On Mon, 3 Dec 2012 16:35:01 -0800 Michel Lespinasse <walken@xxxxxxxxxx> wrote: > On Mon, Dec 3, 2012 at 3:01 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, 30 Nov 2012 22:56:27 -0800 > > Michel Lespinasse <walken@xxxxxxxxxx> wrote: > > > >> expand_stack() runs with a shared mmap_sem lock. Because of this, there > >> could be multiple concurrent stack expansions in the same mm, which may > >> cause problems in the vma gap update code. > >> > >> I propose to solve this by taking the mm->page_table_lock around such vma > >> expansions, in order to avoid the concurrency issue. We only have to worry > >> about concurrent expand_stack() calls here, since we hold a shared mmap_sem > >> lock and all vma modificaitons other than expand_stack() are done under > >> an exclusive mmap_sem lock. > >> > >> I previously tried to achieve the same effect by making sure all > >> growable vmas in a given mm would share the same anon_vma, which we > >> already lock here. However this turned out to be difficult - all of the > >> schemes I tried for refcounting the growable anon_vma and clearing > >> turned out ugly. So, I'm now proposing only the minimal fix. > > > > I think I don't understand the problem fully. Let me demonstrate: > > > > a) vma_lock_anon_vma() doesn't take a lock which is specific to > > "this" anon_vma. It takes anon_vma->root->mutex. That mutex is > > shared with vma->vm_next, yes? If so, we have no problem here? > > (which makes me suspect that the races lies other than where I think > > it lies). > > So, the first thing I need to mention is that this fix is NOT for any > problem that has been reported (and in particular, not for Sasha's > trinity fuzzing issue). It's just me looking at the code and noticing > I haven't gotten locking right for the case of concurrent stack > expansion. > > Regarding vma and vma->vm_next sharing the same root anon_vma mutex - > this will often be the case, but not always. find_mergeable_anon_vma() > will try to make it so, but it could fail if there was another vma > in-between at the time the stack's anon_vmas got assigned (either a > non-stack vma that later gets unmapped, or another stack vma that > didn't get its own anon_vma assigned yet). > > > b) I can see why a broader lock is needed in expand_upwards(): it > > plays with a different vma: vma->vm_next. But expand_downwards() > > doesn't do that - it only alters "this" vma. So I'd have thought > > that vma_lock_anon_vma("this" vma) would be sufficient. > > The issue there is that vma_gap_update() accesses vma->vm_prev, so the > issue is actually symetrical with expand_upwards(). > > > What are the performance costs of this change? > > It's expected to be small. glibc doesn't use expandable stacks for the > threads it creates, so having multiple growable stacks is actually > uncommon (another reason why the problem hasn't been observed in > practice). Because of this, I don't expect the page table lock to get > bounced between threads, so the cost of taking it should be small > (compared to the cost of delivering the #PF, let alone handling it in > software). > > But yes, the initial idea of forcing all growable vmas in an mm to > share the same root anon_vma sounded much more appealing at first. > Unfortunately I haven't been able to make that work in a simple enough > way to be comfortable submitting it this late in the release cycle :/ hm, OK. Could you please cook up a new changelog which explains these things to the next puzzled reader and send it along? Ingo is playing in the same area with "mm/rmap: Convert the struct anon_vma::mutex to an rwsem", but as that patch changes vma_lock_anon_vma() to use down_write(), I expect it won't affect anything. But please check it over. -- 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>