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Subject[RFC 0/2] locking order of mm->mmap_sem and various FS
There had ever been several posts which report a circular locking
problem around mm->mmap_sem and FS. For instance
"INFO: possible circular locking dependency detected 3.1.0-rc2-00190-g3210d19"

While the problem in ext4 evict_inode seems to be already solved, here
I'll try fixing hugetlbfs as first step. The problem in hugetlbfs is

- read(2) -- hugetlbfs_read() -- ... -- __copy_to_user()
hugetlbfs_read() holds i_mutex. So this is i_mutex before mmap_sem

- mmap(2) -- hugetlbfs_file_mmap()
hugetlbfs_file_mmap() holds i_mutex too. But mmap_sem is already held
before hugetlbfs_file_mmap(). This is an AB-BA problem.

While I am not sure whether hugetlbfs_read() really needs to acquire
i_mutex, if it really does, then I'd suggest f_op->{pre,post}_mmap().
These two patches are just to show the approach and not intends to be
merged into mainline now. I don't think it is the best solution, but I
simply have no idea other than this.
I'd like to hear comments from LKML people.

Taking a glance at ->mmap() functions in several FSs. I also found
gfs2_mmap()/gfs2_readdir() which acquires gl->gl_spin and may cause a
similar problem. And ocfs2_mmap()/ocfs2_readdir() too, but I don't
understand it enough.

If it is OK and {pre,post}_mmap() is accepted, then I will step forward
and try fixing below too. All of them acquires mmap_sem and calls
->mmap() (indirectly).

- callers of do_mmap()
arch/x86/ia32/ia32_aout.c:load_aout_binary() and its siblings
drivers/gpu/drm/drm_bufs.c:drm_mapbufs() and others
fs/binfmt_aout.c:load_aout_binary() and its siblings
fs/binfmt_elf.c:elf_map() and its siblings
fs/binfmt_elf_fdpic.c:load_elf_fdpic_binary() and its siblings
fs/binfmt_flat.c:load_flat_file() and its siblings
fs/binfmt_som.c:map_som_binary() and its siblings

- callers of do_mmap_pgoff()
mm/nommu.c:SYSCALL mmap_pgoff
mm/mmap.c:SYSCALL mmap_pgoff

- callers of mmap_region()

Additionally they will need some work too.
- callers of ->mmap()

Oh, the base version is v3.0, not latest mainline.

J. R. Okajima (2):
introduce f_op->{pre,post}_mmap()
hugetlbfs: implement f_op->{pre,post}_mmap()

Documentation/filesystems/Locking | 8 ++++++++
Documentation/filesystems/vfs.txt | 7 +++++++
fs/hugetlbfs/inode.c | 20 +++++++++++++++++---
include/linux/fs.h | 2 ++
include/linux/mm.h | 4 ++++
mm/mmap.c | 27 ++++++++++++++++++++++++---
6 files changed, 62 insertions(+), 6 deletions(-)


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