Re: [PATCH 0/2] mm: swap: mTHP swap allocator base on swap cluster order

[Kairui Song <ryncsn@gmail.com>]

On Thu, May 30, 2024 at 10:54 AM Huang, Ying <ying.huang@intel.com> wrote:
>
> Chris Li <chrisl@kernel.org> writes:
>
> > Hi Ying,
> >
> > On Wed, May 29, 2024 at 1:57 AM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> Chris Li <chrisl@kernel.org> writes:
> >>
> >> > I am spinning a new version for this series to address two issues
> >> > found in this series:
> >> >
> >> > 1) Oppo discovered a bug in the following line:
> >> > +               ci = si->cluster_info + tmp;
> >> > Should be "tmp / SWAPFILE_CLUSTER" instead of "tmp".
> >> > That is a serious bug but trivial to fix.
> >> >
> >> > 2) order 0 allocation currently blindly scans swap_map disregarding
> >> > the cluster->order.
> >>
> >> IIUC, now, we only scan swap_map[] only if
> >> !list_empty(&si->free_clusters) && !list_empty(&si->nonfull_clusters[order]).
> >> That is, if you doesn't run low swap free space, you will not do that.
> >
> > You can still swap space in order 0 clusters while order 4 runs out of
> > free_cluster
> > or nonfull_clusters[order]. For Android that is a common case.
>
> When we fail to allocate order 4, we will fallback to order 0.  Still
> don't need to scan swap_map[].  But after looking at your below reply, I
> realized that the swap space is almost full at most times in your cases.
> Then, it's possible that we run into scanning swap_map[].
> list_empty(&si->free_clusters) &&
> list_empty(&si->nonfull_clusters[order]) will become true, if we put too
> many clusters in si->percpu_cluster.  So, if we want to avoid to scan
> swap_map[], we can stop add clusters in si->percpu_cluster when swap
> space runs low.  And maybe take clusters out of si->percpu_cluster
> sometimes.

Stop adding when it runs low seems too late, there could still be a
free cluster stuck on a CPU, and not getting scanned, right?

> Another issue is nonfull_cluster[order1] cannot be used for
> nonfull_cluster[order2].  In definition, we should not fail order 0
> allocation, we need to steal nonfull_cluster[order>0] for order 0
> allocation.  This can avoid to scan swap_map[] too.  This may be not
> perfect, but it is the simplest first step implementation.  You can
> optimize based on it further.

This can be extended to allow any order < MAX_ORDER to steal from
higher order, which might increase fragmentation though.

So this is looking more and more like a buddy allocator, and that
should be the long term solution.