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

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Message-ID: <CAGsJ_4xON7fYg1VvcjLOsgBb_Wp4ruC+vdA4Q496GH1jXunU1A@mail.gmail.com>
Date: Tue, 2 Sep 2025 19:15:01 +0800
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Chris Li <chrisl@...nel.org>, 
	Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>, 
	David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] mm, swap: implement dynamic allocation of swap table

On Sat, Aug 23, 2025 at 3:21 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.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?

> This way, we can reduce the memory usage of large swap device
> significantly.
>
> This idea to dynamically release unused swap cluster data was initially
> suggested by Chris Li while proposing the cluster swap allocator and
> I found it suits the swap table idea very well.
>

Thanks
Barry