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

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Message-Id: <20180709103159eucas1p1e72d940f6c947769ff7b55cd5bfb4b61~-rLtaSiDM3047830478eucas1p14@eucas1p1.samsung.com>
Date:   Mon, 9 Jul 2018 12:31:57 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 2018-07-02 15:32, Michal Hocko wrote:
> On Mon 02-07-18 15:23:34, Marek Szyprowski wrote:
>> 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);
> Are there any __GFP_NOWARN users? I have quickly hit the indirection
> trap and searching for alloc callback didn't tell me really much.

They might be via dma_alloc_from_contiguous() and dma_alloc_*() path.

>> What about clearing the allocated buffer? Should it be another bool
>> parameter, done unconditionally or moved to the callers?
> That really depends on callers. I have no idea what they actually ask
> for.

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