netdev - Re: [PATCH] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind

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Date:   Wed, 19 Jun 2019 09:21:01 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] mm: mempolicy: handle vma with unmovable pages mapped
 correctly in mbind



On 6/19/19 1:22 AM, Vlastimil Babka wrote:
> On 6/19/19 7:21 AM, Michal Hocko wrote:
>> On Tue 18-06-19 14:13:16, Yang Shi wrote:
>> [...]
>>> I used to have !__PageMovable(page), but it was removed since the
>>> aforementioned reason. I could add it back.
>>>
>>> For the temporary off LRU page, I did a quick search, it looks the most
>>> paths have to acquire mmap_sem, so it can't race with us here. Page
>>> reclaim/compaction looks like the only race. But, since the mapping should
>>> be preserved even though the page is off LRU temporarily unless the page is
>>> reclaimed, so we should be able to exclude temporary off LRU pages by
>>> calling page_mapping() and page_anon_vma().
>>>
>>> So, the fix may look like:
>>>
>>> if (!PageLRU(head) && !__PageMovable(page)) {
>>>      if (!(page_mapping(page) || page_anon_vma(page)))
>>>          return -EIO;
>> This is getting even more muddy TBH. Is there any reason that we have to
>> handle this problem during the isolation phase rather the migration?
> I think it was already said that if pages can't be isolated, then
> migration phase won't process them, so they're just ignored.

Yes，exactly.

> However I think the patch is wrong to abort immediately when
> encountering such page that cannot be isolated (AFAICS). IMHO it should
> still try to migrate everything it can, and only then return -EIO.

It is fine too. I don't see mbind semantics define how to handle such 
case other than returning -EIO.