From: Lorenzo Stoakes Date: Tue, 18 Nov 2025 10:17:47 +0000 (+0000) Subject: mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one X-Git-Tag: v6.19-rc1~112^2~136 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ab04b530e7e8bd5cf9fb0c1ad20e0deee8f569ec;p=thirdparty%2Flinux.git 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 --- diff --git a/include/linux/mm.h b/include/linux/mm.h index fea113d1d723c..af2904aeb1631 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.. diff --git a/mm/memory.c b/mm/memory.c index d1728d0538d64..27bc457b32c2e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1463,25 +1463,15 @@ copy_p4d_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, static bool vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) { + if (src_vma->vm_flags & VM_COPY_ON_FORK) + return true; /* - * 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. + * The presence of an anon_vma indicates an anonymous VMA has page + * tables which naturally cannot be reconstituted on page fault. */ - if (userfaultfd_wp(dst_vma)) - return true; - - if (src_vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)) - return true; - if (src_vma->anon_vma) return true; - /* Guard regions have modified page tables that require copying. */ - if (src_vma->vm_flags & VM_MAYBE_GUARD) - return true; - /* * Don't copy ptes where a page fault will fill them correctly. Fork * becomes much lighter when there are big shared or private readonly diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 73c2025777e6f..233819a9e7ee5 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -145,6 +145,32 @@ extern unsigned long dac_mmap_min_addr; */ #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) + #define FIRST_USER_ADDRESS 0UL #define USER_PGTABLES_CEILING 0UL