On Fri, 23 Apr 2010 17:08:46 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> I'm sorry for my late reply.
>
> On Tue, 20 Apr 2010 18:19:25 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > On Tue, 20 Apr 2010 13:20:50 +0900
> > Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> >
> > > > It will have no meanings for migrating
> > > > file caches, but it may have some meanings for easy debugging.
> > > > I think "mark it always but it's used only for anonymous page" is reasonable
> > > > (if it causes no bug.)
> > > >
> > > Anyway, I don't have any strong objection.
> > > It's all right for me as long as it is well documented or commented.
> > >
> > Okay, before posting as v4, here is draft version.
> >
> Thank you for adding good comments about what it does and why we need it.
> I like the direction that we set MIGRATION flags only on the old page.
> And this patch looks good to me, except that checkpatch warns some problems
> about indent :)
>
(--;
I'm sorry that this patch is delayed. I have to fix migration itself
for testing this. I'd like to post this before long holidayes in the next week.
> I have one question.
>
> > /* remove redundant charge if migration failed*/
> > void mem_cgroup_end_migration(struct mem_cgroup *mem,
> > - struct page *oldpage, struct page *newpage)
> > + struct page *oldpage, struct page *newpage)
> > {
> > - struct page *target, *unused;
> > + struct page *used, *unused;
> > struct page_cgroup *pc;
> > - enum charge_type ctype;
> >
> > if (!mem)
> > return;
> > + /* blocks rmdir() */
> > cgroup_exclude_rmdir(&mem->css);
> > /* at migration success, oldpage->mapping is NULL. */
> > if (oldpage->mapping) {
> > - target = oldpage;
> > - unused = NULL;
> > + used = oldpage;
> > + unused = newpage;
> > } else {
> > - target = newpage;
> > + used = newpage;
> > unused = oldpage;
> > }
> > -
> > - if (PageAnon(target))
> > - ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
> > - else if (page_is_file_cache(target))
> > - ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> > - else
> > - ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> > -
> > - /* unused page is not on radix-tree now. */
> > - if (unused)
> > - __mem_cgroup_uncharge_common(unused, ctype);
> > -
> > - pc = lookup_page_cgroup(target);
> > /*
> > - * __mem_cgroup_commit_charge() check PCG_USED bit of page_cgroup.
> > - * So, double-counting is effectively avoided.
> > + * We disallowed uncharge of pages under migration because mapcount
> > + * of the page goes down to zero, temporarly.
> > + * Clear the flag and check the page should be charged.
> > */
> > - __mem_cgroup_commit_charge(mem, pc, ctype);
> > -
> > + pc = lookup_page_cgroup(unused);
> > + /* This flag itself is not racy, so, check it before lock */
> > + if (PageCgroupMigration(pc)) {
> > + lock_page_cgroup(pc);
> > + ClearPageCgroupMigration(pc);
> > + unlock_page_cgroup(pc);
> > + }
> The reason why "This flag itself is not racy" is that we update the flag only
> while the page is isolated ?
yes and no.
It's not racy because a page is only under a migration thread, not under a few of
migration threads. And only the migration thread mark this MIGRATION.
> Then, we doesn't need page_cgroup lock, do we ? PCG_USED bit will avoid
> double-uncharge.
>
no. there is a chance to update FILE_MAPPED etc..and any other races. I guess.
Thanks,
-Kame
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