On Fri, 11 Sep 2015, Kirill A. Shutemov wrote: > On Thu, Sep 10, 2015 at 03:27:59PM +0200, Andrey Konovalov wrote: > > Can a vma be shared among a few mm's? > > Define "shared". > > vma can belong only to one process (mm_struct), but it can be accessed > from other process like in rmap case below. > > rmap uses anon_vma_lock for anon vma and i_mmap_rwsem for file vma to make > sure that the vma will not disappear under it. > > > If yes, then taking current->mm->mmap_sem to protect vma is not enough. > > Depends on what protection you are talking about. > > > In the first report below both T378 and T398 take > > current->mm->mmap_sem at mm/mlock.c:650, but they turn out to be > > different locks (the addresses are different). > > See i_mmap_lock_read() in T398. It will guarantee that vma is there. > > > In the second report T309 doesn't take any locks at all, since it > > assumes that after checking atomic_dec_and_test(&mm->mm_users) the mm > > has no other users, but then it does a write to vma. > > This one is tricky. I *assume* the mm cannot be generally accessible after > mm_users drops to zero, but I'm not entirely sure about it. > procfs? ptrace? Most of the things (including procfs and ptrace) that need to work on a foreign mm do take a hold on mm_users with get_task_mm(). swapoff uses atomic_inc_not_zero(&mm->mm_users). In KSM I managed to get away with just a hold on the structure itself, atomic_inc(&mm->mm_count), and a check for mm_users 0 wherever it down_reads mmap_sem (but Andrey might like to turn KSM on: it wouldn't be entirely shocking if he were to discover an anomaly from that). > > The VMA is still accessible via rmap at this point. And I think it can be > a problem: > > CPU0 CPU1 > exit_mmap() > // mmap_sem is *not* taken > munlock_vma_pages_all() > munlock_vma_pages_range() > try_to_unmap_one() > down_read_trylock(&vma->vm_mm->mmap_sem)) > !!(vma->vm_flags & VM_LOCKED) == true > vma->vm_flags &= ~VM_LOCKED; > <munlock the page> > mlock_vma_page(page); > // mlocked pages is leaked. > > The obvious solution is to take mmap_sem in exit path, but it would cause > performance regression. > > Any comments? I'm inclined to echo Vlastimil's comment from earlier in the thread: sounds like an overkill, unless we find something more serious than this. I'm not sure whether we'd actually see a regression from taking mmap_sem in exit path; but given that it's mmap_sem, yes, history tells us please not to take it any more than we have to. I do remember wishing, when working out KSM's mm handling, that exit took mmap_sem: it would have made it simpler, but that wasn't a change I dared to make. Maybe an mm_users 0 check after down_read_trylock in try_to_unmap_one() could fix it? But if we were to make a bigger change for this VM_LOCKED issue, and something more serious makes it worth all the effort, I'd say that what needs to be done is to give mlock/munlock proper locking (haha). I have not yet looked at your mlocked THP patch (sorry), but when I was doing the same thing for huge tmpfs, what made it so surprisingly difficult was all the spongy trylocking, which concealed the rules. Maybe I'm completely wrong, but I thought a lot of awkwardness might disappear if they were relying on anon_vma->rwsem and i_mmap_rwsem throughout instead of mmap_sem. Hugh -- 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>