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Subject: Re: [PATCH v2 0/6] Introduce ZONE_CMA
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mel Gorman <mgorman@techsingularity.net>
References: <1461561670-28012-1-git-send-email-iamjoonsoo.kim@lge.com>
 <20160425053653.GA25662@js1304-P5Q-DELUXE>
 <20160428103927.GM2858@techsingularity.net>
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Cc: Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Laura Abbott <lauraa@codeaurora.org>, Minchan Kim <minchan@kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
From: Vlastimil Babka <vbabka@suse.cz>
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Date: Mon, 2 May 2016 09:49:47 +0200
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On 05/02/2016 08:14 AM, Joonsoo Kim wrote:
>>> > >Although it's separate issue, I should mentioned one thing. Related to
>>> > >I/O pinning issue, ZONE_CMA don't get blockdev allocation request so
>>> > >I/O pinning problem is much reduced.
>>> > >
>> >
>> >This is not super-clear from the patch. blockdev is using GFP_USER so it
>> >already should not be classed as MOVABLE. I could easily be looking in
>> >the wrong place or missed which allocation path sets GFP_MOVABLE.
> Okay. Please see sb_bread(), sb_getblk(), __getblk() and __bread() in
> include/linux/buffer_head.h. These are main functions used by blockdev
> and they uses GFP_MOVABLE. To fix permanent allocation case which is
> used by mount and cannot be released until umount, Gioh introduces
> sb_bread_unmovable() but there are many remaining issues that prevent
> migration at the moment and avoid blockdev allocation from CMA area is
> preferable approach.

Hm Patch 3/6 describes the lack of blockdev allocations mostly as a 
limitation, although it does mention the possible advantages later. 
Anyway, this doesn't have to be specific to ZONE_CMA, right? You could 
just change ALLOC_CMA handling to consider GFP_HIGHUSER_MOVABLE instead 
of just __GFP_MOVABLE. For ZONE_CMA it might be inevitable as you 
describe, but it's already possible to do that now, if the advantages 
are larger than the disadvantages.