Re: [PATCH -mm -v7 2/9] mm, memcg: Support to charge/uncharge multiple swap entries

["Huang\, Ying" <ying.huang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org> writes:

> On Tue, Mar 28, 2017 at 01:32:02PM +0800, Huang, Ying wrote:
>> @@ -5908,16 +5907,19 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>>  		css_put(&memcg->css);
>>  }
>>  
>> -/*
>> - * mem_cgroup_try_charge_swap - try charging a swap entry
>> +/**
>> + * mem_cgroup_try_charge_swap - try charging a set of swap entries
>>   * @page: page being added to swap
>> - * @entry: swap entry to charge
>> + * @entry: the first swap entry to charge
>> + * @nr_entries: the number of swap entries to charge
>>   *
>> - * Try to charge @entry to the memcg that @page belongs to.
>> + * Try to charge @nr_entries swap entries starting from @entry to the
>> + * memcg that @page belongs to.
>>   *
>>   * Returns 0 on success, -ENOMEM on failure.
>>   */
>> -int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
>> +int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry,
>> +			       unsigned int nr_entries)
>
> I've pointed this out before,

Yes.  And I have replied to your original comments too :-)

> but there doesn't seem to be a reason to
> pass @nr_entries when we have the struct page. Why can't this function
> just check PageTransHuge() by itself?

Because sometimes we need to charge one swap entry for a THP.  Please
take a look at the original add_to_swap() implementation.  For a THP,
one swap entry will be allocated and charged to the mem cgroup before
the THP is split.  And I think it is not easy to change this, because we
don't want to split THP if the mem cgroup for swap exceeds its limit.

Best Regards,
Huang, Ying