Re: [PATCH gmem FIXUP] mm, compaction: make testing mapping_unmovable() safe
From: Vlastimil Babka <hidden>
Date: 2023-09-06 08:01:03
Also in:
kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-fsdevel, linux-mips, linux-mm, linux-riscv, linux-security-module, lkml
On 9/6/23 01:56, Sean Christopherson wrote:
On Fri, Sep 01, 2023, Vlastimil Babka wrote:quoted
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.
Yeah I guess we could make the helper do that, with a comment, as gmem is the only user right now. And if in the future somebody has case where it makes sense to have unmovable without unevictable, we can discuss what to do about it then.