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mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one

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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 <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Lorenzo Stoakes
2025-11-18 10:17:47 +00:00
committed by Andrew Morton
parent 64212ba02e
commit ab04b530e7
3 changed files with 56 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

26
include/linux/mm.h
View File

@@ -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..

18
mm/memory.c
View File

@@ -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

26
tools/testing/vma/vma_internal.h
View File

@@ -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