Re: [PATCH 1/2] mm: fix bogus VM_BUG_ON_PAGE() in isolate_lru_page()

[Kirill A. Shutemov]

On Mon, Feb 01, 2016 at 03:24:46PM +0100, Michal Hocko wrote:
> On Mon 01-02-16 16:26:08, Kirill A. Shutemov wrote:
> > We don't care if there's a tail pages which is not on LRU. We are not
> > going to isolate them anyway.
> 
> yes we are not going to isolate them but calling this function on a
> tail page is wrong in principle, no? PageLRU check is racy outside of
> lru_lock so what if we are racing here. I know, highly unlikely but not
> impossible. So I am not really sure this is an improvement. When would
> we hit this VM_BUG_ON and it wouldn't be a bug or at least suspicious
> usage?

Yes, there is no point in calling isolate_lru_page() for tail pages, but
we do this anyway -- see the second patch.

And we need to validate all drivers, that they don't forget to set VM_IO
or make vma_migratable() return false in other way.

Alternative approach would be to downgrate the VM_BUG_ON_PAGE() to
WARN_ONCE_ON(). This way we would have chance to catch bad callers.

-- 
 Kirill A. Shutemov