On Wed, Sep 22, 2021 at 01:51:56PM -0400, Peter Xu wrote: > Axel: as I asked in the other thread, please help check whether minor mode will > work properly with shmem thp enabled. If not, I feel like this patch could be > part of that effort at last, but it's also possible that I missed something. Hmm, this seems to be a false-alarm too. UFFDIO_CONTINUE is fine with thp because shmem_getpage() only returns small pages. Khugepaged is fine too on merging small shmem pages into thps, the same reason as uffd-wp: it zaps ptes only, so previous pte_none() ptes will keep the same, it makes sure all old pte_none ptes will not continue until a UFFDIO_CONTINUE. The only last problem is when khugepaged merged small ptes into a thp, then it could zap ptes even if they existed before. So for minor mode fault, the uffd service thread needs to be prepared for false positive messages. That shouldn't be a problem either, because file-backed memory pgtables are unstable and prone to lost, so uffd minor fault handler should always be prepared for false positives anyway.. In summary: please feel free to ignore the above note, and sorry for the noise. -- Peter Xu
