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

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Message-ID: <CANeU7QkZBWFO6SeVHtmm73oLu7r0zavePQEYmQfH8opKPH1QWw@mail.gmail.com>
Date: Wed, 17 Sep 2025 22:03:12 -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 Wed, Sep 17, 2025 at 4:38 PM Barry Song <21cnbao@...il.com> wrote:
>
> > > 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.

Hard to qualify what is "similar", in what way it is similar.
Different readers will have different interpretations of what similar
means to them.

> That is correct for most 32-bit and 64-bit systems,
> but not for every machine.

I think I will leave it as for most 64 bit systems, the swap table
size is exactly one page table page size and that is not coincidental.

> 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.

I feel that is a very corner case. I will leave it out of the
document. I want to present a simplified abstracted view. There is
always more detail to distract the simple abstracted view. That is why
we have physics.

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

Yes, that is what I want to say, for most modern 64 bit systems.

Chris