On Tue, 26 Apr 2011 12:33:48 +0100 Mel Gorman [off-list ref] wrote:

On Tue, 2011-04-26 at 21:15 +1000, NeilBrown wrote:

quoted

On Tue, 26 Apr 2011 08:36:43 +0100 Mel Gorman [off-list ref] wrote:

quoted

+bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
+{
+	return gfp_to_alloc_flags(gfp_mask) & ALLOC_PFMEMALLOC;
+}
+
 static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	struct zonelist *zonelist, enum zone_type high_zoneidx,

@@ -2202,8 +2211,16 @@ nopage:
 got_pg:
 	if (kmemcheck_enabled)
 		kmemcheck_pagealloc_alloc(page, order, gfp_mask);
-	return page;
 
+	/*
+	 * page->pfmemalloc is set when the caller had PFMEMALLOC set or is
+	 * been OOM killed. The expectation is that the caller is taking
+	 * steps that will free more memory. The caller should avoid the
+	 * page being used for !PFMEMALLOC purposes.
+	 */
+	page->pfmemalloc = (alloc_flags & ALLOC_PFMEMALLOC);
+
+	return page;

Linus doesn't seem to be a fan of this construct:
   https://lkml.org/lkml/2011/4/1/255

There is confusion around this topic. Andrew prefers bool for true/false
values and it's self-documenting. I tend to prefer it myself for
readability and there is a slow conversion in the VM from
ints-used-as-bools to bools and my understanding of bool is that any
non-zero value will be treated as true (just as it is for int).

quoted

pfmemalloc is a bool, and the value on the right is either 0 or 0x1000.

If bool happens to be typedefed to 'char' or even 'short', pfmemalloc would
always be set to 0.

It is typedeffed as _Bool though which I thought was able to handle the
cast appropriately or is that wrong?

Yes, I too believe that _Bool does the right thing, so this particular usage
does happen to be safe in the kernel.  But there is a long tradition of
'bool' being typedefed to an int type so the usage can look wrong.  So maybe
it is best avoided.

I have no strong feeling either way - I just thought I would highlight it.
In general, I like bool, but I don't like automatic casts (I often use
  if (pointer != NULL) ...
because I think it reads better).

Thanks,
NeilBrown

quoted

Ditto for the gfp_pfmemalloc_allowed function.

Prefixing with '!!' would make it safe.

Will do that to avoid any oddities. Thanks

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