[RFC][PATCH] __GFP_FAIL (was: Re: [PATCH] fork() failing)

[Rik van Riel]

On Thu, 18 Oct 2001, Marcelo Tosatti wrote:
> On Thu, 18 Oct 2001, Rik van Riel wrote:

> > We just need to make sure that the "wake up kswapd and maybe
> > help free memory" point is EXACTLY the same as the __GFP_FAIL
> > failure point.
>
> Ok, great, that works fine. We can do that for 2.4, no problem.

A quick (untested) patch to demonstrate my idea is below
my signature. Note the comments in __alloc_pages() ...
it's important to only fail our __GFP_FAIL allocation
_after_ having woken up kswapd.

regards,

Rik
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--- linux-2.4.12-ac3/mm/page_alloc.c.nofail	Thu Oct 18 23:24:50 2001
+++ linux-2.4.12-ac3/mm/page_alloc.c	Fri Oct 19 01:34:31 2001
@@ -341,18 +341,25 @@

 	/*
 	 * OK, none of the zones on our zonelist has lots
-	 * of pages free.
-	 *
-	 * We wake up kswapd, in the hope that kswapd will
-	 * resolve this situation before memory gets tight.
-	 *
-	 * We'll also help a bit trying to free pages, this
-	 * way statistics will make sure really fast allocators
-	 * are slowed down more than slow allocators and other
-	 * programs in the system shouldn't be impacted as much
-	 * by the hogs.
+	 * of pages free. Kswapd has work to do ...
 	 */
 	wakeup_kswapd();
+
+	/*
+	 * We don't want to do memory balancing work ourself,
+	 * instead we fail this allocation and hope that kswapd
+	 * will have things in a better shape next time.
+	 */
+	if (gfp_mask & __GFP_FAIL)
+		return NULL;
+
+	/*
+	 * Free some pages ourselves, rather than eating up the
+	 * last few free pages and running the system into the
+	 * ground.  Since this slows down heavy allocators more
+	 * than occasional allocators, it provides some fairness
+	 * and smoother behaviour under heavy load.
+	 */
 	if ((gfp_mask & __GFP_WAIT) && !(current->flags & PF_MEMALLOC))
 		try_to_free_pages(gfp_mask);

--- linux-2.4.12-ac3/include/linux/mm.h.nofail	Fri Oct 19 01:29:19 2001
+++ linux-2.4.12-ac3/include/linux/mm.h	Fri Oct 19 01:33:30 2001
@@ -567,12 +567,14 @@
 #define __GFP_HIGH	0x20	/* Should access emergency pools? */
 #define __GFP_IO	0x40	/* Can start physical IO? */
 #define __GFP_FS	0x80	/* Can call down to low-level FS? */
+#define __GFP_FAIL	0x100	/* Fail early when low on free pages. */

 #define GFP_NOIO	(__GFP_HIGH | __GFP_WAIT)
 #define GFP_NOFS	(__GFP_HIGH | __GFP_WAIT | __GFP_IO)
 #define GFP_ATOMIC	(__GFP_HIGH)
 #define GFP_USER	(             __GFP_WAIT | __GFP_IO | __GFP_FS)
 #define GFP_HIGHUSER	(             __GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HIGHMEM)
+#define GFP_READAHEAD	(             __GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HIGHMEM | __GFP_FAIL)
 #define GFP_KERNEL	(__GFP_HIGH | __GFP_WAIT | __GFP_IO | __GFP_FS)
 #define GFP_NFS		(__GFP_HIGH | __GFP_WAIT | __GFP_IO | __GFP_FS)
 #define GFP_KSWAPD	(                          __GFP_IO | __GFP_FS)