On Tue, Dec 08, 2015 at 06:13:09PM +0100, Michal Hocko wrote: > From: Michal Hocko <mhocko@xxxxxxxx> > > After "mm: rework mapcount accounting to enable 4k mapping of THPs" > it is possible to have a partial THP accessible via ptes. Memcg task > migration code is not prepared for this situation and uncharges the tail > page from the original memcg while the original THP is still charged via > the head page which is not mapped to the moved task. The page counter > of the origin memcg will underflow when the whole THP is uncharged later > on and lead to: > WARNING: CPU: 0 PID: 1340 at mm/page_counter.c:26 page_counter_cancel+0x34/0x40() > reported by Minchan Kim. > > This patch prevents from the underflow by skipping any partial THP pages > in mem_cgroup_move_charge_pte_range. PageTransCompound is checked when > we do pte walk. This means that a process might leave a partial THP > behind in the original memcg if there is no other process mapping it via > pmd but this is considered acceptable because it shouldn't happen often > and this is not considered a memory leak because the original THP is > still accessible and reclaimable. Moreover the task migration has always > been racy and never guaranteed to move all pages. > > Reported-by: Minchan Kim <minchan@xxxxxxxxxx> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > --- > > Hi, > this is a patch tested by Minchan in the original thread [1]. I have > only replaced PageCompound with PageTransCompound because other similar > fixes in mmotm used this one. The underlying implementation is the same. > Johannes, I have kept your a-b but let me know if you are not OK with the > changelog. Looks good to me, thanks Michal. Please keep my Acked-by. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>