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
