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Date:   Wed, 11 Nov 2020 18:28:38 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Vlastimil Babka <vbabka@suse.cz>
cc:     Alex Shi <alex.shi@linux.alibaba.com>, akpm@linux-foundation.org,
        mgorman@techsingularity.net, tj@kernel.org, hughd@google.com,
        khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com,
        willy@infradead.org, hannes@cmpxchg.org, lkp@intel.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, shakeelb@google.com,
        iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com,
        kirill@shutemov.name, alexander.duyck@gmail.com,
        rong.a.chen@intel.com, mhocko@suse.com, vdavydov.dev@gmail.com,
        shy828301@gmail.com
Subject: Re: [PATCH v21 15/19] mm/compaction: do page isolation first in
 compaction
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References: <1604566549-62481-1-git-send-email-alex.shi@linux.alibaba.com> <1604566549-62481-16-git-send-email-alex.shi@linux.alibaba.com> <a0b8c198-6bd0-2ccb-fe55-970895c26a0b@suse.cz>
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On Wed, 11 Nov 2020, Vlastimil Babka wrote:
> On 11/5/20 9:55 AM, Alex Shi wrote:
> 
> > @@ -979,10 +995,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
> >   					goto isolate_abort;
> >   			}
> >   -			/* Recheck PageLRU and PageCompound under lock */
> > -			if (!PageLRU(page))
> > -				goto isolate_fail;
> > -
> >   			/*
> >   			 * Page become compound since the non-locked check,
> >   			 * and it's on LRU. It can only be a THP so the order
> > @@ -990,16 +1002,13 @@ static bool too_many_isolated(pg_data_t *pgdat)

Completely off-topic, and won't matter at all when Andrew rediffs into
mmotm: but isn't it weird that this is showing "too_many_isolated(",
when actually the function is isolate_migratepages_block()?

> >   			 */
> >   			if (unlikely(PageCompound(page) &&
> > !cc->alloc_contig)) {
> >   				low_pfn += compound_nr(page) - 1;
> > -				goto isolate_fail;
> > +				SetPageLRU(page);
> > +				goto isolate_fail_put;
> >   			}
> 
> IIUC the danger here is khugepaged will collapse a THP. For that,
> __collapse_huge_page_isolate() has to succeed isolate_lru_page(). Under the
> new scheme, it shouldn't be possible, right? If that's correct, we can remove
> this part?

I don't think so.  A preliminary check for PageCompound was made much
higher up, before taking a reference on the page, but it can easily have
become PageCompound since then (when racing prep_new_page() calls
prep_compound_page()).

And __collapse_huge_page_isolate() does not turn a non-compound page
into a compound page: it isolates small pages before copying them into
the compound page (in the usual case: I can see there's also allowance
for PageCompound there too, which will do something different).

Hugh