Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

David Hildenbrand
2022-07-21T09:50:57Z

On 21.07.22 11:44, David Hildenbrand wrote:
> On 06.07.22 10:20, Chao Peng wrote:
>> Normally, a write to unallocated space of a file or the hole of a sparse
>> file automatically causes space allocation, for memfd, this equals to
>> memory allocation. This new seal prevents such automatically allocating,
>> either this is from a direct write() or a write on the previously
>> mmap-ed area. The seal does not prevent fallocate() so an explicit
>> fallocate() can still cause allocating and can be used to reserve
>> memory.
>>
>> This is used to prevent unintentional allocation from userspace on a
>> stray or careless write and any intentional allocation should use an
>> explicit fallocate(). One of the main usecases is to avoid memory double
>> allocation for confidential computing usage where we use two memfds to
>> back guest memory and at a single point only one memfd is alive and we
>> want to prevent memory allocation for the other memfd which may have
>> been mmap-ed previously. More discussion can be found at:
>>
>> https://lkml.org/lkml/2022/6/14/1255
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
>> ---
>> include/uapi/linux/fcntl.h | 1 +
>> mm/memfd.c | 3 ++-
>> mm/shmem.c | 16 ++++++++++++++--
>> 3 files changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
>> index 2f86b2ad6d7e..98bdabc8e309 100644
>> --- a/include/uapi/linux/fcntl.h
>> +++ b/include/uapi/linux/fcntl.h
>> @@ -43,6 +43,7 @@
>> #define F_SEAL_GROW 0x0004 /* prevent file from growing */
>> #define F_SEAL_WRITE 0x0008 /* prevent writes */
>> #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
>> +#define F_SEAL_AUTO_ALLOCATE 0x0020 /* prevent allocation for writes */
>
> Why only "on writes" and not "on reads". IIRC, shmem doesn't support the
> shared zeropage, so you'll simply allocate a new page via read() or on
> read faults.

Correction: on read() we don't allocate a fresh page. But on read faults
we would. So this comment here needs clarification.

>
>
> Also, I *think* you can place pages via userfaultfd into shmem. Not sure
> if that would count "auto alloc", but it would certainly bypass fallocate().
>

--
Thanks,

David / dhildenb