On Sat, Jan 12, 2019 at 12:38 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> > > Android uses ashmem for sharing memory regions. We are looking forward to > migrating all usecases of ashmem to memfd so that we can possibly remove > the ashmem driver in the future from staging while also benefiting from > using memfd and contributing to it. Note staging drivers are also not ABI > and generally can be removed at anytime. > > One of the main usecases Android has is the ability to create a region and > mmap it as writeable, then add protection against making any "future" > writes while keeping the existing already mmap'ed writeable-region active. > This allows us to implement a usecase where receivers of the shared > memory buffer can get a read-only view, while the sender continues to > write to the buffer. See CursorWindow documentation in Android for more > details: > https://developer.android.com/reference/android/database/CursorWindow > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal > which prevents any future mmap and write syscalls from succeeding while > keeping the existing mmap active. > > A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week > where we don't need to modify core VFS structures to get the same > behavior of the seal. This solves several side-effects pointed by Andy. > self-tests are provided in later patch to verify the expected semantics. > > [1] https://lore.kernel.org/lkml/20181111173650.GA256781@xxxxxxxxxx/ > > [Thanks a lot to Andy for suggestions to improve code] > Cc: Andy Lutomirski <luto@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > fs/hugetlbfs/inode.c | 2 +- > include/uapi/linux/fcntl.h | 1 + > mm/memfd.c | 3 ++- > mm/shmem.c | 25 ++++++++++++++++++++++--- > 4 files changed, 26 insertions(+), 5 deletions(-) Acked-by: John Stultz <john.stultz@xxxxxxxxxx>
