linux-kernel - Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

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Date:   Tue, 30 Aug 2022 21:54:55 +0000
From:   "Alex Zhu (Kernel)" <alexlzhu@...com>
To:     David Hildenbrand <david@...hat.com>
CC:     Rik van Riel <riel@...riel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled
 tail pages


> 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.