Messages in this thread | | | Date | Tue, 27 Apr 2021 17:03:17 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCH v5 04/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem |
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On Tue, 27 Apr 2021, Axel Rasmussen wrote:
> With this change, userspace can resolve a minor fault within a > shmem-backed area with a UFFDIO_CONTINUE ioctl. The semantics for this > match those for hugetlbfs - we look up the existing page in the page > cache, and install a PTE for it. > > This commit introduces a new helper: mcopy_atomic_install_pte. > > Why handle UFFDIO_CONTINUE for shmem in mm/userfaultfd.c, instead of in > shmem.c? The existing userfault implementation only relies on shmem.c > for VM_SHARED VMAs. However, minor fault handling / CONTINUE work just > fine for !VM_SHARED VMAs as well. We'd prefer to handle CONTINUE for > shmem in one place, regardless of shared/private (to reduce code > duplication). > > Why add a new mcopy_atomic_install_pte helper? A problem we have with > continue is that shmem_mcopy_atomic_pte() and mcopy_atomic_pte() are > *close* to what we want, but not exactly. We do want to setup the PTEs > in a CONTINUE operation, but we don't want to e.g. allocate a new page, > charge it (e.g. to the shmem inode), manipulate various flags, etc. Also > we have the problem stated above: shmem_mcopy_atomic_pte() and > mcopy_atomic_pte() both handle one-half of the problem (shared / > private) continue cares about. So, introduce mcontinue_atomic_pte(), to > handle all of the shmem continue cases. Introduce the helper so it > doesn't duplicate code with mcopy_atomic_pte(). > > In a future commit, shmem_mcopy_atomic_pte() will also be modified to > use this new helper. However, since this is a bigger refactor, it seems > most clear to do it as a separate change. > > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
> --- > mm/userfaultfd.c | 172 ++++++++++++++++++++++++++++++++++------------- > 1 file changed, 127 insertions(+), 45 deletions(-)
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