Messages in this thread | | | Date | Thu, 8 Mar 2012 18:33:14 -0800 (PST) | From | Hugh Dickins <> | Subject | Re: [PATCH v3 2/2] memcg: avoid THP split in task migration |
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On Fri, 9 Mar 2012, KAMEZAWA Hiroyuki wrote: > > + > > + page = pmd_page(pmd); > > + VM_BUG_ON(!page || !PageHead(page)); > > + if (!move_anon() || page_mapcount(page) != 1) > > + return 0; > > Could you add this ? > == > static bool move_check_shared_map(struct page *page) > { > /* > * Handling of shared pages between processes is a big trouble in memcg. > * Now, we never move shared-mapped pages between memcg at 'task' moving because > * we have no hint which task the page is really belongs to. For example, > * When a task does "fork()-> move to the child other group -> exec()", the charges > * should be stay in the original cgroup. > * So, check mapcount to determine we can move or not. > */ > return page_mapcount(page) != 1; > }
That's a helpful elucidation, thank you. However...
That is not how it has actually been behaving for the last 18 months (because of the "> 2" bug), so in practice you are asking for a change in behaviour there.
And it's not how it has been and continues to behave with file pages.
Isn't getting that behaviour in fork-move-exec just a good reason not to set move_charge_at_immigrate?
I think there are other scenarios where you do want all the pages to move if move_charge_at_immigrate: and that's certainly easier to describe and to understand and to code.
But if you do insist on not moving the shared, then it needs to involve something like mem_cgroup_count_swap_user() on PageSwapCache pages, rather than just the bare page_mapcount().
I'd rather delete than add code here!
Hugh
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