Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing extra unlock check

[Kairui Song <ryncsn@gmail.com>]

Chris Li <chrisl@kernel.org> 于2023年11月23日周四 04:57写道：
>
> Hi Kairui,
>
> On Wed, Nov 22, 2023 at 9:33 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> > > There are two different charges. Memcg charging and memcg swapin charging.
> > > The folio_put will do the memcg discharge, the corresponding memcg
> > > charge is in follio allocation.
> >
> > Hi Chris,
> >
> > I didn't get your idea here... By "memcg swapin charge", do you mean
> > "memory.swap.*"? And "memcg charging" means "memory.*"?. There is no
>
> Sorry I should have used the function name then there is no ambiguity.
> "memcg swapin charge" I mean function mem_cgroup_swapin_charge_folio().
> This function will look up the swap entry and find the memcg by swap entry then
> charge to that memcg.
>
> > memcg charge related code in folio allocation (alloc_pages_mpol),
> > actually the mem_cgroup_swapin_charge_folio here is doing memcg charge
> > not memcg swapin charge. Swapin path actually need to uncharge
> > "memory.swap" by mem_cgroup_swapin_uncharge_swap in later part of this
> > function.
>
> I still think you have a bug there.
>
> Take this make up example:
> Let say the for loop runs 3 times and the 3rd time breaks out the for loop.
> The original code will call:
> filemap_get_folio() 3 times
> folio_put() 2 times
> mem_cgroup_swapin_charge_folio() 1 time.
>
> With your patch, it will call:
> filemap_get_folio() 3 times
> folio_put() 2 times
> mem_cgroup_swapin_charge_folio() 3 times.
>
> Do you see the behavior difference there?

Hi Chris.

folio_put will discharge a page if it's charged, in original code the
2 folio_put call simply free the page since it's not charged. But in
this patch, folio_put will cancel previous
mem_cgroup_swapin_charge_folio call, so actually the 3
mem_cgroup_swapin_charge_folio calls will only charge once. (2 calls
was cancelled by folio_put).

I think this is making it confusing indeed and causing more trouble in
error path (the uncharge could be more expensive than unlock check),
will rework this part.