Re: [RFC PATCH] hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined

[Michal Hocko <mhocko@kernel.org>]

On Thu 06-12-18 09:15:53, Naoya Horiguchi wrote:
> On Thu, Dec 06, 2018 at 09:32:06AM +0100, Michal Hocko wrote:
> > On Thu 06-12-18 05:21:38, Naoya Horiguchi wrote:
> > > On Wed, Dec 05, 2018 at 05:57:16PM +0100, Michal Hocko wrote:
> > > > On Wed 05-12-18 13:29:18, Michal Hocko wrote:
> > > > [...]
> > > > > After some more thinking I am not really sure the above reasoning is
> > > > > still true with the current upstream kernel. Maybe I just managed to
> > > > > confuse myself so please hold off on this patch for now. Testing by
> > > > > Oscar has shown this patch is helping but the changelog might need to be
> > > > > updated.
> > > > 
> > > > OK, so Oscar has nailed it down and it seems that 4.4 kernel we have
> > > > been debugging on behaves slightly different. The underlying problem is
> > > > the same though. So I have reworded the changelog and added "just in
> > > > case" PageLRU handling. Naoya, maybe you have an argument that would
> > > > make this void for current upstream kernels.
> > > 
> > > The following commit (not in 4.4.x stable tree) might explain the
> > > difference you experienced:
> > > 
> > >   commit 286c469a988fbaf68e3a97ddf1e6c245c1446968                          
> > >   Author: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>                      
> > >   Date:   Wed May 3 14:56:22 2017 -0700                                    
> > >                                                                            
> > >       mm: hwpoison: call shake_page() after try_to_unmap() for mlocked page
> > > 
> > > This commit adds shake_page() for mlocked pages to make sure that the target
> > > page is flushed out from LRU cache. Without this shake_page(), subsequent
> > > delete_from_lru_cache() (from me_pagecache_clean()) fails to isolate it and
> > > the page will finally return back to LRU list.  So this scenario leads to
> > > "hwpoisoned by still linked to LRU list" page.
> > 
> > OK, I see. So does that mean that the LRU handling is no longer needed
> > and there is a guanratee that all kernels with the above commit cannot
> > ever get an LRU page?
> 
> Theoretically no such gurantee, because try_to_unmap() doesn't have a
> guarantee of success and then memory_failure() returns immediately
> when hwpoison_user_mappings fails.
> Or the following code (comes after hwpoison_user_mappings block) also implies
> that the target page can still have PageLRU flag.
> 
>         /*
>          * Torn down by someone else?
>          */
>         if (PageLRU(p) && !PageSwapCache(p) && p->mapping == NULL) {
>                 action_result(pfn, MF_MSG_TRUNCATED_LRU, MF_IGNORED);
>                 res = -EBUSY;
>                 goto out;
>         }
> 
> So I think it's OK to keep "if (WARN_ON(PageLRU(page)))" block in
> current version of your patch.
> 
> Feel free to add my ack.
> 
> Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks a lot Naoya! I will extend the changelog with your wording.

-- 
Michal Hocko
SUSE Labs