On Sat, Nov 5, 2011 at 8:21 AM, Nai Xia <nai.xia@xxxxxxxxx> wrote: > 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 OK, I think I need to be more concise: Your last reasoning only ensures that mremap as a whole entity cannot interleave with rmap_walk(). But I think nothing can prevent move_page_tables() from doing this. As long as copy_vma() gives an wrong ordering, the racing between rmap_walk() & move_page_tables() afterwards may trigger the bug. Do you agree? > > 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