Re: [RFC PATCH 1/3] mm: teach mm by current context info to not do I/O during memory allocation

[Minchan Kim <minchan@kernel.org>]

On Tue, Oct 16, 2012 at 03:08:41PM +0800, Ming Lei wrote:
> On Tue, Oct 16, 2012 at 1:49 PM, Minchan Kim <minchan@kernel.org> wrote:
> >
> > Fair enough but it wouldn't be a good idea that add new unlikely branch
> > in allocator's fast path. Please move the check into slow path which could
> > be in __alloc_pages_slowpath.
> 
> Thanks for your comment.
> 
> I have considered to add the branch into gfp_to_alloc_flags() before,
> but didn't do it because I see that get_page_from_freelist() may use
> the GFP_IO or GFP_FS flag at least in zone_reclaim() path.

Good point. You can check it in __zone_reclaim and change gfp_mask of scan_control
because it's never hot path.

> 
> So could you make sure it is safe to move the branch into
> __alloc_pages_slowpath()?  If so, I will add the check into
> gfp_to_alloc_flags().

How about this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d976957..b3607fa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2614,10 +2614,16 @@ retry_cpuset:
 	page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
 			zonelist, high_zoneidx, alloc_flags,
 			preferred_zone, migratetype);
-	if (unlikely(!page))
+	if (unlikely(!page)) {
+		/*
+		 * Resume path can deadlock because block device
+		 * isn't active yet.
+		 */
+		if (unlikely(tsk_memalloc_no_io(current)))
+			gfp_mask &= ~GFP_IOFS;
 		page = __alloc_pages_slowpath(gfp_mask, order,
 				zonelist, high_zoneidx, nodemask,
 				preferred_zone, migratetype);
+	}
 
 	trace_mm_page_alloc(page, order, gfp_mask, migratetype);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b5e45f4..6c2ccdd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3290,6 +3290,16 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 	};
 	unsigned long nr_slab_pages0, nr_slab_pages1;
 
+	if (unlikely(tsk_memalloc_no_io(current))) {
+		sc.gfp_mask &= ~GFP_IOFS;
+		shrink.gfp_mask = sc.gfp_mask;
+		/*
+		 * We allow to reclaim only clean pages.
+		 * It can affect RECLAIM_SWAP and RECLAIM_WRITE mode
+		 * but this is really rare event and allocator can
		 * fallback to other zones.
+		 */
+		sc.may_writepage = 0;
+		sc.may_swap = 0;
+	}
+
 	cond_resched();
 	/*
 	 * We need to be able to allocate from the reserves for RECLAIM_SWAP


> 
> 
> Thanks,
> --
> Ming Lei
> 
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-- 
Kind Regards,
Minchan Kim

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