On Mon, Mar 22, 2021 at 08:48:49PM -0400, Peter Xu wrote: > This patchset is based on tag v5.12-rc3-mmots-2021-03-17-22-26. To run the > selftest, need to apply the two patches to fix minor mode page leak: > > https://lore.kernel.org/lkml/20210322175132.36659-1-peterx@xxxxxxxxxx/ > https://lore.kernel.org/lkml/20210322204836.1650221-1-axelrasmussen@xxxxxxxxxx/ > > Since I didn't get any NACK in the previous RFC series for months, I decided to > remove the RFC tag starting from this version, so this is v1 of uffd-wp support > on shmem & hugetlb. > > The whole series can also be found online [1]. > > The major comment I'd like to get is on the new idea of swap special pte. That > comes from suggestions from both Hugh and Andrea and I appreciated a lot for > those discussions. > > In short, the so-called "swap special pte" in this patchset is a new type of > pte that doesn't exist in the past, but it got used initially in this series in > file-backed memories. It is used to persist information even if the ptes got > dropped meanwhile when the page cache still existed. For example, when > splitting a file-backed huge pmd, we could be simply dropping the pmd entry > then wait until another fault coming. It's okay in the past since all > information in the pte can be retained from the page cache when the next page > fault triggers. However in this case, uffd-wp is per-pte information which > cannot be kept in page cache, so that information needs to be maintained > somehow still in the pgtable entry, even if the pgtable entry is going to be > dropped. Here instead of replacing with a none entry, we used the "swap > special pte". Then when the next page fault triggers, we can observe orig_pte > to retain this information. > > I'm copy-pasting some commit message from the patch "mm/swap: Introduce the > idea of special swap ptes", where it tried to explain this pte in another angle: > > We used to have special swap entries, like migration entries, hw-poison > entries, device private entries, etc. > > Those "special swap entries" reside in the range that they need to be at least > swap entries first, and their types are decided by swp_type(entry). > > This patch introduces another idea called "special swap ptes". > > It's very easy to get confused against "special swap entries", but a speical > swap pte should never contain a swap entry at all. It means, it's illegal to > call pte_to_swp_entry() upon a special swap pte. > > Make the uffd-wp special pte to be the first special swap pte. > > Before this patch, is_swap_pte()==true means one of the below: > > (a.1) The pte has a normal swap entry (non_swap_entry()==false). For > example, when an anonymous page got swapped out. > > (a.2) The pte has a special swap entry (non_swap_entry()==true). For > example, a migration entry, a hw-poison entry, etc. > > After this patch, is_swap_pte()==true means one of the below, where case (b) is > added: > > (a) The pte contains a swap entry. > > (a.1) The pte has a normal swap entry (non_swap_entry()==false). For > example, when an anonymous page got swapped out. > > (a.2) The pte has a special swap entry (non_swap_entry()==true). For > example, a migration entry, a hw-poison entry, etc. > > (b) The pte does not contain a swap entry at all (so it cannot be passed > into pte_to_swp_entry()). For example, uffd-wp special swap pte. > > Hugetlbfs needs similar thing because it's also file-backed. I directly reused > the same special pte there, though the shmem/hugetlb change on supporting this > new pte is different since they don't share code path a lot. > > Patch layout > ============ > > Part (1): Shmem support, this is where the special swap pte is introduced. > Some zap rework is needed within the process: > > shmem/userfaultfd: Take care of UFFDIO_COPY_MODE_WP > mm: Clear vmf->pte after pte_unmap_same() returns > mm/userfaultfd: Introduce special pte for unmapped file-backed mem > mm/swap: Introduce the idea of special swap ptes > shmem/userfaultfd: Handle uffd-wp special pte in page fault handler > mm: Drop first_index/last_index in zap_details > mm: Introduce zap_details.zap_flags > mm: Introduce ZAP_FLAG_SKIP_SWAP > mm: Pass zap_flags into unmap_mapping_pages() > shmem/userfaultfd: Persist uffd-wp bit across zapping for file-backed > shmem/userfaultfd: Allow wr-protect none pte for file-backed mem > shmem/userfaultfd: Allows file-back mem to be uffd wr-protected on thps > shmem/userfaultfd: Handle the left-overed special swap ptes > shmem/userfaultfd: Pass over uffd-wp special swap pte when fork() > > Part (2): Hugetlb support, we need to disable huge pmd sharing for uffd-wp > because not compatible just like uffd minor mode. The rest is the changes > required to teach hugetlbfs understand the special swap pte too that introduced > with the uffd-wp change: > > hugetlb/userfaultfd: Hook page faults for uffd write protection > hugetlb/userfaultfd: Take care of UFFDIO_COPY_MODE_WP > hugetlb/userfaultfd: Handle UFFDIO_WRITEPROTECT > hugetlb: Pass vma into huge_pte_alloc() > hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled > mm/hugetlb: Introduce huge version of special swap pte helpers > mm/hugetlb: Move flush_hugetlb_tlb_range() into hugetlb.h > hugetlb/userfaultfd: Unshare all pmds for hugetlbfs when register wp > hugetlb/userfaultfd: Handle uffd-wp special pte in hugetlb pf handler > hugetlb/userfaultfd: Allow wr-protect none ptes > hugetlb/userfaultfd: Only drop uffd-wp special pte if required > > Part (3): Enable both features in code and test > > userfaultfd: Enable write protection for shmem & hugetlbfs > userfaultfd/selftests: Enable uffd-wp for shmem/hugetlbfs > > Tests > ========= > > I've tested it using either userfaultfd kselftest program, but also with > umapsort [2] which should be even stricter. Tested page swapping in/out during > umapsort. > > If anyone would like to try umapsort, need to use an extremely hacked version > of umap library [3], because by default umap only supports anonymous. So to > test it we need to build [3] then [2]. > > Any comment would be greatly welcomed. Thanks, > > [1] https://github.com/xzpeter/linux/tree/uffd-wp-shmem-hugetlbfs > [2] https://github.com/LLNL/umap-apps > [3] https://github.com/xzpeter/umap/tree/peter-shmem-hugetlbfs Hugh, Mike, Andrew, Any comment for this series? Thanks, -- Peter Xu
