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

[Chris Li <chrisl@kernel.org>]

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.
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.

Chris