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Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create
From: |
Huang, Kai |
Subject: |
Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory |
Date: |
Mon, 19 Dec 2022 08:48:10 +0000 |
User-agent: |
Evolution 3.44.4 (3.44.4-2.fc36) |
On Mon, 2022-12-19 at 15:53 +0800, Chao Peng wrote:
> >
> > [...]
> >
> > > +
> > > + /*
> > > + * These pages are currently unmovable so don't place them into
> > > movable
> > > + * pageblocks (e.g. CMA and ZONE_MOVABLE).
> > > + */
> > > + mapping = memfd->f_mapping;
> > > + mapping_set_unevictable(mapping);
> > > + mapping_set_gfp_mask(mapping,
> > > + mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);
> >
> > But, IIUC removing __GFP_MOVABLE flag here only makes page allocation from
> > non-
> > movable zones, but doesn't necessarily prevent page from being migrated. My
> > first glance is you need to implement either a_ops->migrate_folio() or just
> > get_page() after faulting in the page to prevent.
>
> The current api restrictedmem_get_page() already does this, after the
> caller calling it, it holds a reference to the page. The caller then
> decides when to call put_page() appropriately.
I tried to dig some history. Perhaps I am missing something, but it seems Kirill
said in v9 that this code doesn't prevent page migration, and we need to
increase page refcount in restrictedmem_get_page():
https://lore.kernel.org/linux-mm/20221129112139.usp6dqhbih47qpjl@box.shutemov.name/
But looking at this series it seems restrictedmem_get_page() in this v10 is
identical to the one in v9 (except v10 uses 'folio' instead of 'page')?
Anyway if this is not fixed, then it should be fixed. Otherwise, a comment at
the place where page refcount is increased will be helpful to help people
understand page migration is actually prevented.
- [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM, Chao Peng, 2022/12/02
- [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/02
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Fuad Tabba, 2022/12/06
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Huang, Kai, 2022/12/13
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/19
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory,
Huang, Kai <=
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/20
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Huang, Kai, 2022/12/20
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/21
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Huang, Kai, 2022/12/21
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/23
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Sean Christopherson, 2022/12/22
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Huang, Kai, 2022/12/22
- Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory, Chao Peng, 2022/12/23
[PATCH v10 2/9] KVM: Introduce per-page memory attributes, Chao Peng, 2022/12/02