linux-kernel - Re: [PATCH v4 01/15] docs/mm: add document for swap table

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Message-ID: <CACePvbUabb+L6Z9Nb-41fLR-FMhj--cDWSbnXtCj3rpqXModiQ@mail.gmail.com>
Date: Wed, 17 Sep 2025 09:48:46 -0700
From: Chris Li <chrisl@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Hugh Dickins <hughd@...gle.com>, 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, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v4 01/15] docs/mm: add document for swap table

On Tue, Sep 16, 2025 at 4:48 PM Barry Song <21cnbao@...il.com> wrote:
> > In your above two examples, what is the respected swap cluster swap entry size?
> > In other words, how much entry does one swap cluster hold?
> >
> > Sorry I am not very familiar with the ARM page tables.
>
> Oh, my mistake—I recalculated:
>
> For a 16 KB page size, SWAPCLUSTER_SIZE will be 2^11 = 2048, so the swap
> table is 2048 * 8 = 16 KB.

So my original description is correct in the sense that, the 16KB page
size, swap table is 16KB, this is not coincidental.

> For a 64 KB page size, SWAPCLUSTER_SIZE will be 2^13 = 8192, so the swap
> table is 8192 * 8 = 64 KB.

Same here. For 64 KB page size, the swap table is 64KB as you just told me.
I am just trying to give a bit of a glimpse of where I get the
intuition for swap tables.

>
> This approach still seems to work, so the 32-bit system appears to be
> the only exception. However, I’m not entirely sure that your description
> of “the second last level” is correct. I believe it refers to the PTE,
> which corresponds to the last level, not the second-to-last.
> In other words, how do you define the second-to-last level page table?

The second-to-last level page table page holds the PMD. The last level
page table holds PTE.
Cluster size is HPAGE_PMD_NR = 1<<HPAGE_PMD_ORDER
I was thinking of a PMD entry but the actual page table page it points
to is the last level.
That is a good catch. Let me see how to fix it.

What I am trying to say is that, swap table size should match to the
PTE page table page size which determines the cluster size. An
alternative to understanding the swap table is that swap table is a
shadow PTE page table containing the shadow PTE matching to the page
that gets swapped out to the swapfile. It is arranged in the swapfile
swap offset order. The intuition is simple once you find the right
angle to view it. However it might be a mouthful to explain.

I am fine with removing it, on the other hand it removes the only bit
of secret sauce which I try to give the reader a glimpse of my
intuition of the swap table.

Thanks for catching that.

Chris