linux-kernel - Re: [PATCH] mm: cma: honor __GFP_ZERO flag in cma

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Message-Id: <20180702132335eucas1p1323fbf51cd5e82a59939d72097acee04~9kAizDyji0466904669eucas1p1w@eucas1p1.samsung.com>
Date:   Mon, 2 Jul 2018 15:23:34 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Nazarewicz <mina86@...a86.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: cma: honor __GFP_ZERO flag in cma_alloc()

Hi Michal,

On 2018-06-13 15:39, Michal Hocko wrote:
> On Wed 13-06-18 05:55:46, Christoph Hellwig wrote:
>> On Wed, Jun 13, 2018 at 02:40:00PM +0200, Marek Szyprowski wrote:
>>> It is not only the matter of the spinlocks. GFP_ATOMIC is not supported
>>> by the
>>> memory compaction code, which is used in alloc_contig_range(). Right, this
>>> should be also noted in the documentation.
>> Documentation is good, asserts are better.  The code should reject any
>> flag not explicitly supported, or even better have its own flags type
>> with the few actually supported flags.
> Agreed. Is the cma allocator used for anything other than GFP_KERNEL
> btw.? If not then, shouldn't we simply drop the gfp argument altogether
> rather than give users a false hope for differen gfp modes that are not
> really supported and grow broken code?

Nope, all cma_alloc() callers are expected to use it with GFP_KERNEL gfp 
mask.
The only flag which is now checked is __GFP_NOWARN. I can change the 
function
signature of cma_alloc to:
struct page *cma_alloc(struct cma *cma, size_t count, unsigned int 
align, bool no_warn);

What about clearing the allocated buffer? Should it be another bool 
parameter,
done unconditionally or moved to the callers?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland