On Tue, Jan 11, 2011 at 12:45:51PM -0800, 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.
>
Subtle but we can't have reached here if PF_MEMALLOC was previous set.
It gets caught by
/* Avoid recursion of direct reclaim */
if (p->flags & PF_MEMALLOC)
goto nopage;
> It's risky, and general bad practice. The default operation here
> should be to push the old value and to later restore it.
>
Arguably we would do this in case that check was ever removed but I
can't imagine why we would allow direct reclaim or compaction to recurse
into direct reclaim or compaction.
> 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.
>
If the code evolves in that direction, it's pretty dangerous.
> One way of doing that would be to add a WARN_ON(p->flags & PF_MEMALLOC)
> on entry.
>
That would be reasonable.
> Oh, and since when did we use `p' to identify task_structs?
>
It's pretty stupid all right but it was the name chosen for other parts
of page_alloc.c. A patch that either removed all local caching or
renamed p to tsk throughout page_alloc.c would be reasonable.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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