Thank you for explaining in detail. On Mon, 19 Apr 2010 13:18:17 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > On Mon, 19 Apr 2010 12:42:25 +0900 > Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote: > > > Hmm, before going further, will you explain why we need a new PCG_MIGRATION flag ? > > What's the problem of v2 ? > > > > v2 can't handle migration-failure case of freed swapcache and the used page > was swapped-out case. I think. > > All "page" in following is ANON. > > > mem_cgroup_prepare_migration() > charge against new page. > > try_to_unmap() > -> mapcount goes down to 0. > -> an old page is unchaged > But old page isn't uncharged iff PageSwapCache, is it ? > move_to_new_page() > -> may fail. (in some case.) ----(*1) > > remap the old page to pte. > > mem_cgroup_end_migration() > (at success *1) > check charge for newpage is valid or not (*2) > > (at fail *1) > uncharge new page. > What we should do for an old page. ---(*3) > > At (*2). (*3), there are several cases. > > (*2) migration was succeeded. > 1. The new page was successfully remapped. > -> Nothing to do. > 2. The new page was remapped but finally unmapped before (*3) > -> page_remove_rmap() will catch the event. > 3. The new page was not remapped. > -> page_remove_rmap() can't catch the event. end_migraion() has to > uncharge it. > > (*3) migration was failed. > 1. The old page was successfully remapped. > -> We have to recharge against the old page. (But it may hit OOM.) > 2. The old page wasn't remapped. > -> mapcount is 0. No new charge will happen. > 3. The old page wasn't remapped but SwapCache. > -> mapcount is 0. We have to recharge against the old page (But it may hit OOM) > hmm, we've done try_charge at this point, so why can we cause oom here ? > Maybe other seqence I couldn't write will exist......IMHO, "we have to recharge it because > it's uncharged.." is bad idea. It seems hard to maintainace.. > > > When we use MIGRATION flag. > After migaration. > > 1. Agaisnt new page, we remove MIGRATION flag and try to uncharge() it again. > > 2. Agaisnt old page, we remove MIGRATION flag and try to uncharge it again. > > NOTE: I noticed my v3 patch is buggy when the page-is-swapped-out case. It seems > mem_cgroup_uncharge_swapcache() has to wait for migration ends or some > other case handling. (Anyway, this race exists only after unlock_page(newpage). > So, wait for MIGRATION ends in spin will not be very bad.) > > > To me, things are much simpler than now, we have to know what kind of magics behind us... > > Maybe I can think of other tricks for handling them...but using a FLAG and prevent uncharge > is the simplest, I think. > Anyway, I agree that current implementation is complicated and there might be some cases we are missing. MIGRATION flag can make it simpler. I have one concern for now. Reading the patch, the flag have influence on only anonymous pages, so we'd better to note it and I feel it strange to set(and clear) the flag of "old page" always(iow, even when !PageAnon) in prepare_migration. Thanks, Daisuke Nishimura. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>