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

[Andrew Morton]

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.