On Sun, Dec 16, 2012 at 4:39 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote: > On Fri, Dec 14, 2012 at 6:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> This is a serious cause of mmap_sem contention. MAP_POPULATE >> and MCL_FUTURE, in particular, are disastrous in multithreaded programs. >> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> --- >> >> Changes from v1: >> >> The non-unlocking versions of do_mmap_pgoff and mmap_region are still >> available for aio_setup_ring's benefit. In theory, aio_setup_ring >> would do better with a lock-downgrading version, but that would be >> somewhat ugly and doesn't help my workload. > > Hi Andy, > > I agree that the long mmap_sem hold times when using MAP_POPULATE, > MAP_LOCKED or MCL_FUTURE are a problem. However, I'm not entirely > happy with your proposed solution. > > My main concern is that just downgrading the mmap_sem only hides the > problem: as soon as a writer gets queued on that mmap_sem, > reader/writer fairness kicks in and blocks any new readers, which > makes the problem reappear. So in order to completely fix the issue, > we should look for a way that doesn't require holding the mmap_sem > (even in read mode) for the entire duration of the populate or mlock > operation. Ugh. At least with my patch, mmap in MCL_FUTURE mode is no worse than mmap + mlock. I suspect I haven't hit this because all my mmaping is done by one thread, so it never ends up waiting for itself, and the other thread have very short mmap_sem hold times. > > I think this could be done by extending the mlock work I did as part > of v2.6.38-rc1. The commit message for > c explains the idea; basically > mlock() was split into do_mlock() which just sets the VM_LOCKED flag > on vmas as needed, and do_mlock_pages() which goes through a range of > addresses and actually populates/mlocks each individual page that is > part of a VM_LOCKED vma. > Doesn't this have the same problem? It holds mmap_sem for read for a long time, and if another writer comes in then r/w starvation prevention will kick in. > This could be easily extended for mlocks that happen in mmap_region() > due to MAP_LOCKED or MCL_FUTURE: mmap_region() would just set the > VM_LOCKED flag and defer the work of actually populating/mlocking the > individual pages. I think the only constraint here is that the pages > must be locked before returning to userspace, so we may be able to use > the task_work mechanism to achieve that. Later on (but before > returning to userspace) we would notice we have some mlock work to do > and call do_mlock_pages() to achieve that. > > I think the benefits of this approach would be: > - no mmap_sem locking changes around mmap_region() - which also means > that all code paths to mmap_region() can instantly benefit > - do_mlock_pages() doesn't need to hold a read lock on mmap_sem for > the entire duration of the operation, so we can fully solve the > problem instead of just making it harder to trigger > > Now for handling MAP_POPULATE, we would probably want to use a similar > mechanism as well, so that we don't need to hold the mmap_sem for the > entire duration of the populate. This is similar in principle to the > MAP_LOCKED case; however this may require the introduction of a new > VM_POPULATE vma flag in order to avoid the possibility of a race where > someone replaces our vma with another before we get a chance to > populate it. > > I don't have an implementation for this idea yet; however I'm hoping > to come up with one before xmas. Before then, any comments on the idea > ? IMO this all sucks. What we want is some way to lock a single vma or (better?) a range of virtual addresses. mmap_region (or some equivalent) ought to be able to return a read-locked vma, and the caller can (while still holding that lock) do whatever they want and take their time at it. If mmap_sem were only held while actually modifying the tree of vmas, hold times could be nice and short. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>