Re: [patch] mm: fix deferred congestion timeout if preferred zone is not allowed

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > > This patch resets preferred_zone to an allowed zone in the slowpath if
> > > the allocation context is constrained by current's cpuset. 
> > 
> > Well, preferred_zone has meaning. If it's not possible to allocate from
> > that zone in the current cpuset context, it's not really preferred. Why
> > not set it in the fast path so there isn't a useless call to
> > get_page_from_freelist()?
> > 
> 
> It may be the preferred zone even if it isn't allowed by current's cpuset 
> such as if the allocation is __GFP_WAIT or the task has been oom killed 
> and has the TIF_MEMDIE bit set, so the preferred zone in the fastpath is 
> accurate in these cases.  In the slowpath, the former is protected by 
> checking for ALLOC_CPUSET and the latter is usually only set after the 
> page allocator has looped at least once and triggered the oom killer to be 
> killed.
> 
> I didn't want to add a branch to test for these possibilities in the 
> fastpath, however, since preferred_zone isn't of critical importance until 
> it's used in the slowpath (ignoring the statistical usage).

I'm glad to you are keeping fastpath concern. However you don't need
nodemask-and in this case. Because zonelist->zref[0] is always in nodemask.
Please see policy_zonelist(). So, you can just replace nodemask with cpuset_mems_allowed.

This is not only simple, but also improve a consisteny of mempolicy.

---
 mm/page_alloc.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 07a6544..876de04 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2146,7 +2146,8 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 
 	get_mems_allowed();
 	/* The preferred zone is used for statistics later */
-	first_zones_zonelist(zonelist, high_zoneidx, nodemask, &preferred_zone);
+	first_zones_zonelist(zonelist, high_zoneidx,
+			     &cpuset_current_mems_allowed, &preferred_zone);
 	if (!preferred_zone) {
 		put_mems_allowed();
 		return NULL;
-- 
1.6.5.2




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