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. -- Mike Kravetz > > 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=ilto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a> > -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
