On Mon, Feb 4, 2013 at 10:04 AM, Jan Kara <jack@xxxxxxx> wrote: > 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. I'm not sure if there is enough interest for an MM topic there; however I would like to at least discuss this privately with you - I have a lot of mmap_sem frustrations too :) >> > 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. There is really nothing x86 specific about FAULT_FLAG_ALLOW_RETRY - upstream code already uses it (on all archs) to drop mmap_sem during large mlocks that hit disk; and patches in -mm extend this to handle MAP_POPULATE mmaps as well. Using it during page faults is currently only done on x86, but doing that on other arch page fault handlers wouldn't be hard - the code is easy to write, it's just a matter of getting it tested on all archs. This leaves the issue of all the other gup users. I don't think dropping and regrabbing mmap_sem within gup is realistic in general, as the call sites don't expect VMAs to change in the middle of the gup call. >> 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... Yes. FAULT_FLAG_ALLOW_RETRY is my attempt at giving a way for call sites which can deal with mmap_sem being dropped to signal that, so that we don't need to convert every call sites at once. But if you have a better way to go about it, I would be open to discuss it :) -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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
