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SubjectRe: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap
On Mon, 9 Jan 2012, KOSAKI Motohiro wrote:
> (1/9/12 5:25 PM), Hugh Dickins wrote:
> > On Mon, 9 Jan 2012, KOSAKI Motohiro wrote:
> >
> > > PG_mlocked operation is not protected by LRU lock. So, I think we
> >
> > Right. But I don't see that I've made a significant change there.
> >
> > I may be being lazy, and rushing back to answer you, without giving
> > constructive thought to what the precise race is that you see, and
> > how we might fix it. If the case you have in mind is easy for you
> > to describe in detail, please do so; but don't hesitate to tell me
> > to my own work for myself!
>
> Bah! I was moron. I now think your code is right.
>
> spin_lock(lru_lock)
> if (page_evictable(page))
> blah blah blah
> spin_unlock(lru_lock)
>
> is always safe. Counter part should have following code and
> waiting spin_lock(lru_lock) in isolate_lru_page().
>
> if (!isolate_lru_page(page))
> putback_lru_page(page);
>
> then, even if check_move_unevictable_pages() observed wrong page status,
> putback_lru_page() should put back the page into right lru.
>
> I'm very sorry for annoying you.

Far from it, thank you again for giving it serious thought.

I am not going to pretend to have thought down these paths myself,
not recently - I was just relying on not changing the behaviour.
But I am reassured to know that you have worked through it again
and are now satisfied.

>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Thank you.

>
> Probably, page_evictable() might be needed some additional comments. But
> I have no idea what comment clearly explain this complex rule.....

I don't know any language that can make it clear: when forced to,
one just has to think through it back and forth by oneself; and
even then, it's so quickly forgotten.

Hugh


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