From ab04b530e7e8bd5cf9fb0c1ad20e0deee8f569ec Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Tue, 18 Nov 2025 10:17:47 +0000 Subject: mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one Gather all the VMA flags whose presence implies that page tables must be copied on fork into a single bitmap - VM_COPY_ON_FORK - and use this rather than specifying individual flags in vma_needs_copy(). We also add VM_MAYBE_GUARD to this list, as it being set on a VMA implies that there may be metadata contained in the page tables (that is - guard markers) which would will not and cannot be propagated upon fork. This was already being done manually previously in vma_needs_copy(), but this makes it very explicit, alongside VM_PFNMAP, VM_MIXEDMAP and VM_UFFD_WP all of which imply the same. Note that VM_STICKY flags ought generally to be marked VM_COPY_ON_FORK too - because equally a flag being VM_STICKY indicates that the VMA contains metadat that is not propagated by being faulted in - i.e. that the VMA metadata does not fully describe the VMA alone, and thus we must propagate whatever metadata there is on a fork. However, for maximum flexibility, we do not make this necessarily the case here. Link: https://lkml.kernel.org/r/5d41b24e7bc622cda0af92b6d558d7f4c0d1bc8c.1763460113.git.lorenzo.stoakes@oracle.com Signed-off-by: Lorenzo Stoakes Reviewed-by: Pedro Falcato Reviewed-by: Vlastimil Babka Acked-by: David Hildenbrand (Red Hat) Cc: Andrei Vagin Cc: Baolin Wang Cc: Barry Song Cc: Dev Jain Cc: Jann Horn Cc: Jonathan Corbet Cc: Lance Yang Cc: Liam Howlett Cc: "Masami Hiramatsu (Google)" Cc: Mathieu Desnoyers Cc: Michal Hocko Cc: Mike Rapoport Cc: Nico Pache Cc: Ryan Roberts Cc: Steven Rostedt Cc: Suren Baghdasaryan Cc: Zi Yan Signed-off-by: Andrew Morton --- include/linux/mm.h | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) (limited to 'include/linux') diff --git a/include/linux/mm.h b/include/linux/mm.h index fea113d1d723..af2904aeb163 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -555,6 +555,32 @@ extern unsigned int kobjsize(const void *objp); */ #define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY) +/* + * Flags which should result in page tables being copied on fork. These are + * flags which indicate that the VMA maps page tables which cannot be + * reconsistuted upon page fault, so necessitate page table copying upon + * + * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be + * reasonably reconstructed on page fault. + * + * VM_UFFD_WP - Encodes metadata about an installed uffd + * write protect handler, which cannot be + * reconstructed on page fault. + * + * We always copy pgtables when dst_vma has uffd-wp + * enabled even if it's file-backed + * (e.g. shmem). Because when uffd-wp is enabled, + * pgtable contains uffd-wp protection information, + * that's something we can't retrieve from page cache, + * and skip copying will lose those info. + * + * VM_MAYBE_GUARD - Could contain page guard region markers which + * by design are a property of the page tables + * only and thus cannot be reconstructed on page + * fault. + */ +#define VM_COPY_ON_FORK (VM_PFNMAP | VM_MIXEDMAP | VM_UFFD_WP | VM_MAYBE_GUARD) + /* * mapping from the currently active vm_flags protection bits (the * low four bits) to a page protection mask.. -- cgit v1.2.3