Re: [patch] mm: tiny-shmem fix lor, mmap_sem vs i_mutex

[David Howells <dhowells@redhat.com>]

Hugh Dickins <hugh@veritas.com> wrote:

> But now looking into it further, I see this is all a red herring,
> your rearrangement is not the significant difference: before that
> there was David Howells' Jan 2006 commit
> b0e15190ead07056ab0c3844a499ff35e66d27cc
> [PATCH] NOMMU: Make SYSV IPC SHM use ramfs facilities on NOMMU
> which is the one which adds do_truncate() into tiny-shmem.c's
> shmem_file_setup() but not into shmem.c's - presumably because
> config SHMEM depends on MMU so it was irrelevant in shmem.c.
>
> *That* is the relevant commit, which introduced the bad i_mutex
> within mmap_sem lock ordering, and it seems that Nick's current
> patch is wrong just to remove that do_truncate(), a significant
> change hidden inside his restoration of the original arrangement.

That would break SYSV IPC SHM under CONFIG_MMU=n conditions.

The truncate is necessary as I explained in my patch:

     (2) ramfs files now need resizing using do_truncate() rather than by
         modifying the inode size directly (see shmem_file_setup()). This
         causes ramfs to attempt to bind a block of pages of sufficient size to
         the inode.

What I didn't belabour in the patch, and perhaps I should have, is that to do
SYSV IPC SHM under NOMMU conditions, it is necessary to allocate a *contiguous*
set of pages - something that ramfs has been taught to do under NOMMU when
truncating a file upwards from zero size.  This makes POSIX SHM on ramfs files
viable also.

David