Re: [PATCH 1/3] mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE

[Laura Abbott]

On 08/31/2017 07:32 AM, Vlastimil Babka wrote:
> On 08/31/2017 03:40 AM, Joonsoo Kim wrote:
> > On Tue, Aug 29, 2017 at 11:16:18AM +0200, Vlastimil Babka wrote:
> > > On 08/24/2017 08:36 AM, js1304@xxxxxxxxx wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > >
> > > > 0. History
> > > >
> > > > This patchset is the follow-up of the discussion about the
> > > > "Introduce ZONE_CMA (v7)" [1]. Please reference it if more information
> > > > is needed.
> > >
> > > [...]
> > >
> > > > [1]: lkml.kernel.org/r/1491880640-9944-1-git-send-email-iamjoonsoo.kim@xxxxxxx
> > > > [2]: https://lkml.org/lkml/2014/10/15/623
> > > > [3]: http://www.spinics.net/lists/linux-mm/msg100562.html
> > > >
> > > > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> > >
> > > The previous version has introduced ZONE_CMA, so I would think switching
> > > to ZONE_MOVABLE is enough to drop previous reviews. Perhaps most of the
> > > code involved is basically the same, though?
> >
> > Yes, most of the code involved is the same. I considered to drop
> > previous review tags but most of the code and concept is the same so I
> > decide to keep review tags. I should mention it in cover-letter but I
> > forgot to mention it. Sorry about that.
> >
> > > Anyway I checked the current patch and did some basic tests with qemu,
> > > so you can keep my ack.
> >
> > Thanks!
> >
> > > BTW, if we dropped NR_FREE_CMA_PAGES, could we also drop MIGRATE_CMA and
> > > related hooks? Is that counter really that useful as it works right now?
> > > It will decrease both by CMA allocations (which has to be explicitly
> > > freed) and by movable allocations (which can be migrated). What if only
> > > CMA alloc/release touched it?
> >
> > I think that NR_FREE_CMA_PAGES would not be as useful as previous. We
> > can remove it.
> >
> > However, removing MIGRATE_CMA has a problem. There is an usecase to
> > check if the page comes from the CMA area or not. See
> > check_page_span() in mm/usercopy.c. I can implement it differently by
> > iterating whole CMA area and finding the match, but I'm not sure it's
> > performance effect. I guess that it would be marginal.
>
> +CC Kees Cook
>
> Hmm, seems like this check is to make sure we don't copy from/to parts
> of kernel memory we're not supposed to? Then I believe checking that
> pages are in ZONE_MOVABLE should then give the same guarantees as
> MIGRATE_CMA.

The check is to make sure we are copying only to a single page unless
that page is allocated with __GFP_COMP. CMA needs extra checks since
its allocations have nothing to do with compound page. Checking
ZONE_MOVABLE might cause us to miss some cases of copying to vanilla
ZONE_MOVABLE pages.

> BTW the comment says "Reject if range is entirely either Reserved or
> CMA" but the code does the opposite thing. I assume the comment is wrong?

Yes, I think that needs clarification.

Thanks,
Laura