linux-kernel - Re: [PATCH 6/9] mm, swap: use the swap table for the swap cache and switch API

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Message-ID: <CAGsJ_4z7BagquRVHoJq612h-_n87F_VuTqRxOnSCoox2qpYmRg@mail.gmail.com>
Date: Wed, 3 Sep 2025 14:31:40 +1200
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm <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>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/9] mm, swap: use the swap table for the swap cache and
 switch API

On Wed, Sep 3, 2025 at 2:12 PM Kairui Song <ryncsn@...il.com> wrote:
>
> Barry Song <21cnbao@...il.com> 于 2025年9月3日周三 07:44写道：
> >
> > On Tue, Sep 2, 2025 at 11:59 PM Kairui Song <ryncsn@...il.com> wrote:
> > >
> > > On Tue, Sep 2, 2025 at 6:46 PM Barry Song <21cnbao@...il.com> wrote:
> > > >
> > > > > +
> > > > > +/*
> > > > > + * Helpers for accessing or modifying the swap table of a cluster,
> > > > > + * the swap cluster must be locked.
> > > > > + */
> > > > > +static inline void __swap_table_set(struct swap_cluster_info *ci,
> > > > > +                                   unsigned int off, unsigned long swp_tb)
> > > > > +{
> > > > > +       VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
> > > > > +       atomic_long_set(&ci->table[off], swp_tb);
> > > > > +}
> > > > > +
> > > > > +static inline unsigned long __swap_table_get(struct swap_cluster_info *ci,
> > > > > +                                            unsigned int off)
> > > > > +{
> > > > > +       VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
> > > > > +       return atomic_long_read(&ci->table[off]);
> > > > > +}
> > > > > +
> > > >
> > > > Why should this use atomic_long instead of just WRITE_ONCE and
> > > > READ_ONCE?
> > >
> > > Hi Barry,
> > >
> > > That's a very good question. There are multiple reasons: I wanted to
> > > wrap all access to the swap table to ensure there is no non-atomic
> > > access, since it's almost always wrong to read a folio or shadow value
> > > non-atomically from it. And users should never access swap tables
> > > directly without the wrapper helpers. And in another reply, as Chris
> > > suggested, we can use atomic operations to catch potential issues
> > > easily too.
> >
> > I still find it odd that for writing we have the si_cluster lock,
> > but for reading a long, atomic operations don’t seem to provide
> > valid protection against anything. For example, you’re still
> > checking folio_lock and folio_test_swapcache() in such cases.
> >
> >
> > >
> > > And most importantly, later phases can make use of things like
> > > atomic_cmpxchg as a fast path to update the swap count of a swap
> > > entry. That's a bit hard to explain for now, short summary is the swap
> > > table will be using a single atomic for both count and folio tracking,
> > > and we'll clean up the folio workflow with swap, so it should be
> > > possible to get an final consistency of swap count by simply locking
> > > the folio, and doing atomic_cmpxchg on swap table with folio locked
> > > will be safe.
> >
> > I’m still missing this part: if the long stores a folio pointer,
> > how could it further save the swap_count?
>
> We use PFN here, it works very well, saves more memory and the
> performance is very good, tested using the 28 series patch which have
> already implemented this:
> https://lore.kernel.org/linux-mm/20250514201729.48420-25-ryncsn@gmail.com/

Alright, I see. With the PFN, we already have the folio.

>
> >
> > >
> > > For now using atomic doesn't bring any overhead or complexity, only
> > > make it easier to implement other code. So I think it should be good.
> >
> > I guess it depends on the architecture. On some arches, it might
> > require irq_disable plus a spinlock.
>
> If an arch can't provide atomic for basic access to a long, then that
> justified the usage of atomic here even more.. The read has to be
> atomic since swap cache lookup is lockless, so the write should be
> atomic too.

I actually confused atomic_64 with atomic_long. After double-checking, I
found that on almost all architectures, atomic_long_set/read are effectively
write_once and read_once. However, many architectures override these in the
common header file. This seems like a spot worth cleaning up for those
architectures.

>
> Xchg / cmpxchg is a bit more complex on some arches, they are optional
> in the swap table anyway. We can use them only on arches that provide
> better performance with atomic. I believe most arches do. For the xchg
> debug check, it can be dropped once we are confident enough that there
> is no hidden bug.

Thanks
Barry