be too
>>> much gain.
>>
>> Sure, I can remove it.
>
> Please leave it. It's a function call that cannot be optimized out otherwise.
OK. If you think it is worth optimizing that function call, I will keep it.
--
Best Regards,
Yan, Zi
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]
prev parent reply other threads:[~2024-04-25 14:51 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-24 21:10 [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list Zi Yan
2024-04-24 22:32 ` Yang Shi
2024-04-24 22:39 ` Zi Yan
2024-04-24 22:53 ` Yang Shi
2024-04-25 7:15 ` David Hildenbrand
2024-04-25 14:50 ` Zi Yan [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=935E19A6-752C-49E7-8D25-8091195E0104@nvidia.com \
--to=ziy@nvidia.com \
--cc=21cnbao@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=david@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=ryan.roberts@arm.com \
--cc=shy828301@gmail.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).