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@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
