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SubjectRe: [PATCH 1/2] mm, oom: do not rely on TIF_MEMDIE for memory reserves access
On Tue 01-08-17 18:52:42, Michal Hocko wrote:
> On Wed 02-08-17 00:30:33, Tetsuo Handa wrote:
[...]
> > > - if (gfp_pfmemalloc_allowed(gfp_mask))
> > > - alloc_flags = ALLOC_NO_WATERMARKS;
> > > + /*
> > > + * Distinguish requests which really need access to whole memory
> > > + * reserves from oom victims which can live with their own reserve
> > > + */
> > > + reserves = gfp_pfmemalloc_allowed(gfp_mask);
> > > + if (reserves) {
> > > + if (tsk_is_oom_victim(current))
> > > + alloc_flags = ALLOC_OOM;
> >
> > If reserves == true due to reasons other than tsk_is_oom_victim(current) == true
> > (e.g. __GFP_MEMALLOC), why dare to reduce it?
>
> Well the comment above tries to explain. I assume that the oom victim is
> special here. a) it is on the way to die and b) we know that something
> will be freeing memory on the background so I assume this is acceptable.

I was thinking about this some more. It is not that hard to achive the
original semantic. The code is slightly uglier but acceptable I guess
What do you think about the following?
---
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3510e06b3bf3..7ae0f6d45614 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3627,21 +3627,31 @@ static bool oom_reserves_allowed(struct task_struct *tsk)
return true;
}

-bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
+/*
+ * Distinguish requests which really need access to full memory
+ * reserves from oom victims which can live with a portion of it
+ */
+static inline int __gfp_pfmemalloc_flags(gfp_t gfp_mask)
{
if (unlikely(gfp_mask & __GFP_NOMEMALLOC))
- return false;
-
+ return 0;
if (gfp_mask & __GFP_MEMALLOC)
- return true;
+ return ALLOC_NO_WATERMARKS;
if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
- return true;
- if (!in_interrupt() &&
- ((current->flags & PF_MEMALLOC) ||
- oom_reserves_allowed(current)))
- return true;
+ return ALLOC_NO_WATERMARKS;
+ if (!in_interrupt()) {
+ if (current->flags & PF_MEMALLOC)
+ return ALLOC_NO_WATERMARKS;
+ else if (oom_reserves_allowed(current))
+ return ALLOC_OOM;
+ }

- return false;
+ return 0;
+}
+
+bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
+{
+ return __gfp_pfmemalloc_flags(gfp_mask) > 0;
}

/*
@@ -3794,7 +3804,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
unsigned long alloc_start = jiffies;
unsigned int stall_timeout = 10 * HZ;
unsigned int cpuset_mems_cookie;
- bool reserves;
+ int reserves;

/*
* In the slowpath, we sanity check order to avoid ever trying to
@@ -3900,17 +3910,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
if (gfp_mask & __GFP_KSWAPD_RECLAIM)
wake_all_kswapds(order, ac);

- /*
- * Distinguish requests which really need access to whole memory
- * reserves from oom victims which can live with their own reserve
- */
- reserves = gfp_pfmemalloc_allowed(gfp_mask);
- if (reserves) {
- if (tsk_is_oom_victim(current))
- alloc_flags = ALLOC_OOM;
- else
- alloc_flags = ALLOC_NO_WATERMARKS;
- }
+ reserves = __gfp_pfmemalloc_flags(gfp_mask);
+ if (reserves)
+ alloc_flags = reserves;

/*
* Reset the zonelist iterators if memory policies can be ignored.
--
Michal Hocko
SUSE Labs
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