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 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 ?
Then, we doesn't need page_cgroup lock, do we ? PCG_USED bit will avoid
double-uncharge.
Thanks,
Daisuke Nishimura.
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