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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>