On Thu, Mar 15, 2012 at 04:27:11PM -0700, Andrew Morton wrote: > On Fri, 16 Mar 2012 00:15:56 +0100 > Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > > > On Thu, Mar 15, 2012 at 03:45:04PM -0700, Andrew Morton wrote: > > > Or do we still need pdm_trans_unstable() checking in > > > mem_cgroup_count_precharge_pte_range() and > > > mem_cgroup_move_charge_pte_range()? > > > > I think we need a pmd_trans_unstable check before the > > pte_offset_map_lock in both places. Otherwise with only the mmap_sem > > hold for reading, the pmd may have been transhuge, > > mem_cgroup_move_charge_pte_range could be called, and then > > MADV_DONTNEED would transform the pmd to none from another thread just > > before pmd_trans_huge_lock runs, and we would end up doing > > pmd_offset_map_lock on a none pmd (or a transhuge pmd if it becomes > > huge again before we get there). > > page_table_lock doesn't prevent the race? pmd_trans_huge_lock() > rechecks after taking that lock... pmd_trans_huge_lock makes the THP path safe. No change needed in that path, after taking the page_table_lock we're safe there and it'll stop changing. The problem is when the pmd_trans_huge isn't set when pmd_trans_huge_lock runs, so we fallback in the pte walk without holding the page_table_lock. And the pte walk then needs a pmd_trans_unstable check before calling pte_offset_map_lock on the pmd, to skip the pmd in case a race triggered and the pmd may have become none (or trans huge again). The pmd_trans_unstable check is a noop for builds with THP disabled. It can transition to none to transhuge freely under any code with mmap_sem in read mode. It stops changing only if it becomes a regular pmd pointing to a pte (that's because free_pgtables is only run with mmap_sem in write mode). Only if it is a regular pmd we can start the pte walk and take the PT lock. > > Only if pmd_trans_unstable is false, the pmd can't change from under > > us, so we can proceed safely with the pte level walk (and it just need > > to be checked with a compiler barrier, as the real pmd changes freely > > from under us). > > > > pmd_trans_unstable will never actually trigger unless we're hitting > > the race, if the pmd was none when we started the walk we'd abort at > > the higher level (method not called), if the pmd was transhuge we'd > > stop at the pmd_trans_huge_lock() == 1 branch. So the only way to run > > pmd_trans_unstable is when the result is undefined, i.e. the pmd was > > not none initially but it become none or transhuge or none again at > > some point, so we can just simply consider it still none and skip for > > the undefined case. > > Naoya, could you please take a look into this? That would help thanks. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>