Moving a page out of a userfaultfd registered region and into a userland anonymous vma is needed by the use case of uncooperatively limiting the resident size of the userfaultfd region. Reverse the direction of the original userfaultfd_remap() to the out direction. Now after memory has been removed, subsequent accesses will generate uffdio page fault events. Signed-off-by: Blake Caldwell <blake.caldwell@xxxxxxxxxxxx> --- Documentation/admin-guide/mm/userfaultfd.rst | 10 ++++++++++ fs/userfaultfd.c | 6 +++--- 2 files changed, 13 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index 5048cf6..714af49 100644 --- a/Documentation/admin-guide/mm/userfaultfd.rst +++ b/Documentation/admin-guide/mm/userfaultfd.rst @@ -108,6 +108,16 @@ UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an half copied page since it'll keep userfaulting until the copy has finished. +To move pages out of a userfault registered region and into a user vma +the UFFDIO_REMAP ioctl can be used. This is only possible for the +"OUT" direction. For the "IN" direction, UFFDIO_COPY is preferred +since UFFDIO_REMAP requires a TLB flush on the source range at a +greater penalty than copying the page. With +UFFDIO_REGISTER_MODE_MISSING set, subsequent accesses to the same +region will generate a page fault event. This allows non-cooperative +removal of memory in a userfaultfd registered vma, effectively +limiting the amount of resident memory in such a region. + QEMU/KVM ======== diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index cf68cdb..8099da2 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1808,10 +1808,10 @@ static int userfaultfd_remap(struct userfaultfd_ctx *ctx, sizeof(uffdio_remap)-sizeof(__s64))) goto out; - ret = validate_range(ctx->mm, uffdio_remap.dst, uffdio_remap.len); + ret = validate_range(current->mm, uffdio_remap.dst, uffdio_remap.len); if (ret) goto out; - ret = validate_range(current->mm, uffdio_remap.src, uffdio_remap.len); + ret = validate_range(ctx->mm, uffdio_remap.src, uffdio_remap.len); if (ret) goto out; ret = -EINVAL; @@ -1819,7 +1819,7 @@ static int userfaultfd_remap(struct userfaultfd_ctx *ctx, UFFDIO_REMAP_MODE_DONTWAKE)) goto out; - ret = remap_pages(ctx->mm, current->mm, + ret = remap_pages(current->mm, ctx->mm, uffdio_remap.dst, uffdio_remap.src, uffdio_remap.len, uffdio_remap.mode); if (unlikely(put_user(ret, &user_uffdio_remap->remap))) -- 1.8.3.1
