Re: [PATCH v21 15/19] mm/compaction: do page isolation first in compaction

[Hugh Dickins <hughd@google.com>]

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