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

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Message-ID: <CAGsJ_4y8yTX48ESHKgLNCvM1M1_gY9uGnD4qiz8n+gD47Zd1Hg@mail.gmail.com>
Date: Thu, 18 Sep 2025 07:37:50 +0800
From: Barry Song <21cnbao@...il.com>
To: Chris Li <chrisl@...nel.org>
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

> > 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.

Perhaps you could describe the swap table as similar to a PTE page table
representing the swap cache mapping.
That is correct for most 32-bit and 64-bit systems,
but not for every machine.

The only exception is a 32-bit system with a 64-bit physical address
(Large Physical Address Extension, LPAE), which uses a 4 KB PTE table
but a 2 KB swap table because the pointer is 32 bit while each page
table entry is 64 bit.

Maybe we can simply say that the number of entries in the swap table
is the same as in a PTE page table?

>
> Thanks for catching that.
>
> Chris

Thanks
Barry