On Thu, Aug 31, 2017 at 4:32 AM, Vlastimil Babka [off-list ref] wrote:

On 08/31/2017 03:40 AM, Joonsoo Kim wrote:

quoted

On Tue, Aug 29, 2017 at 11:16:18AM +0200, Vlastimil Babka wrote:

quoted

On 08/24/2017 08:36 AM, js1304@gmail.com wrote:

quoted

From: Joonsoo Kim <redacted>

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.

[...]

quoted

[1]: lkml.kernel.org/r/1491880640-9944-1-git-send-email-iamjoonsoo.kim@lge.com
[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 <redacted>
Acked-by: Vlastimil Babka <redacted>

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.

quoted

Anyway I checked the current patch and did some basic tests with qemu,
so you can keep my ack.

Thanks!

quoted

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.

Yeah, as Laura said, the idea is to make sure that a copy doesn't
exceed the bounds of the allocation (and that means a single page when
not __GFP_COMP nor CMA nor Reserved).

The trouble with this check, which I'd like see fixed, is that there
are portions of the kernel that make separate adjacent page
allocations and then copy across individual allocations in a single
usercopy. It's not clear to me if that is fixable just by adding
__GFP_COMP or not, though.

-Kees

-- 
Kees Cook
Pixel Security

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