linux-kernel - Re: [PATCH 1/2] mm/cma: remove unsupported gfp_mask parameter from cma

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Message-ID: <20180711085407.GB20050@dhcp22.suse.cz>
Date:   Wed, 11 Jul 2018 10:54:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        iommu@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Nazarewicz <mina86@...a86.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@....de>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Chris Zankel <chris@...kel.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Joerg Roedel <joro@...tes.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Robin Murphy <robin.murphy@....com>,
        Laura Abbott <labbott@...hat.com>,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 1/2] mm/cma: remove unsupported gfp_mask parameter from
 cma_alloc()

On Wed 11-07-18 16:35:28, Joonsoo Kim wrote:
> 2018-07-10 18:50 GMT+09:00 Michal Hocko <mhocko@...nel.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@...sung.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