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. -- 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
