Re: [PATCH 8/9] mm, swap: implement dynamic allocation of swap table

[Kairui Song <ryncsn@gmail.com>]

Barry Song <21cnbao@gmail.com> 于 2025年9月3日周三 08:03写道：
>
> On Wed, Sep 3, 2025 at 1:17 AM Chris Li <chrisl@kernel.org> wrote:
> >
> > On Tue, Sep 2, 2025 at 4:15 AM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Sat, Aug 23, 2025 at 3:21 AM Kairui Song <ryncsn@gmail.com> wrote:
> > > >
> > > > From: Kairui Song <kasong@tencent.com>
> > > >
> > > > Now swap table is cluster based, which means free clusters can free its
> > > > table since no one should modify it.
> > > >
> > > > There could be speculative readers, like swap cache look up, protect
> > > > them by making them RCU safe. All swap table should be filled with null
> > > > entries before free, so such readers will either see a NULL pointer or
> > > > a null filled table being lazy freed.
> > > >
> > > > On allocation, allocate the table when a cluster is used by any order.
> > > >
> > >
> > > Might be a silly question.
> > >
> > > Just curious—what happens if the allocation fails? Does the swap-out
> > > operation also fail? We sometimes encounter strange issues when memory is
> > > very limited, especially if the reclamation path itself needs to allocate
> > > memory.
> > >
> > > Assume a case where we want to swap out a folio using clusterN. We then
> > > attempt to swap out the following folios with the same clusterN. But if
> > > the allocation of the swap_table keeps failing, what will happen?
> >
> > I think this is the same behavior as the XArray allocation node with no memory.
> > The swap allocator will fail to isolate this cluster, it gets a NULL
> > ci pointer as return value. The swap allocator will try other cluster
> > lists, e.g. non_full, fragment etc.
>
> What I’m actually concerned about is that we keep iterating on this
> cluster. If we try others, that sounds good.
>
> > If all of them fail, the folio_alloc_swap() will return -ENOMEM. Which
> > will propagate back to the try to swap out, then the shrink folio
> > list. It will put this page back to the LRU.
> >
> > The shrink folio list either free enough memory (happy path) or not
> > able to free enough memory and it will cause an OOM kill.
> >
> > I believe previously XArray will also return -ENOMEM at insert a
> > pointer and not be able to allocate a node to hold that ponter. It has
> > the same error poperation path. We did not change that.
>
> Yes, I agree there was an -ENOMEM, but the difference is that we
> are allocating much larger now :-)
>
> One option is to organize every 4 or 8 swap slots into a group for
> allocating or freeing the swap table. This way, we avoid the worst
> case where a single unfreed slot consumes a whole swap table, and
> the allocation size also becomes smaller. However, it’s unclear
> whether the memory savings justify the added complexity and effort.
>
> Anyway, I’m glad to see the current swap_table moving towards merge
> and look forward to running it on various devices. This should help
> us see if it causes any real issues.

Thanks for the insightful review.

I do plan to implement a shrinker to compact the swap table of idle /
full clusters when under pressure. It will be done at the very end.
Things will be much cleaner by then so it's easier to do. And
currently it seems the memory usage is quite good already.

>>
> Thanks
> Barry
>