From: "Alex Zhu (Kernel)" <alexlzhu@fb.com>
To: David Hildenbrand <david@redhat.com>
Cc: Rik van Riel <riel@surriel.com>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"willy@infradead.org" <willy@infradead.org>,
"hannes@cmpxchg.org" <hannes@cmpxchg.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
Kernel Team <Kernel-team@fb.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages
Date: Tue, 30 Aug 2022 21:54:55 +0000 [thread overview]
Message-ID: <A92C4953-F9BC-4687-BB03-2202D94D6F5D@fb.com> (raw)
In-Reply-To: <00f2dee2-ebc1-e732-f230-bc5b17da9f80@redhat.com>
> If you unmap something (resulting in pte_none()) where previously
> something used to be mapped in a page table, you might suddenly inform
> the user space fault handler about a page fault that it doesn't expect,
> because it previously placed a page and did not zap that page itself
> (MADV_DONTNEED).
>
> So at least with userfaultfd I think we have to be careful. Not sure if
> there are other corner cases (again, KSM behavior is interesting)
>
> --
> Thanks,
>
> David / dhildenb
We can implement it such that if userfaultfd is enabled on a VMA then instead of unmapping the zero page,
we will map to a read only zero page.
The original patch from Yu Zhao frees zero pages only on reclaim, I am not sure
it needs to be this restricted though. In use cases where immediately freeing
zero pages does not work we can dedupe similar to how KSM does it.
next prev parent reply other threads:[~2022-08-30 22:14 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-25 21:30 [RFC 0/3] THP Shrinker alexlzhu
2022-08-25 21:30 ` [RFC 1/3] mm: add thp_utilization metrics to debugfs alexlzhu
2022-08-27 0:11 ` Zi Yan
2022-08-29 20:19 ` Alex Zhu (Kernel)
2022-08-25 21:30 ` [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages alexlzhu
2022-08-26 10:18 ` David Hildenbrand
2022-08-26 18:34 ` Alex Zhu (Kernel)
2022-08-26 21:18 ` Rik van Riel
2022-08-29 10:02 ` David Hildenbrand
2022-08-29 13:17 ` Rik van Riel
2022-08-30 12:33 ` David Hildenbrand
2022-08-30 21:54 ` Alex Zhu (Kernel) [this message]
2022-08-25 21:30 ` [RFC 3/3] mm: THP low utilization shrinker alexlzhu
2022-08-27 0:25 ` Zi Yan
2022-08-29 20:49 ` Alex Zhu (Kernel)
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