On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@xxxxxxxxxxxxxx> wrote: > 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(). Yes. This is my understanding of the source code too. And I described it in the original patch description too. I just want to make sure whether it is possible that !pmd_none() and !pmd_present() for a PMD in userfaultfd_must_wait(). And, whether it is possible for us to implement PMD mapping copying in UFFDIO_COPY in the future? Best Regards, Huang, Ying > Am I missing anything here? > > > -- > Best Regards > Yan Zi -- 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>
