Re: [PATCH] mm: fix list corruptions on shmem shrinklist

[Kirill A. Shutemov]

On Thu, Aug 03, 2017 at 04:53:07PM -0700, Andrew Morton wrote:
> On Thu, 3 Aug 2017 16:25:46 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, Aug 3, 2017 at 4:11 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Where is this INIT_LIST_HEAD()?
> > 
> > I think it's this one:
> > 
> >         list_del_init(&info->shrinklist);
> > 
> > in shmem_unused_huge_shrink().
> 
> OK.
> 
> > > I'm not sure I'm understanding this.  AFAICT all the list operations to
> > > which you refer are synchronized under spin_lock(&sbinfo->shrinklist_lock)?
> > 
> > No, notice how shmem_unused_huge_shrink() does the
> > 
> >         list_move(&info->shrinklist, &to_remove);
> > 
> > and
> > 
> >         list_move(&info->shrinklist, &list);
> > 
> > to move to (two different) private lists under the shrinklist_lock,
> > but once it is on that private "list/to_remove" list, it is then
> > accessed outside the locked region.
> 
> So the code is using sbinfo->shrinklist_lock to protect
> sbinfo->shrinklist AND to protect all the per-inode info->shrinklist's.
> Except it didn't get the coverage complete.
> 
> Presumably it's too expensive to extend sbinfo->shrinklist_lock
> coverage in shmem_unused_huge_shrink() (or is it?  - this is huge
> pages).  An alternative would be to add a new
> shmem_inode_info.shrinklist_lock whose mandate is to protect
> shmem_inode_info.shrinklist.
> 
> > Honestly, I don't love this situation, or the patch, but I think the
> > patch is likely the right thing to do.
> 
> Well, we could view the premature droppage of sbinfo->shrinklist_lock
> in shmem_unused_huge_shrink() to be a performance optimization and put
> some big fat comments in there explaining what's going on.  But it's
> tricky and it's not known that such an optimization is warranted.

The reason we need to drop shrinklist_lock is that we need to perform
sleeping operations for both of the lists:

 - 'to_remove' was added to get iput() running outside spin lock.
    See 253fd0f02040 ("shmem: fix sleeping from atomic context")

 - on handling 'list' we need to take lock_page() and also call iput().

 The fix looks fine to me.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov