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When we fork anonymous pages, apply a guard page then remove it, the
previous CoW mapping is cleared.
This might not be obvious to an outside observer without taking some time
to think about how the overall process functions, so document that this is
the case through a test, which also usefully asserts that the behaviour is
as we expect.
This is grouped with other, more important, fork tests that ensure that
guard pages are correctly propagated on fork.
Fix a typo in a nearby comment at the same time.
[ryan.roberts@arm.com: static process_madvise() wrapper for guard-pages]
Link: https://lkml.kernel.org/r/20250107142937.1870478-1-ryan.roberts@arm.com
Link: https://lkml.kernel.org/r/20241205190748.115656-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Utilise the kselftest harmness to implement tests for the guard page
implementation.
We start by implement basic tests asserting that guard pages can be
installed, removed and that touching guard pages result in SIGSEGV. We
also assert that, in removing guard pages from a range, non-guard pages
remain intact.
We then examine different operations on regions containing guard markers
behave to ensure correct behaviour:
* Operations over multiple VMAs operate as expected.
* Invoking MADV_GUARD_INSTALL / MADV_GUARD_REMOVE via process_madvise() in
batches works correctly.
* Ensuring that munmap() correctly tears down guard markers.
* Using mprotect() to adjust protection bits does not in any way override
or cause issues with guard markers.
* Ensuring that splitting and merging VMAs around guard markers causes no
issue - i.e. that a marker which 'belongs' to one VMA can function just
as well 'belonging' to another.
* Ensuring that madvise(..., MADV_DONTNEED) and madvise(..., MADV_FREE)
do not remove guard markers.
* Ensuring that mlock()'ing a range containing guard markers does not
cause issues.
* Ensuring that mremap() can move a guard range and retain guard markers.
* Ensuring that mremap() can expand a guard range and retain guard
markers (perhaps moving the range).
* Ensuring that mremap() can shrink a guard range and retain guard markers.
* Ensuring that forking a process correctly retains guard markers.
* Ensuring that forking a VMA with VM_WIPEONFORK set behaves sanely.
* Ensuring that lazyfree simply clears guard markers.
* Ensuring that userfaultfd can co-exist with guard pages.
* Ensuring that madvise(..., MADV_POPULATE_READ) and
madvise(..., MADV_POPULATE_WRITE) error out when encountering
guard markers.
* Ensuring that madvise(..., MADV_COLD) and madvise(..., MADV_PAGEOUT) do
not remove guard markers.
If any test is unable to be run due to lack of permissions, that test is
skipped.
Link: https://lkml.kernel.org/r/c3dcca76b736bac0aeaf1dc085927536a253ac94.1730123433.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: Jann Horn <jannh@google.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Helge Deller <deller@gmx.de>
Cc: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Vlastimil Babka <vbabkba@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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