Re: [PATCH v4 4/9] mm: implement sticky VMA flags

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 11/18/25 11:17, 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.
> 
> Also update the comments for vma_modify_flags() to directly reference
> sticky flags now we have established the concept.
> 
> We also update the VMA userland tests to account for the changes.
> 
> 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>

-- 
Cheers

David