Hi Andrew,
Sorry to be a pain here, and can respin if it's easier, but can we update the text of the comments below? As in discussion with Liam off-list we agreed that the current wording is rather unclear and we can do a lot better.
I provide the improved version inline below:
On Fri, Nov 07, 2025 at 04:11:48PM +0000, Lorenzo Stoakes wrote:
It is useful to be able to designate that certain flags are 'sticky', that is, if two VMAs are merged one with a flag of this nature and one without, the merged VMA sets this flag.
As a result we ignore these flags for the purposes of determining VMA flag differences between VMAs being considered for merge.
This patch therefore updates the VMA merge logic to perform this action, with flags possessing this property being described in the VM_STICKY bitmap.
Those flags which ought to be ignored for the purposes of VMA merge are described in the VM_IGNORE_MERGE bitmap, which the VMA merge logic is also updated to use.
As part of this change we place VM_SOFTDIRTY in VM_IGNORE_MERGE as it already had this behaviour, alongside VM_STICKY as sticky flags by implication must not disallow merge.
Ultimately it seems that we should make VM_SOFTDIRTY a sticky flag in its own right, but this change is out of scope for this series.
The only sticky flag designated as such is VM_MAYBE_GUARD, so as a result of this change, once the VMA flag is set upon guard region installation, VMAs with guard ranges will now not have their merge behaviour impacted as a result and can be freely merged with other VMAs without VM_MAYBE_GUARD set.
We also update the VMA userland tests to account for the changes.
Reviewed-by: Pedro Falcato pfalcato@suse.de Reviewed-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com
include/linux/mm.h | 29 +++++++++++++++++++++++++++++ mm/vma.c | 22 ++++++++++++---------- tools/testing/vma/vma_internal.h | 29 +++++++++++++++++++++++++++++ 3 files changed, 70 insertions(+), 10 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 699566c21ff7..6c1c459e9acb 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -527,6 +527,35 @@ extern unsigned int kobjsize(const void *objp); #endif #define VM_FLAGS_CLEAR (ARCH_VM_PKEY_FLAGS | VM_ARCH_CLEAR)
+/*
- Flags which should be 'sticky' on merge - that is, flags which, when one VMA
- possesses it but the other does not, the merged VMA should nonetheless have
- applied to it:
- VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
mapped page tables may contain metadata not described by the
VMA and thus any merged VMA may also contain this metadata,
and thus we must make this flag sticky.- */
+#define VM_STICKY VM_MAYBE_GUARD
+/*
- VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
- of these flags and the other not does not preclude a merge.
- VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
dirty bit -- the caller should mark merged VMA as dirty. If
dirty bit won't be excluded from comparison, we increase
pressure on the memory system forcing the kernel to generate
new VMAs when old one could be extended instead.
Could you replace this:
- VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
which should propagate to all VMAs, but the other does not,
the merge should still proceed with the merge logic applying
sticky flags to the final VMA.
With this:
* VM_STICKY - When merging VMAs, VMA flags must match, unless they are * 'sticky'. If any sticky flags exist in either VMA, we simply * set all of them on the merged VMA.
- */
+#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
/*
- mapping from the currently active vm_flags protection bits (the
- low four bits) to a page protection mask..
diff --git a/mm/vma.c b/mm/vma.c index 0c5e391fe2e2..6cb082bc5e29 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -89,15 +89,7 @@ static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_nex
if (!mpol_equal(vmg->policy, vma_policy(vma))) return false;
- /*
* VM_SOFTDIRTY should not prevent from VMA merging, if we* match the flags but dirty bit -- the caller should mark* merged VMA as dirty. If dirty bit won't be excluded from* comparison, we increase pressure on the memory system forcing* the kernel to generate new VMAs when old one could be* extended instead.*/- if ((vma->vm_flags ^ vmg->vm_flags) & ~VM_SOFTDIRTY)
- if ((vma->vm_flags ^ vmg->vm_flags) & ~VM_IGNORE_MERGE) return false; if (vma->vm_file != vmg->file) return false;
@@ -808,6 +800,7 @@ static bool can_merge_remove_vma(struct vm_area_struct *vma) static __must_check struct vm_area_struct *vma_merge_existing_range( struct vma_merge_struct *vmg) {
- vm_flags_t sticky_flags = vmg->vm_flags & VM_STICKY; struct vm_area_struct *middle = vmg->middle; struct vm_area_struct *prev = vmg->prev; struct vm_area_struct *next;
@@ -900,11 +893,13 @@ static __must_check struct vm_area_struct *vma_merge_existing_range( if (merge_right) { vma_start_write(next); vmg->target = next;
sticky_flags |= (next->vm_flags & VM_STICKY);}
if (merge_left) { vma_start_write(prev); vmg->target = prev;
sticky_flags |= (prev->vm_flags & VM_STICKY);}
if (merge_both) {
@@ -974,6 +969,7 @@ static __must_check struct vm_area_struct *vma_merge_existing_range( if (err || commit_merge(vmg)) goto abort;
- vm_flags_set(vmg->target, sticky_flags); khugepaged_enter_vma(vmg->target, vmg->vm_flags); vmg->state = VMA_MERGE_SUCCESS; return vmg->target;
@@ -1124,6 +1120,10 @@ int vma_expand(struct vma_merge_struct *vmg) bool remove_next = false; struct vm_area_struct *target = vmg->target; struct vm_area_struct *next = vmg->next;
vm_flags_t sticky_flags;
sticky_flags = vmg->vm_flags & VM_STICKY;
sticky_flags |= target->vm_flags & VM_STICKY;
VM_WARN_ON_VMG(!target, vmg);
@@ -1133,6 +1133,7 @@ int vma_expand(struct vma_merge_struct *vmg) if (next && (target != next) && (vmg->end == next->vm_end)) { int ret;
sticky_flags |= next->vm_flags & VM_STICKY;@@ -1159,6 +1160,7 @@ int vma_expand(struct vma_merge_struct *vmg) if (commit_merge(vmg)) goto nomem;
- vm_flags_set(target, sticky_flags); return 0;
nomem: @@ -1902,7 +1904,7 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct * return a->vm_end == b->vm_start && mpol_equal(vma_policy(a), vma_policy(b)) && a->vm_file == b->vm_file &&
!((a->vm_flags ^ b->vm_flags) & ~(VM_ACCESS_FLAGS | VM_SOFTDIRTY)) &&
!((a->vm_flags ^ b->vm_flags) & ~(VM_ACCESS_FLAGS | VM_IGNORE_MERGE)) &&}
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 46acb4df45de..a54990aa3009 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -117,6 +117,35 @@ extern unsigned long dac_mmap_min_addr; #define VM_SEALED VM_NONE #endif
+/*
- Flags which should be 'sticky' on merge - that is, flags which, when one VMA
- possesses it but the other does not, the merged VMA should nonetheless have
- applied to it:
- VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
mapped page tables may contain metadata not described by the
VMA and thus any merged VMA may also contain this metadata,
and thus we must make this flag sticky.- */
+#define VM_STICKY VM_MAYBE_GUARD
+/*
- VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
- of these flags and the other not does not preclude a merge.
- VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
dirty bit -- the caller should mark merged VMA as dirty. If
dirty bit won't be excluded from comparison, we increase
pressure on the memory system forcing the kernel to generate
new VMAs when old one could be extended instead.
Also here, we dupicate this for the VMA unit tests, could we replace:
- VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
which should propagate to all VMAs, but the other does not,
the merge should still proceed with the merge logic applying
sticky flags to the final VMA.
With:
* VM_STICKY - When merging VMAs, VMA flags must match, unless they are * 'sticky'. If any sticky flags exist in either VMA, we simply * set all of them on the merged VMA.
- */
+#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
#define FIRST_USER_ADDRESS 0UL #define USER_PGTABLES_CEILING 0UL
-- 2.51.0
Thanks, Lorenzo