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
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