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SubjectRe: [PATCH v21 15/19] mm/compaction: do page isolation first in compaction
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On 11/12/20 3:28 AM, Hugh Dickins wrote:
> 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).

Right, on both points, got too confused.

> Hugh
>

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