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?
Only to show what has changed since this was last proposed, but yes, that change to fs/userfaultfd.c should be squashed with patch 3. The purpose of patch 4 will only be documenting UFFDIO_REMAP.
I will make those changes for the next revision. Thanks for looking this over. 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
-- Sincerely yours,Mike.
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