Re: [RFC PATCH v2 11/16] mm,hwpoison: Rework soft offline for in-use pages

[Oscar Salvador]

On Mon, Oct 21, 2019 at 04:06:19PM +0200, Michal Hocko wrote:
> On Mon 21-10-19 15:48:48, Oscar Salvador wrote:
> > We can only perform actions on LRU/Movable pages or hugetlb pages.
> 
> What would prevent other pages mapped via page tables to be handled as
> well?

What kind of pages?
I mean, I guess it could be done, it was just not implemented, and I
did not want to add more "features" as my main goal was to re-work
the interface to be more deterministic.

> > 1) we would need to hook in enqueue_hugetlb_page so the page is not enqueued
> >    into hugetlb freelists
> > 2) when trying to free a hugetlb page, we would need to do as we do for gigantic
> >    pages now, and that is breaking down the pages into order-0 pages and release
> >    them to the buddy (so the check in free_papges_prepare would skip the
> >    hwpoison page).
> >    Trying to handle a higher-order hwpoison page in free_pages_prepare is
> >    a bit complicated.
> 
> I am not sure I see the problem. If you dissolve the hugetlb page then
> there is no hugetlb page anymore and so you make it a regular high-order
> page.

Yes, but the problem comes when trying to work with a hwpoison high-order page
in free_pages_prepare, it gets more complicated, and when I weigthed
code vs benefits, I was not really sure to go down that road.

If we get a hwpoison high-order page in free_pages_prepare, we need to
break it down to smaller pages, so we can skip the "bad" to not be sent
into buddy allocator.

> If the page is free then it shouldn't pin the memcg or any other state.

Well, it is not really free, as it is not usable, is it?
Anyway, I do agree that we should clean the bondings to other subsystems like memcg.

-- 
Oscar Salvador
SUSE L3