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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>