linux-kernel - Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

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Date:   Mon, 19 Dec 2022 08:48:10 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>
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Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to
 create restricted user memory

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.