On Sat, Nov 5, 2011 at 7:56 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > On Fri, Nov 04, 2011 at 12:31:04AM -0700, Hugh Dickins wrote: >> On Mon, 31 Oct 2011, Andrea Arcangeli wrote: >> > diff --git a/mm/mmap.c b/mm/mmap.c >> > index a65efd4..a5858dc 100644 >> > --- a/mm/mmap.c >> > +++ b/mm/mmap.c >> > @@ -2339,7 +2339,15 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, >> > */ >> > if (vma_start >= new_vma->vm_start && >> > vma_start < new_vma->vm_end) >> > + /* >> > + * No need to call anon_vma_order_tail() in >> > + * this case because the same PT lock will >> > + * serialize the rmap_walk against both src >> > + * and dst vmas. >> > + */ >> >> Really? Please convince me: I just do not see what ensures that >> the same pt lock covers both src and dst areas in this case. > > Right, vma being the same for src/dst doesn't mean the PT lock is the > same, it might be if source pte entry fit in the same pagetable but > maybe not if the vma is >2M (the max a single pagetable can point to). > >> > *vmap = new_vma; >> > + else >> > + anon_vma_order_tail(new_vma); >> >> And if this puts new_vma in the right position for the normal >> move_page_tables(), as anon_vma_clone() does in the block below, >> aren't they both in exactly the wrong position for the abnormal >> move_page_tables(), called to put ptes back where they were if >> the original move_page_tables() fails? > > Failure paths. Good point, they'd need to be reversed again in that > case. > >> It might be possible to argue that move_page_tables() can only >> fail by failing to allocate memory for pud or pmd, and that (perhaps) >> could only happen if the task was being OOM-killed and ran out of >> reserves at this point, and if it's being OOM-killed then we don't >> mind losing a migration entry for a moment... perhaps. > > Hmm no it wouldn't be ok, or I wouldn't want to risk that. > >> Certainly I'd agree that it's a very rare case. But it feels wrong >> to be attempting to fix the already unlikely issue, while ignoring >> this aspect, or relying on such unrelated implementation details. > > Agreed. > >> Perhaps some further anon_vma_ordering could fix it up, >> but that would look increasingly desperate. > > I think what Nai didn't consider in explaining this theoretical race > that I noticed now is the anon_vma root lock taken by adjust_vma. > > If the merge succeeds adjust_vma will take the lock and flush away > from all others CPUs any sign of rmap_walk before the move_page_tables > can start. > > So it can't happen that you do rmap_walk, check vma1, mremap moves > stuff from vma2 to vma1 (wrong order), and then rmap_walk continues > checking vma2 where the pte won't be there anymore. It can't happen > because mremap would block in vma_merge waiting the rmap_walk to > complete. Before proceeding moving any pte. Thanks to the anon_vma > lock already taken by adjust_vma. Still, I think it's not rmap_walk() ---> mremap() --> rmap_walk() that trigger the bug, but this events would: copy_vma() ---> rmap_walk() scan dst VMA --> move_page_tables() moves src to dst ---> rmap_walk() scan src VMA. :D I might be wrong. But thank you all for the time and patience for playing this racing game with me. It's really an honor to exhaust my mind on a daunting thing with you. :) Best Regards, Nai -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href