Re: [PATCH V1 0/5] memfd-pin huge page fixes

[Steven Sistare <steven.sistare@oracle.com>]

On 9/3/2024 9:12 PM, Kasireddy, Vivek wrote:
> Hi Steve,
> 
>> Subject: [PATCH V1 0/5] memfd-pin huge page fixes
>>
>> Fix multiple bugs that occur when using memfd_pin_folios with hugetlb
>> pages
>> and THP.  The hugetlb bugs only bite when the page is not yet faulted in
>> when memfd_pin_folios is called.  The THP bug bites when the starting offset
>> passed to memfd_pin_folios is not huge page aligned.  See the commit
>> messages
>> for details.
> Thank you for fixing these bugs. I have Acked all patches except for patch #1,
> as my understanding of xarrays is limited at this point.
> 
> Also, could you please briefly describe how you have exercised memfd_alloc_folio()
> code path or what tests you have run to uncover these bugs? I'd like to
> figure out ways to augment the list of udmabuf tests to validate scenarios
> where hugetlb pages are not faulted in and memfd_pin_folios() is called.

I am extending iommufd to support memfd pinning, so I added a new ioctl which
takes an fd, offset, and length to pin.  I am just getting started, so currently
it does nothing more than call memfd_pin_folios immediately followed by unpin_folios.
Then exit the process.  That is sufficient to trigger all the bugs except the alloc
race panic. I tested with these files:
   /dev/hugepages/file
   /dev/shm/file    with: mount -o remount,huge=always /dev/shm
   memfd_create(MFD_HUGETLB)
   memfd_create(0)  with: echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled

- Steve

>> Steve Sistare (5):
>>    mm/filemap: fix filemap_get_folios_contig THP panic
>>    mm/hugetlb: fix memfd_pin_folios free_huge_pages leak
>>    mm/hugetlb: fix memfd_pin_folios resv_huge_pages leak
>>    mm/gup: fix memfd_pin_folios hugetlb page allocation
>>    mm/gup: fix memfd_pin_folios alloc race panic
>>
>>   include/linux/hugetlb.h | 10 ++++++++++
>>   mm/filemap.c            |  4 ++++
>>   mm/gup.c                |  5 ++++-
>>   mm/hugetlb.c            | 17 +++++++++++++++++
>>   mm/memfd.c              | 15 +++++++++------
>>   5 files changed, 44 insertions(+), 7 deletions(-)
>>
>> --
>> 1.8.3.1
>