Re: [PATCH v2 0/6] Introduce ZONE_CMA

[Vlastimil Babka]

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.