[LSF/MM TOPIC] mmap_sem in ->fault and ->page_mkwrite

[Jan Kara <jack@suse.cz>]

  Hi,

  I'm not sure if this is such a great topic but it's a question which
I came across a few times already and LSF/MM is a good place for
brainstorming somewhat crazy ideas ;).

So currently ->fault() and ->page_mkwrite() are called under mmap_sem held
for reading. Now this creates sometimes unpleasant locking dependencies for
filesystems (modern filesystems have to do an equivalent of ->write_begin
in ->page_mkwrite and that is a non-trivial operation). Just to mention my
last itch, I had to split reader side of filesystem freezing lock into two
locks - one which ranks above mmap_sem and one which ranks below it. Then
writer side has to wait for both locks. It works but ...

So I was wondering: Would it be somehow possible we could drop mmap_sem in
these two callbacks (especially ->page_mkwrite())? I understand process'
mapping can change under us once we drop the semaphore so we'd have to
somehow recheck we have still the right page after re-taking mmap_sem. Like
if we protected VMAs with SRCU so that they don't disappear under us once
we drop mmap_sem and after retaking mmap_sem we would recheck whether VMA
still applies to our fault.

And I know there's VM_FAULT_RETRY but that really seems like a special hack
for x86 architecture page fault code. Making it work for all architectures
and callers such as get_user_pages() didn't really seem plausible to me.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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