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SubjectRe: [RFC][-mm PATCH 4/8] Memory controller memory accounting (v3)
Paul Menage wrote:
> On 7/20/07, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>> +void __always_inline unlock_meta_page(struct page *page)
>> +{
>> + bit_spin_unlock(PG_metapage, &page->flags);
>> +}
>
> Maybe add a BUG_ON(!test_bit(PG_metapage, &page->flags)) at least for
> development?
>

I'd rather make that a VM_BUG_ON, but that's a good suggestion

>> + mem = rcu_dereference(mm->mem_container);
>> + /*
>> + * For every charge from the container, increment reference
>> + * count
>> + */
>> + css_get(&mem->css);
>> + rcu_read_unlock();
>
> It's not clear to me that this is safe.
>
> If
>
>> +
>> + /*
>> + * If we created the meta_page, we should free it on exceeding
>> + * the container limit.
>> + */
>> + if (res_counter_charge(&mem->res, 1)) {
>> + css_put(&mem->css);
>> + goto free_mp;
>> + }
>> +
>> + lock_meta_page(page);
>> + /*
>> + * Check if somebody else beat us to allocating the meta_page
>> + */
>> + if (page_get_meta_page(page)) {
>
> I think you need to add something like
>
> kfree(mp);
> mp = page_get_meta_page(page);
>
> otherwise you're going to leak the new but unneeded metapage.
>

Yes, good catch! I am surprised I did not check for that.

>> + atomic_inc(&mp->ref_cnt);
>> + res_counter_uncharge(&mem->res, 1);
>> + goto done;
>> + }
>> +
>> + atomic_set(&mp->ref_cnt, 1);
>> + mp->mem_container = mem;
>> + mp->page = page;
>> + page_assign_meta_page(page, mp);
>
> Would it make sense to have the "mp->page = page" be part of
> page_assign_meta_page() for consistency?
>

Yes, that could be done easily.

>> +err:
>> + unlock_meta_page(page);
>> + return -ENOMEM;
>
> The only jump to err: is from a location where the metapage is already
> unlocked. Maybe scrap err: and just do a return -ENOMEM when the
> allocation fails?
>

Sounds good, let me revisit the code.

>> +out_uncharge:
>> + mem_container_uncharge(page_get_meta_page(page));
>
> Wanting to call mem_container_uncharge() on a page and hence having to
> call page_get_meta_page() seems to be more common than wanting to call
> it on a meta page that you already have available. Maybe make
> mem_container_uncharge() be a wrapper that take a struct page and does
> something like mem_container_uncharge_mp(page_get_meta_page(page))
> where mem_container_uncharge_mp() is the raw meta-page version?
>


Yes.. right! Will do, I'll write a wrapper.

> Paul


--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL
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