On Fri, Feb 27, 2015 at 9:34 PM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote: > 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. That's history: exe_file had direct relation to mm->mmap_sem, that was file from first executable vma. After my patch it's less related to vmas. > 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. Please don't. What's wrong with mmap_sem? Do you want optimize reading mm->exe_file? Then you should use rcu for that: struct file is rcu-protected thing. See fget(), you could do something like that. > > 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> -- 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>