On 4/13/22 18:39, David Hildenbrand wrote:
>>> @@ -3035,10 +3083,19 @@ void set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>>>
>>> flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
>>> pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
>>> +
>>> + anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
>>> + if (anon_exclusive && page_try_share_anon_rmap(page)) {
>>> + set_pmd_at(mm, address, pvmw->pmd, pmdval);
>>> + return;
>>
>> I am admittedly not too familiar with this code, but looks like this means
>> we fail to migrate the THP, right? But we don't seem to be telling the
>> caller, which is try_to_migrate_one(), so it will continue and not terminate
>> the walk and return false?
>
> Right, we're not returning "false". Returning "false" would be an
> optimization to make rmap_walk_anon() fail faster.
Ah right, that's what I missed, it's an optimization and we will realize
elsewhere afterwards that the page has still mappings and we can't migrate...
> But, after all, the THP is exclusive (-> single mapping), so
> anon_vma_interval_tree_foreach() would most probably not have a lot work
> to do either way I'd assume?
>
> In any case, once we return from try_to_migrate(), the page will still
> be mapped.
>
