Commit bdd804f4 authored by Paul Jackson's avatar Paul Jackson Committed by Linus Torvalds
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[PATCH] Cpuset: might sleep checking zones allowed fix

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 gfp_flags).

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: default avatarPaul Jackson <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 593ee207
......@@ -951,7 +951,7 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
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));
......@@ -970,7 +970,8 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
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
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