On Jan 20, 2019, at 4:07 PM, Mike Rapoport <rppt@linux.ibm.com> wrote:

Hi,

On Sat, Jan 12, 2019 at 12:36:29AM +0000, Blake Caldwell wrote:

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.

It took me a while but better late then never :)

Why did you keep this as a separate patch? If the primary use case for
UFFDIO_REMAP to move pages out of userfaultfd region, why not make it so
from the beginning?

Signed-off-by: Blake Caldwell <blake.caldwell@colorado.edu>
---
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

-- 
Sincerely yours,
Mike.