Hi On Sat, Nov 4, 2017 at 12:31 AM, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > On 11/03/2017 10:56 AM, Mike Kravetz wrote: >> On 11/03/2017 10:41 AM, David Herrmann wrote: >>> Hi >>> >>> On Fri, Nov 3, 2017 at 6:12 PM, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: >>>> On 11/03/2017 10:03 AM, David Herrmann wrote: >>>>> Hi >>>>> >>>>> On Tue, Oct 31, 2017 at 7:40 PM, Marc-André Lureau >>>>> <marcandre.lureau@xxxxxxxxxx> wrote: >>>>>> Implements memfd sealing, similar to shmem: >>>>>> - WRITE: deny fallocate(PUNCH_HOLE). mmap() write is denied in >>>>>> memfd_add_seals(). write() doesn't exist for hugetlbfs. >>>>>> - SHRINK: added similar check as shmem_setattr() >>>>>> - GROW: added similar check as shmem_setattr() & shmem_fallocate() >>>>>> >>>>>> Except write() operation that doesn't exist with hugetlbfs, that >>>>>> should make sealing as close as it can be to shmem support. >>>>> >>>>> SEAL, SHRINK, and GROW look fine to me. >>>>> >>>>> Regarding WRITE >>>> >>>> The commit message may not be clear. However, hugetlbfs does not support >>>> the write system call (or aio). The only way to modify contents of a >>>> hugetlbfs file is via mmap or hole punch/truncate. So, we do not really >>>> need to worry about those special (a)io cases for hugetlbfs. >>> >>> This is not about the write(2) syscall. Please consider this scenario >>> about shmem: >>> >>> You create a memfd via memfd_create() and map it writable. You now >>> call another kernel syscall that takes as input _any mapped page >>> range_. You pass your mapped memfd-addresses to it. Those syscalls >>> tend to use get_user_pages() to pin arbitrary user-mapped pages, as >>> such this also affects shmem. In this case, those pages might stay >>> mapped even if you munmap() your memfd! >>> >>> One example of this is using AIO-read() on any other file that >>> supports it, passing your mapped memfd as buffer to _read into_. The >>> operations supported on the memfd are irrelevant here. >>> The selftests contain a FUSE-based test for this, since FUSE allows >>> user-space to GUP pages for an arbitrary amount of time. >>> >>> The original fix for this is: >>> >>> commit 05f65b5c70909ef686f865f0a85406d74d75f70f >>> Author: David Herrmann <dh.herrmann@xxxxxxxxx> >>> Date: Fri Aug 8 14:25:36 2014 -0700 >>> >>> shm: wait for pins to be released when sealing >>> >>> Please have a look at this. Your patches use shmem_add_seals() almost >>> unchanged, and as such you call into shmem_wait_for_pins() on >>> hugetlbfs. I would really like to see an explicit ACK that this works >>> on hugetlbfs. >> >> Thanks for the explanation. I missed that in your first reply. I'll >> look into this for hugetlbfs. > > I reviewed the routines in the above commit and did not see anything that > would prevent them from working properly with hugetlbfs. I modified the > fuse test to use hugetlbfs based mapping. I also instrumented the above > routines and verified that tags were set/checked/cleared as designed for > hugetlb pages. So, that is an ACK on working with hugetlbfs. > > This does bring up the point that the fuse seals test should also be > modified to work with hugetlbfs as part of this series. Perfect! Looks all good to me then! Thanks David -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href
