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.
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