Re: [PATCH v2 29/33] mm/vma: introduce and use vma_set_pgoff()

On Fri, Jul 10, 2026 at 09:17:10PM +0100, Lorenzo Stoakes wrote:
> In order to lay the foundation for work that permits us to track the
> virtual page offset of MAP_PRIVATE file-backed mappings, we abstract the
> assignment of vma->vm_pgoff to vma_set_pgoff().
> 
> We additionally add a lock check here using the newly introduced
> vma_assert_can_modify(). This asserts the VMA write lock if the VMA is
> attached.
> 
> We also assert that, if this is an anonymous VMA and unfaulted, that its
> (virtual) page offset is equal to the page offset of the VMA's address.
> 
> We must be careful about MAP_PRIVATE-/dev/zero which violates fundamental
> assumptions about anonymous memory, so we check for !vma->vm_file after
> using vma_is_anonymous() which these mappings satisfy.
> 
> Additionally, we only perform the assert if CONFIG_MMU is defined, as nommu
> does not set vma->vm_pgoff = addr >> PAGE_SHIFT. This isn't really relevant
> to rmap as it has no anon rmap (nor needs it), but we must avoid it
> asserting falsely.
> 
> All of this logic is kept in assert_sane_pgoff() to keep things clear.
> 
> In order to maintain correctness given this assert, we also update
> __install_special_mapping() to invoke vma_set_range() after it's set
> vma->vm_ops (which determine whether the VMA is anonymous or not).
> 
> We do not use vma_set_pgoff() in vm_area_init_from(), as at the point of
> forking, we don't necessarily have correct locking state.
> 
> Updating vma_set_range() covers most cases, but in addition to this we also
> update insert_vm_struct(), compat_set_vma_from_desc() and nommu callers.
> 
> We also update vma_add_pgoff() and vma_sub_pgoff() to use vma_set_pgoff().
> 
> While we're here, we drop a BUG_ON() and update insert_vm_struct()'s
> comment to reflect the fact anonymous mappings can be added here.
> 
> Finally, we update the CONFIG_MMU, CONFIG_PER_VMA_LOCK defines in the VMA
> userland tests so IS_ENABLED() will work correctly with them.
> 
> No functional change intended.
> 
> Reviewed-by: Pedro Falcato <[email protected]>
> Signed-off-by: Lorenzo Stoakes <[email protected]>

carry-over from v1

Reviewed-by: Gregory Price <[email protected]>