[4/4] userfaultfd: change the direction for UFFDIO_REMAP to out
diff mbox series

Message ID ab1b6be85254e111935104cf4a2293ab2fa4a8d6.1547251023.git.blake.caldwell@colorado.edu
State New
Headers show
Series
  • RFC: userfaultfd remap
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Commit Message

Blake Caldwell Jan. 12, 2019, 12:36 a.m. UTC
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@colorado.edu>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 10 ++++++++++
 fs/userfaultfd.c                             |  6 +++---
 2 files changed, 13 insertions(+), 3 deletions(-)

Comments

Mike Rapoport Jan. 20, 2019, 9:07 p.m. UTC | #1
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
>

Patch
diff mbox series

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)))