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    SubjectRe: [PATCH 6/10] mm/memcg: take care over pc->mem_cgroup
    On Tue, 21 Feb 2012, Konstantin Khlebnikov wrote:
    > >
    > > Be particularly careful in compaction's isolate_migratepages() and
    > > vmscan's lumpy handling in isolate_lru_pages(): those approach the
    > > page by its physical location, and so can encounter pages which
    > > would not be found by any logical lookup. For those cases we have
    > > to change __isolate_lru_page() slightly: it must leave ClearPageLRU
    > > to the caller, because compaction and lumpy cannot safely interfere
    > > with a page until they have first isolated it and then locked lruvec.
    >
    > Yeah, this is most complicated part.

    Yes, I found myself leaving this patch until last when commenting.

    And was not entirely convinced by what I then said of move_account().
    Indeed, I wondered if it might be improved and simplified by taking
    lruvec locks itself, in the manner that commit_charge lrucare ends
    up doing.

    > I found one race here, see below.

    Thank you!

    > > @@ -386,10 +364,24 @@ static isolate_migrate_t isolate_migrate
    > > continue;
    > >
    > > page_relock_lruvec(page,&lruvec);
    >
    > Here race with mem_cgroup_move_account() we hold lock for old lruvec,
    > while move_account() recharge page and put page back into other lruvec.
    > Thus we see PageLRU(), but below we isolate page from wrong lruvec.

    I expect you'll prove right on that, but I'm going to think about it
    more later.

    >
    > In my patch-set this is fixed with __wait_lru_unlock() [ spin_unlock_wait() ]
    > in mem_cgroup_move_account()

    Right now, after finishing mail, I want to concentrate on getting your
    series working under my swapping load.

    It's possible that I screwed up rediffing it to my slightly later
    base (though the only parts that appeared to need fixing up were as
    expected, near update_isolated_counts and move_active_pages_to_lru);
    but if so I'd expect that to show up differently.

    At present, although it runs fine with cgroup_disable=memory, with memcg
    two machines very soon hit that BUG at include/linux/mm_inline.h:41!
    when the lru_size or pages_count wraps around; on another it hit that
    precisely when I stopped the test.

    In all cases it's in release_pages from free_pages_and_swap_cache from
    tlb_flush_mmu from tlb_finish_mmu from exit_mmap from mmput from exit_mm
    from do_exit (but different processes exiting).

    Hugh


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