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

[Chao Peng]

On Mon, Dec 19, 2022 at 08:48:10AM +0000, Huang, Kai wrote:
> 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')?

restrictedmem_get_page() increases page refcount several versions ago so
no change in v10 is needed. You probably missed my reply:

https://lore.kernel.org/linux-mm/20221129135844.GA902164@chaop.bj.intel.com/

The current solution is clear: unless we have better approach, we will
let restrictedmem user (KVM in this case) to hold the refcount to
prevent page migration.

Thanks,
Chao
> 
> 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.
>