[PATCH 01/03] Cpuset: might sleep checking zones allowed fix

From: Paul Jackson
Date: Thu May 18 2006 - 00:35:34 EST

From: Paul Jackson <pj@xxxxxxx>

Fix a couple of infrequently encountered 'sleeping function
called from invalid context' in the cpuset hooks in
__alloc_pages. Could sleep while interrupts disabled.

The routine cpuset_zone_allowed() is called by code in
mm/page_alloc.c __alloc_pages() to determine if a zone is
allowed in the current tasks cpuset. This routine can sleep,
for certain GFP_KERNEL allocations, if the zone is on a memory
node not allowed in the current cpuset, but might be allowed
in a parent cpuset.

But we can't sleep in __alloc_pages() if in interrupt, nor
if called for a GFP_ATOMIC request (__GFP_WAIT not set in

The rule was intended to be:
Don't call cpuset_zone_allowed() if you can't sleep, unless you
pass in the __GFP_HARDWALL flag set in gfp_flag, which disables
the code that might scan up ancestor cpusets and sleep.

This rule was being violated in a couple of places, due to a
bogus change made (by myself, pj) to __alloc_pages() as part
of the November 2005 effort to cleanup its logic, and also due
to a later fix to constrain which swap daemons were awoken.

The bogus change can be seen at:
[PATCH 01/05] mm fix __alloc_pages cpuset ALLOC_* flags

This was first noticed on a tight memory system, in code that
was disabling interrupts and doing allocation requests with
__GFP_WAIT not set, which resulted in __might_sleep() writing
complaints to the log "Debug: sleeping function called ...",
when the code in cpuset_zone_allowed() tried to take the
callback_sem cpuset semaphore.

We haven't seen a system hang on this 'might_sleep' yet, but
we are at decent risk of seeing it fairly soon, especially
since the additional cpuset_zone_allowed() check was added,
conditioning wakeup_kswapd(), in March 2006.

Special thanks to Dave Chinner, for figuring this out,
and a tip of the hat to Nick Piggin who warned me of this
back in Nov 2005, before I was ready to listen.

Signed-off-by: Paul Jackson <pj@xxxxxxx>


Andrew - this patch is worth pushing along a little faster. -pj

mm/page_alloc.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- 2.6.17-rc4-mm1.orig/mm/page_alloc.c 2006-05-17 18:57:58.400484681 -0700
+++ 2.6.17-rc4-mm1/mm/page_alloc.c 2006-05-17 19:04:59.177414192 -0700
@@ -1038,7 +1038,7 @@ restart:
goto got_pg;

do {
- if (cpuset_zone_allowed(*z, gfp_mask))
+ if (cpuset_zone_allowed(*z, gfp_mask|__GFP_HARDWALL))
wakeup_kswapd(*z, order);
} while (*(++z));

@@ -1057,7 +1057,8 @@ restart:
alloc_flags |= ALLOC_HARDER;
if (gfp_mask & __GFP_HIGH)
alloc_flags |= ALLOC_HIGH;
- alloc_flags |= ALLOC_CPUSET;
+ if (wait)
+ alloc_flags |= ALLOC_CPUSET;

* Go through the zonelist again. Let __GFP_HIGH and allocations

I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@xxxxxxx> 1.650.933.1373
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