(2014/05/01 5:25), Johannes Weiner wrote: > The memcg uncharging code that is involved towards the end of a page's > lifetime - truncation, reclaim, swapout, migration - is impressively > complicated and fragile. > > Because anonymous and file pages were always charged before they had > their page->mapping established, uncharges had to happen when the page > type could be known from the context, as in unmap for anonymous, page > cache removal for file and shmem pages, and swap cache truncation for > swap pages. However, these operations also happen well before the > page is actually freed, and so a lot of synchronization is necessary: > > - On page migration, the old page might be unmapped but then reused, > so memcg code has to prevent an untimely uncharge in that case. > Because this code - which should be a simple charge transfer - is so > special-cased, it is not reusable for replace_page_cache(). > > - Swap cache truncation happens during both swap-in and swap-out, and > possibly repeatedly before the page is actually freed. This means > that the memcg swapout code is called from many contexts that make > no sense and it has to figure out the direction from page state to > make sure memory and memory+swap are always correctly charged. > > But now that charged pages always have a page->mapping, introduce > mem_cgroup_uncharge(), which is called after the final put_page(), > when we know for sure that nobody is looking at the page anymore. > > For page migration, introduce mem_cgroup_migrate(), which is called > after the migration is successful and the new page is fully rmapped. > Because the old page is no longer uncharged after migration, prevent > double charges by decoupling the page's memcg association (PCG_USED > and pc->mem_cgroup) from the page holding an actual charge. The new > bits PCG_MEM and PCG_MEMSW represent the respective charges and are > transferred to the new page during migration. > > mem_cgroup_migrate() is suitable for replace_page_cache() as well. > > Swap accounting is massively simplified: because the page is no longer > uncharged as early as swap cache deletion, a new mem_cgroup_swapout() > can transfer the page's memory+swap charge (PCG_MEMSW) to the swap > entry before the final put_page() in page reclaim. > > Finally, because pages are now charged under proper serialization > (anon: exclusive; cache: page lock; swapin: page lock; migration: page > lock), and uncharged under full exclusion, they can not race with > themselves. Because they are also off-LRU during charge/uncharge, > charge migration can not race, with that, either. Remove the crazily > expensive the page_cgroup lock and set pc->flags non-atomically. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> The whole series seems wonderful to me. Thank you. I'm not sure whether I have enough good eyes now but this seems good. One thing in my mind is batched uncharge rework. Because uncharge() is done in final put_page() path, mem_cgroup_uncharge_start()/mem_cgroup_uncharge_end() placement may not be good enough. swap.c::release_pages() may be good to have mem_cgroup_uncharge_start()/end(). (and you may be able to remove unnecessary calls of mem_cgroup_uncharge_start/end()) Thanks, -Kame -- 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>