Re: [PATCH 03/18] mm: memcontrol: drop @compound parameter from memcg charging API

[Joonsoo Kim]

On Mon, Apr 20, 2020 at 06:11:11PM -0400, Johannes Weiner wrote:
> The memcg charging API carries a boolean @compound parameter that
> tells whether the page we're dealing with is a hugepage.
> mem_cgroup_commit_charge() has another boolean @lrucare that indicates
> whether the page needs LRU locking or not while charging. The majority
> of callsites know those parameters at compile time, which results in a
> lot of naked "false, false" argument lists. This makes for cryptic
> code and is a breeding ground for subtle mistakes.
> 
> Thankfully, the huge page state can be inferred from the page itself
> and doesn't need to be passed along. This is safe because charging
> completes before the page is published and somebody may split it.
> 
> Simplify the callsites by removing @compound, and let memcg infer the
> state by using hpage_nr_pages() unconditionally. That function does
> PageTransHuge() to identify huge pages, which also helpfully asserts
> that nobody passes in tail pages by accident.
> 
> The following patches will introduce a new charging API, best not to
> carry over unnecessary weight.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>