Re: [PATCH mm-unstable] mm: Fix uffd-wp bit loss when batching file folio unmapping

[David Hildenbrand (Red Hat)]

On 1/16/26 10:40, Dev Jain wrote:
> On 16/01/26 2:09 pm, Lorenzo Stoakes wrote:
> > On Fri, Jan 16, 2026 at 01:57:21PM +0530, Dev Jain wrote:
> > > The recently added file folio unmap batching support forgets to update
> > > pte_install_uffd_wp_if_needed(), which still updates a single pte.
> > > We end up jumping to the end of the folio in page_vma_mapped_walk(), thus
> > > setting the uffd-wp marker only on a single pte in the batch. Fix this by
> > > passing nr_pages into the function, and set the uffd-wp marker on all ptes.
> > >
> > > Note that, since the nr_pages passed to this function is always derived by
> > > some sort of batching, it is guaranteed that the set of old ptevals of the
> > > batch have uffd-wp bit on all ptes or no ptes, therefore it is safe to derive
> > > the value of the local variable "arm_uffd_pte" from only the particular
> > > pteval passed to this function, but apply the result on all ptes of the batch.
> > >
> > > Use set_pte_at() in a loop to set the markers - we cannot use set_ptes()
> > > as that will increment the PFN, but we don't have any PFN to update here.
> > >
> > > The userspace visible effect of the bug is inaccuracy observed by workloads
> > > relying on uffd-wp regions to install their own pages.
> > >
> > > Fixes: 8798e255b5ec ("mm: rmap: support batched unmapping for file large folios")
> > Hmm this patch isn't upstream yet, so wouldn't this patch be better as a comment
> > replying to the series so it can be respun?
> >
> > You only do a fixes tag for either upstream things that obviously have to stay
> > in place or perhaps things in mm-stable immediately prior to merge window that
> > can't be adjusted.
>
> I saw that the last comment on that series was more than a week back, so best
> thought to just do a folded fix on top of it - and I had formed the impression
> (from the conversations on list) that akpm prefers fixes over respins : )

That series is still on my TODO list and I hope Andrew will keep it out 
of stable until I get to it.

Hope is all I got ;)

--
Cheers

David