On 3 Nov 2017, at 3:52, Huang, Ying wrote: > From: Huang Ying <ying.huang@xxxxxxxxx> > > If THP migration is enabled, the following situation is possible, > > - A THP is mapped at source address > - Migration is started to move the THP to another node > - Page fault occurs > - The PMD (migration entry) is copied to the destination address in mremap > You mean the page fault path follows the source address and sees pmd_none() now because mremap() clears it and remaps the page with dest address. Otherwise, it seems not possible to get into handle_userfault(), since it is called in pmd_none() branch inside do_huge_pmd_anonymous_page(). > That is, it is possible for handle_userfault() encounter a PMD entry > which has been handled but !pmd_present(). In the current > implementation, we will wait for such PMD entries, which may cause > unnecessary waiting, and potential soft lockup. handle_userfault() should only see pmd_none() in the situation you describe, whereas !pmd_present() (migration entry case) should lead to pmd_migration_entry_wait(). Am I missing anything here? -- Best Regards Yan Zi
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