linux-kernel - Re: [PATCH gmem FIXUP] mm, compaction: make testing mapping

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Message-ID: <ZPfAL0D95AwXD9tg@google.com>
Date:   Tue, 5 Sep 2023 16:56:31 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     ackerleytng@...gle.com, akpm@...ux-foundation.org,
        anup@...infault.org, aou@...s.berkeley.edu,
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Subject: Re: [PATCH gmem FIXUP] mm, compaction: make testing
 mapping_unmovable() safe

On Fri, Sep 01, 2023, Vlastimil Babka wrote:
> As Kirill pointed out, mapping can be removed under us due to
> truncation. Test it under folio lock as already done for the async
> compaction / dirty folio case. To prevent locking every folio with
> mapping to do the test, do it only for unevictable folios, as we can
> expect the unmovable mapping folios are also unevictable - it is the
> case for guest memfd folios.

Rather than expect/assume that unmovable mappings are always unevictable, how about
requiring that?  E.g. either through a VM_WARN_ON in mapping_set_unmovable(), or by
simply having that helper forcefully set AS_UNEVICTABLE as well.