On Fri, 2015-02-27 at 18:36 +0100, Oleg Nesterov wrote: > On 02/26, Cyrill Gorcunov wrote: > > > > On Thu, Feb 26, 2015 at 11:36:57AM -0800, Davidlohr Bueso wrote: > > > We currently use the mmap_sem to serialize the mm exe_file. > > > This is atrocious and a clear example of the misuses this > > > lock has all over the place, making any significant changes > > > to the address space locking that much more complex and tedious. > > > This also has to do of how we used to check for the vma's vm_file > > > being VM_EXECUTABLE (much of which was replaced by 2dd8ad81e31). > > > > > > This patch, therefore, removes the mmap_sem dependency and > > > introduces a specific lock for the exe_file (rwlock_t, as it is > > > read mostly and protects a trivial critical region). As mentioned, > > > the motivation is to cleanup mmap_sem (as opposed to exe_file > > > performance). > > Well, I didn't see the patch, can't really comment. > > But I have to admit that this looks as atrocious and a clear example of > "lets add yet another random lock which we will regret about later" ;) > > rwlock_t in mm_struct just to serialize access to exe_file? I don't see why this is a random lock nor how would we regret this later. I regret having to do these kind of patches because people were lazy and just relied on mmap_sem without thinking beyond their use case. As mentioned I'm also planning on creating an own sort of exe_file_struct, which would be an isolated entity (still in the mm though), with its own locking and prctl bits, that would tidy mm_struct a bit. RCU was something else I considered, but it doesn't suite well in all paths and we would still need a spinlock when updating the file anyway. If you have a better suggestion please do tell. > > > A nice side effect of this is that we avoid taking > > > the mmap_sem (shared) in fork paths for the exe_file handling > > > (note that readers block when the rwsem is taken exclusively by > > > another thread). > > Yes, this is ugly. Can't we kill this dup_mm_exe_file() and copy change > dup_mmap() to also dup ->exe_file ? > > > Hi Davidlohr, it would be interesting to know if the cleanup > > bring some performance benefit? > > To me the main question is whether the patch makes this code simpler > or uglier ;) Its much beyond that. As mentioned, for any significant changes to the mmap_sem locking scheme, this sort of thing needs to be addressed first. Thanks, Davidlohr -- 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>