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Date: Wed, 11 Jul 2018 10:54:07 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH 1/2] mm/cma: remove unsupported gfp_mask parameter from
 cma_alloc()
Message-ID: <20180711085407.GB20050@dhcp22.suse.cz>
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To: Joonsoo Kim <js1304@gmail.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>, Linux Memory Management List <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, linux-arm-kernel@lists.infradead.org, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, iommu@lists.linux-foundation.org, Andrew Morton <akpm@linux-foundation.org>, Michal Nazarewicz <mina86@mina86.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Vlastimil Babka <vbabka@suse.cz>, Christoph Hellwig <hch@lst.de>, Russell King <linux@armlinux.org.uk>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>, Paul Mackerras <paulus@ozlabs.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Chris Zankel <chris@zankel.net>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Joerg Roedel <joro@8bytes.org>, Sumit Semwal <sumit.semwal@linaro.org>, Robin Murphy <robin.murphy@arm.com>, Laura Abbott <labbott@redhat.com>, linaro-mm-sig@lists.linaro.org

On Wed 11-07-18 16:35:28, Joonsoo Kim wrote:
> 2018-07-10 18:50 GMT+09:00 Michal Hocko <mhocko@kernel.org>:
> > On Tue 10-07-18 16:19:32, Joonsoo Kim wrote:
> >> Hello, Marek.
> >>
> >> 2018-07-09 21:19 GMT+09:00 Marek Szyprowski <m.szyprowski@samsung.com>:
> >> > cma_alloc() function doesn't really support gfp flags other than
> >> > __GFP_NOWARN, so convert gfp_mask parameter to boolean no_warn parameter.
> >>
> >> Although gfp_mask isn't used in cma_alloc() except no_warn, it can be used
> >> in alloc_contig_range(). For example, if passed gfp mask has no __GFP_FS,
> >> compaction(isolation) would work differently. Do you have considered
> >> such a case?
> >
> > Does any of cma_alloc users actually care about GFP_NO{FS,IO}?
> 
> I don't know. My guess is that cma_alloc() is used for DMA allocation so
> block device would use it, too. If fs/block subsystem initiates the
> request for the device,
> it would be possible that cma_alloc() is called with such a flag.
> Again, I don't know
> much about those subsystem so I would be wrong.

The patch converts existing users and none of them really tries to use
anything other than GFP_KERNEL [|__GFP_NOWARN] so this doesn't seem to
be the case. Should there be a new user requiring more restricted
gfp_mask we should carefuly re-evaluate and think how to support it.

Until then I would simply stick with the proposed approach because my
experience tells me that a wrong gfp mask usage is way too easy so the
simpler the api is the less likely we will see an abuse.
-- 
Michal Hocko
SUSE Labs