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Message-ID: <cf58c518-05d0-494f-8fe4-571879834031@lucifer.local>
Date: Mon, 17 Nov 2025 20:02:03 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
Lance Yang <lance.yang@...ux.dev>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH v3 3/8] mm: implement sticky VMA flags
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@...e.de>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.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;
> remove_next = true;
> /* This should already have been checked by this point. */
> VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
> @@ -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)) &&
> b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
> }
>
> 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
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