Re: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined

[Mel Gorman]

On Mon, Dec 03, 2012 at 10:52:27AM +0800, Lin Feng wrote:
> 
> 
> On 11/30/2012 07:00 PM, Mel Gorman wrote:
> >>
> >> Well, that's a fairly low-level implementation detail.  A more typical
> >> approach would be to add a new get_user_pages_non_movable() or such. 
> >> That would probably have the same signature as get_user_pages(), with
> >> one additional argument.  Then get_user_pages() becomes a one-line
> >> wrapper which passes in a particular value of that argument.
> >>
> > 
> > That is going in the direction that all pinned pages become MIGRATE_UNMOVABLE
> > allocations.  That will impact THP availability by increasing the number
> > of MIGRATE_UNMOVABLE blocks that exist and it would hit every user --
> > not just those that care about ZONE_MOVABLE.
> > 
> > I'm likely to NAK such a patch if it's only about node hot-remove because
> > it's much more of a corner case than wanting to use THP.
> > 
> > I would prefer if get_user_pages() checked if the page it was about to
> > pin was in ZONE_MOVABLE and if so, migrate it at that point before it's
> > pinned. It'll be expensive but will guarantee ZONE_MOVABLE availability
> > if that's what they want. The CMA people might also want to take
> > advantage of this if the page happened to be in the MIGRATE_CMA
> > pageblock.
> > 
> hi Mel,
> 
> Thanks for your suggestion. 
> My initial idea is also to restrict the impact as little as possible so 
> migrate such pages as we need. 
> But even to such "going to pin pages", most of them are going to be released 
> soon, so deal with them all in the same way is really *expensive*. 
> 

Then you need to somehow distinguish between short-lived pins and
long-lived pins and only migrate the long-lived pins. I didn't research
how this could be implemented

-- 
Mel Gorman
SUSE Labs
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