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Patch series "mm/memory: cleanly support zeropage in vm_insert_page*(), vm_map_pages*() and vmf_insert_mixed()", v2. There is interest in mapping zeropages via vm_insert_pages() [1] into MAP_SHARED mappings. For now, we only get zeropages in MAP_SHARED mappings via vmf_insert_mixed() from FSDAX code, and I think it's a bit shaky in some cases because we refcount the zeropage when mapping it but not necessarily always when unmapping it ... and we should actually never refcount it. It's all a bit tricky, especially how zeropages in MAP_SHARED mappings interact with GUP (FOLL_LONGTERM), mprotect(), write-faults and s390x forbidding the shared zeropage (rewrite [2] s now upstream). This series tries to take the careful approach of only allowing the zeropage where it is likely safe to use (which should cover the existing FSDAX use case and [1]), preventing that it could accidentally get mapped writable during a write fault, mprotect() etc, and preventing issues with FOLL_LONGTERM in the future with other users. Tested with a patch from Vincent that uses the zeropage in context of [1]. [1] https://lkml.kernel.org/r/20240430111354.637356-1-vdonnefort@google.com [2] https://lkml.kernel.org/r/20240411161441.910170-1-david@redhat.com This patch (of 3): We'll now also cover the case where insert_page() is called from __vm_insert_mixed(), which sounds like the right thing to do. Link: https://lkml.kernel.org/r/20240522125713.775114-2-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Vincent Donnefort <vdonnefort@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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