Re: [PATCH] mm/vma: predicate setting mmap_prepare VMA fields on new vma alloc

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 23, 2026 at 06:45:41PM +0100, Lorenzo Stoakes (ARM) wrote:
> It only makes sense to manipulate VMA fields if a new VMA was allocated,
> rather than merged.
> 
> VMA merging does not compare vm_ops or vm_private_data, so a merged VMA
> keeps its own, which is also what the legacy f_op->mmap path does since it
> never touches an existing VMA.
> 
> Currently, these fields will get overwritten by whatever state is
> established in the mmap_prepare hook, and if the VMA was merged,
> vm_ops->mapped will not have been called, so this could destructively clear
> existing state without replacing it with anything valid.
> 
> There is an implicit requirement that vm_private_data and vm_ops are
> fungible across VMAs which means that losing the 'new' state is
> fine.
> 
> However in this case the 'old' state is being overwritten by potentially
> invalid 'new' state, so this must be rectified.
> 
> Additionally constify have_mmap_prepare while here.
> 
> All existing in-tree users either derive state for the tree or are
> unmergeable due to VMA flags, so this has no direct impact.
> 
> Fixes: c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>