Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

From: Vlastimil Babka
Date: Tue Aug 27 2019 - 08:02:00 EST

On 8/27/19 1:02 PM, Kirill A. Shutemov wrote:
> On Tue, Aug 27, 2019 at 08:01:39AM +0200, Michal Hocko wrote:
>> On Mon 26-08-19 16:15:38, Kirill A. Shutemov wrote:
>>> Unmapped completely pages will be freed with current code. Deferred split
>>> only applies to partly mapped THPs: at least on 4k of the THP is still
>>> mapped somewhere.
>> Hmm, I am probably misreading the code but at least current Linus' tree
>> reads page_remove_rmap -> [page_remove_anon_compound_rmap ->\ deferred_split_huge_page even
>> for fully mapped THP.
> Well, you read correctly, but it was not intended. I screwed it up at some
> point.
> See the patch below. It should make it work as intened.
> It's not bug as such, but inefficientcy. We add page to the queue where
> it's not needed.

But that adding to queue doesn't affect whether the page will be freed
immediately if there are no more partial mappings, right? I don't see
deferred_split_huge_page() pinning the page.
So your patch wouldn't make THPs freed immediately in cases where they
haven't been freed before immediately, it just fixes a minor
inefficiency with queue manipulation?