On Thu, 25 Oct 2012, Ni zhan Chen wrote: > On 10/25/2012 02:59 PM, Hugh Dickins wrote: > > On Thu, 25 Oct 2012, Ni zhan Chen wrote: > > > > > > I think it maybe caused by your commit [d189922862e03ce: shmem: fix > > > negative > > > rss in memcg memory.stat], one question: > > Well, yes, I added the VM_BUG_ON in that commit. > > > > > if function shmem_confirm_swap confirm the entry has already brought back > > > from swap by a racing thread, > > The reverse: true confirms that the swap entry has not been brought back > > from swap by a racing thread; false indicates that there has been a race. > > > > > then why call shmem_add_to_page_cache to add > > > page from swapcache to pagecache again? > > Adding it to pagecache again, after such a race, would set error to > > -EEXIST (originating from radix_tree_insert); but we don't do that, > > we add it to pagecache when it has not already been added. > > > > Or that's the intention: but Dave seems to have found an unexpected > > exception, despite us holding the page lock across all this. > > > > (But if it weren't for the memcg and replace_page issues, I'd much > > prefer to let shmem_add_to_page_cache discover the race as before.) > > > > Hugh > > Hi Hugh > > Thanks for your response. You mean the -EEXIST originating from > radix_tree_insert, in radix_tree_insert: > if (slot != NULL) > return -EEXIST; > But why slot should be NULL? if no race, the pagecache related radix tree > entry should be RADIX_TREE_EXCEPTIONAL_ENTRY+swap_entry_t.val, where I miss? I was describing what would happen in a case that should not exist, that you had thought the common case. In actuality, the entry should not be NULL, it should be as you say there. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>