Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations

[Andrew Morton]

On Thu, 12 Mar 2020 10:41:28 +0900 Joonsoo Kim <js1304@xxxxxxxxx> wrote:

> Hello, Roman.
> 
> 2020ë 3ì 12ì (ë) ìì 2:35, Roman Gushchin <guro@xxxxxx>ëì ìì:
> >
> > On Wed, Mar 11, 2020 at 09:51:07AM +0100, Vlastimil Babka wrote:
> > > On 3/6/20 9:01 PM, Rik van Riel wrote:
> > > > Posting this one for Roman so I can deal with any upstream feedback and
> > > > create a v2 if needed, while scratching my head over the next piece of
> > > > this puzzle :)
> > > >
> > > > ---8<---
> > > >
> > > > From: Roman Gushchin <guro@xxxxxx>
> > > >
> > > > Currently a cma area is barely used by the page allocator because
> > > > it's used only as a fallback from movable, however kswapd tries
> > > > hard to make sure that the fallback path isn't used.
> > >
> > > Few years ago Joonsoo wanted to fix these kinds of weird MIGRATE_CMA corner
> > > cases by using ZONE_MOVABLE instead [1]. Unfortunately it was reverted due to
> > > unresolved bugs. Perhaps the idea could be resurrected now?
> >
> > Hi Vlastimil!
> >
> > Thank you for this reminder! I actually looked at it and also asked Joonsoo in private
> > about the state of this patch(set). As I understand, Joonsoo plans to resubmit
> > it later this year.
> >
> > What Rik and I are suggesting seems to be much simpler, however it's perfectly
> > possible that Joonsoo's solution is preferable long-term.
> >
> > So if the proposed patch looks ok for now, I'd suggest to go with it and return
> > to this question once we'll have a new version of ZONE_MOVABLE solution.
> 
> Hmm... utilization is not the only matter for CMA user. The more
> important one is
> success guarantee of cma_alloc() and this patch would have a bad impact on it.
> 
> A few years ago, I have tested this kind of approach and found that increasing
> utilization increases cma_alloc() failure. Reason is that the page
> allocated with
> __GFP_MOVABLE, especially, by sb_bread(), is sometimes pinned by someone.
> 
> Until now, cma memory isn't used much so this problem doesn't occur easily.
> However, with this patch, it would happen.

So I guess we keep Roman's patch on hold pending clarification of this
risk?