memcg: fix mis-accounting of file mapped racy with migration
FILE_MAPPED per memcg of migrated file cache is not properly updated, because our hook in page_add_file_rmap() can't know to which memcg FILE_MAPPED should be counted. Basically, this patch is for fixing the bug but includes some big changes to fix up other messes. Now, at migrating mapped file, events happen in following sequence. 1. allocate a new page. 2. get memcg of an old page. 3. charge ageinst a new page before migration. But at this point, no changes to new page's page_cgroup, no commit for the charge. (IOW, PCG_USED bit is not set.) 4. page migration replaces radix-tree, old-page and new-page. 5. page migration remaps the new page if the old page was mapped. 6. Here, the new page is unlocked. 7. memcg commits the charge for newpage, Mark the new page's page_cgroup as PCG_USED. Because "commit" happens after page-remap, we can count FILE_MAPPED at "5", because we should avoid to trust page_cgroup->mem_cgroup. if P...
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315c1998
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