On Tue, 11 Jan 2011 13:03:22 -0800 (PST)
Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 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).
mutter.
> >
> > 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!
Only bad people do that. "p". Really?
z:/usr/src/linux-2.6.37> grep -r " \*p;" . | wc -l
2329
z:/usr/src/linux-2.6.37> grep -r "task_struct \*p" . | wc -l
824
bah.
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