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

[Barry Song <21cnbao@gmail.com>]

On Tue, Sep 2, 2025 at 11:59 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> On Tue, Sep 2, 2025 at 6:46 PM Barry Song <21cnbao@gmail.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?

>
> 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.

Thanks
Barry