Re: [PATCH 0/2] mm/memfd: add F_SEAL_PUNCH_HOLE

[David Hildenbrand (Arm)]

On 10/26/26 23:05, Samiullah Khawaja wrote:
> This series introduces F_SEAL_PUNCH_HOLE, a new memfd seal that prevents
> holes from being punched into a shmem or hugetlbfs memfd while leaving
> the file fully writable.
> 
> Background
> ==========
> 
> Drivers such as udmabuf and iommufd (via IOMMU_IOAS_MAP_FILE) use
> memfd_pin_folios() to pin the folios backing a memfd and hand them to a
> device for DMA. Pinning keeps those folios allocated, but it does not
> keep them in the file. If userspace later calls
> fallocate(FALLOC_FL_PUNCH_HOLE) or madvise(MADV_REMOVE) on the range,
> the folios are dropped from the page cache while the device continues
> to use them. The pin holder is never notified, and from that point on
> the device and the file silently diverge.

We have the exact same situation when userspace maps anonymous folios into vfio
and then desides to MADV_DONTNEED the pages. Or any other memory backend that is
used for such purposes.

User space must not zap/punch things that might be pinned elsewhere.

That's not specific to memfd, udmabuf, iommufd or anything else.

I don't see a reason to make userspace (that should not shoot itself into the
foot) a kernel feature it must activate itself to not shoot itself into the foot.

-- 
Cheers,

David