On Thu 31-01-13 23:03:27, Al Viro wrote: > On Thu, Jan 31, 2013 at 11:23:35PM +0100, Jan Kara wrote: > > 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. > > Please, *please*, don't. VMA locking is complete horror without SRCU > mess thrown in. It's a bloody bad idea, at least without a very massive > cleanup prior to that thing. > > Start with drawing the call graph for vma-related code - at least the > parts from relevant locks grabbed to accesses of fields protected by > said locks. VMAs are protected by mmap_sem AFAIK so that doesn't look all that complex. But I guess you are pointing at the fact that sometimes mmap_sem is acquired rather far (sometimes even in arch code) from the places which use the protection of mmap_sem and so it would be difficult (if possible at all) to verify that once we drop mmap_sem, all these places will happily handle that fact. I agree it would be a mess unless we somehow simplify things first... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
