Re: [PATCH v2 3/5] mm: implement sticky, copy on fork VMA flags

[Pedro Falcato <pfalcato@suse.de>]

On Thu, Nov 06, 2025 at 10:46:14AM +0000, Lorenzo Stoakes wrote:
> It's useful to be able to force a VMA to be copied on fork outside of the
> parameters specified by vma_needs_copy(), which otherwise only copies page
> tables if:
> 
> * The destination VMA has VM_UFFD_WP set
> * The mapping is a PFN or mixed map
> * The mapping is anonymous and forked in (i.e. vma->anon_vma is non-NULL)
> 
> Setting this flag implies that the page tables mapping the VMA are such
> that simply re-faulting the VMA will not re-establish them in identical
> form.
> 
> We introduce VM_COPY_ON_FORK to clearly identify which flags require this
> behaviour, which currently is only VM_MAYBE_GUARD.
> 
> Any VMA flags which require this behaviour are inherently 'sticky', that
> is, should we merge two VMAs together, this implies that the newly merged
> VMA maps a range that requires page table copying on fork.
> 
> In order to implement this we must both introduce the concept of a 'sticky'
> VMA flag and adjust the VMA merge logic accordingly, and also have VMA
> merge still successfully succeed should one VMA have the flag set and
> another not.

Perhaps we should separate this patch into two? It looks like we're doing two
things at once for no great reason. But it's a bit of a sticky situation...

> 
> Note that we update the VMA expand logic to handle new VMA merging, as this
> function is the one ultimately called by all instances of merging of new
> VMAs.
> 
> This patch implements this, establishing VM_STICKY to contain all such
> flags and VM_IGNORE_MERGE for those flags which should be ignored when
> comparing adjacent VMA's flags for the purposes of merging.
> 
> 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.
> 
> As a result of this change, VMAs with guard ranges will now not have their
> merge behaviour impacted by doing so 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.
> 
> Note that VM_MAYBE_GUARD being set atomically remains correct as
> vma_needs_copy() is invoked with the mmap and VMA write locks held,
> excluding any race with madvise_guard_install().
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Overall the patch LGTM.

Feel free to add:
Reviewed-by: Pedro Falcato <pfalcato@suse.de>

and maybe print it out on a sticker.

-- 
Pedro