Re: [PATCH] oom: handle overflow in mem_cgroup_out_of_memory()

[Johannes Weiner]

On Wed, Jan 26, 2011 at 09:33:09AM -0800, Greg Thelen wrote:
> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
> 
> > On Wed, Jan 26, 2011 at 12:29:15AM -0800, Greg Thelen wrote:
> >> mem_cgroup_get_limit() returns a byte limit as a unsigned 64 bit value,
> >> which is converted to a page count by mem_cgroup_out_of_memory().  Prior
> >> to this patch the conversion could overflow on 32 bit platforms
> >> yielding a limit of zero.
> >
> > Balbir: It can truncate, because the conversion shrinks the required
> > bits of this 64-bit number by only PAGE_SHIFT (12).  Trying to store
> > the resulting up to 52 significant bits in a 32-bit integer will cut
> > up to 20 significant bits off.
> >
> >> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
> >> ---
> >>  mm/oom_kill.c |    2 +-
> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> >> 
> >> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> >> index 7dcca55..3fcac51 100644
> >> --- a/mm/oom_kill.c
> >> +++ b/mm/oom_kill.c
> >> @@ -538,7 +538,7 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
> >>  	struct task_struct *p;
> >>  
> >>  	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, 0, NULL);
> >> -	limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
> >> +	limit = min(mem_cgroup_get_limit(mem) >> PAGE_SHIFT, (u64)ULONG_MAX);
> >
> > I would much prefer using min_t(u64, ...).  To make it really, really
> > explicit that this is 64-bit arithmetic.  But that is just me, no
> > correctness issue.
> >
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> I agree that min_t() is clearer.  Does the following look better?

Sweet, thank you Greg!

> Author: Greg Thelen <gthelen@xxxxxxxxxx>
> Date:   Wed Jan 26 00:05:59 2011 -0800
> 
>     oom: handle truncation in mem_cgroup_out_of_memory()
>     
>     mem_cgroup_get_limit() returns a byte limit as an unsigned 64 bit value.
>     mem_cgroup_out_of_memory() converts this byte limit to an unsigned long
>     page count.  Prior to this patch, the 32 bit version of
>     mem_cgroup_out_of_memory() would silently truncate the most significant
>     20 bits from byte limit when constructing the limit as a page count.
>     For byte limits with the lowest 44 bits set to zero, this truncation
>     would compute a page limit of zero.
>     
>     This patch checks for such large byte limits that cannot be converted to
>     page counts without loosing information.  In such situations, where a 32
>     bit page counter is too small to represent the corresponding byte count,
>     select a maximal page count.
>     
>     Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

That being said, does this have any practical impact at all?  I mean,
this code runs when the cgroup limit is breached.  But if the number
of allowed pages (not bytes!) can not fit into 32 bits, it means you
have a group of processes using more than 16T.  On a 32-bit machine.
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