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

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Date:   Mon, 2 Jul 2018 15:32:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
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()

On Mon 02-07-18 15:23:34, Marek Szyprowski wrote:
> 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);

Are there any __GFP_NOWARN users? I have quickly hit the indirection
trap and searching for alloc callback didn't tell me really much.

> 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.
-- 
Michal Hocko
SUSE Labs