On Tue, 11 Jan 2011, Andrew Morton wrote:
> On Tue, 11 Jan 2011 11:45:21 +0000
> Mel Gorman <mel@xxxxxxxxx> wrote:
>
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1809,12 +1809,15 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> > bool sync_migration)
> > {
> > struct page *page;
> > + struct task_struct *p = current;
> >
> > if (!order || compaction_deferred(preferred_zone))
> > return NULL;
> >
> > + p->flags |= PF_MEMALLOC;
> > *did_some_progress = try_to_compact_pages(zonelist, order, gfp_mask,
> > nodemask, sync_migration);
> > + p->flags &= ~PF_MEMALLOC;
>
> Thus accidentally wiping out PF_MEMALLOC if it was already set.
>
> It's risky, and general bad practice. The default operation here
> should be to push the old value and to later restore it.
>
> If it is safe to micro-optimise that operation then we need to make
> sure that it's really really safe and that there is no risk of
> accidentally breaking things later on as code evolves.
>
> One way of doing that would be to add a WARN_ON(p->flags & PF_MEMALLOC)
> on entry.
True. Though one of the nice things about Mel's patch is that it is
precisely copying in __alloc_pages_direct_compact what is already
done in __alloc_pages_direct_reclaim (both being called from
__alloc_pages_slowpath after it checked for PF_MEMALLOC).
>
> Oh, and since when did we use `p' to identify task_structs?
Tsk, tsk: we've been using `p' for task_structs for years and years!
Hugh
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