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

[Samiullah Khawaja <skhawaja@google.com>]

On Mon, Oct 26, 2026 at 03:46:34PM -0700, Andy Lutomirski wrote:
>On Mon, Oct 26, 2026 at 3:06 PM Samiullah Khawaja <skhawaja@google.com> 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.
>>
>> The existing seals only partially address this. F_SEAL_SHRINK prevents
>> pages from being dropped by truncation, which is why udmabuf requires
>> it. Hole punching, however, is only blocked by F_SEAL_WRITE and
>> F_SEAL_FUTURE_WRITE, and both of those also forbid writes. That makes
>> them unusable for memory that must stay writable, such as guest RAM or
>> DMA buffers, and udmabuf explicitly rejects memfds that carry them.
>>
>> Proposal
>> ========
>>
>> F_SEAL_PUNCH_HOLE fills this gap. Once it is set,
>> fallocate(FALLOC_FL_PUNCH_HOLE) and madvise(MADV_REMOVE) fail with
>> -EPERM, while reads, writes and resizing are unaffected. Combined with
>> F_SEAL_SHRINK, it guarantees that userspace cannot remove pages from
>> the memfd while the memfd remains writable.
>
>How does this solve the problem?  F_SEAL is about a userspace
>contract, and if there are existing users of the APIs in question, it
>makes very little sense to start making those users fail because they
>haven't taken the appropriate seal.

It doesn't change any existing behaviour, nothing starts failing for
users that don't set the seal, and drivers wouldn't require it. It's
opt-in. Userspace that wants the guarantee seals the memfd, and a kernel
user that pins it can check for the seal and rely on the pages staying
in the file. For example, the iommufd live update path could skip
verifying the pinned pages at preserve time when the seal is present,
and do the full check otherwise.

https://lore.kernel.org/all/arVYa_90j0oN1hnu@google.com/
>
>Would it make more sense for the drivers in question to set some flag
>(refcount?) on the file to block hole punching?  Or perhaps to have
>the hole punch code notice that it's trying to punch out a folio that
>is pinned or otherwise referenced and to refuse to do it?

The pins already keep the memory alive while it's used for DMA; the
problem is only that the page silently stops being part of the file.
Refusing to punch pinned folios on every file would change behaviour for
existing users: fallocate() would start failing whenever a page is
pinned, even briefly by O_DIRECT. A flag set on the file by long-term
pinners would avoid the short-term case, but it would still make punches
start failing for existing users of those drivers, which is why I went
for an opt-in seal.

Another option, closer to what you mentioned, but opt-in, is instead of
sealing the whole file against hole punching, a seal that only protects
pinned pages. Punching, shrinking and truncating keep working on a
sealed memfd, except where they would remove a page that is currently
pinned; those would fail with -EBUSY. Files without the seal behave
exactly as today, so existing users are unaffected, and pinned memory is
guaranteed to stay part of the file.
>
>To some extent, the issue you're describing is maybe broader than just
>the APIs you're contemplating.  If I open a file and then do something
>in thread A that GUP's the file and punch a hole in the file in thread
>B, I'm doing something weird, and I can imagine plenty of ways it
>could go wrong.  (Especially if it's GUP for write.  I hope we already
>get that case right.)
>
>-- 
>Andy Lutomirski
>AMA Capital Management, LLC

Thanks,
Sami