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mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to install WP PTEs

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UFFDIO_COPY already has UFFDIO_COPY_MODE_WP, so when installing a new PTE
to resolve a missing fault, one can install a write-protected one.  This
is useful when using UFFDIO_REGISTER_MODE_{MISSING,WP} in combination.

This was motivated by testing HugeTLB HGM [1], and in particular its
interaction with userfaultfd features.  Existing userfaultfd code supports
using WP and MINOR modes together (i.e.  you can register an area with
both enabled), but without this CONTINUE flag the combination is in
practice unusable.

So, add an analogous UFFDIO_CONTINUE_MODE_WP, which does the same thing as
UFFDIO_COPY_MODE_WP, but for *minor* faults.

Update the selftest to do some very basic exercising of the new flag.

Update Documentation/ to describe how these flags are used (neither the
COPY nor the new CONTINUE versions of this mode flag were described there
before).

[1]: https://patchwork.kernel.org/project/linux-mm/cover/20230218002819.1486479-1-jthoughton@google.com/

Link: https://lkml.kernel.org/r/20230314221250.682452-5-axelrasmussen@google.com
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nadav Amit <namit@vmware.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Axel Rasmussen
2023-03-14 15:12:50 -07:00
committed by Andrew Morton
parent d971293703
commit 0289184476
6 changed files with 30 additions and 5 deletions
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7
include/uapi/linux/userfaultfd.h
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@@ -305,6 +305,13 @@ struct uffdio_writeprotect {
struct uffdio_continue {
struct uffdio_range range;
#define UFFDIO_CONTINUE_MODE_DONTWAKE ((__u64)1<<0)
/*
* UFFDIO_CONTINUE_MODE_WP will map the page write protected on
* the fly. UFFDIO_CONTINUE_MODE_WP is available only if the
* write protected ioctl is implemented for the range
* according to the uffdio_register.ioctls.
*/
#define UFFDIO_CONTINUE_MODE_WP ((__u64)1<<1)
__u64 mode;
/*