Re: [PATCH v4] mm/swap: fix race when skipping swapcache

[Barry Song]

On Tue, Feb 20, 2024 at 4:42 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Tue, Feb 20, 2024 at 9:31 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, 19 Feb 2024 16:20:40 +0800 Kairui Song <ryncsn@xxxxxxxxx> wrote:
> >
> > > From: Kairui Song <kasong@xxxxxxxxxxx>
> > >
> > > When skipping swapcache for SWP_SYNCHRONOUS_IO, if two or more threads
> > > swapin the same entry at the same time, they get different pages (A, B).
> > > Before one thread (T0) finishes the swapin and installs page (A)
> > > to the PTE, another thread (T1) could finish swapin of page (B),
> > > swap_free the entry, then swap out the possibly modified page
> > > reusing the same entry. It breaks the pte_same check in (T0) because
> > > PTE value is unchanged, causing ABA problem. Thread (T0) will
> > > install a stalled page (A) into the PTE and cause data corruption.
> > >
> > > @@ -3867,6 +3868,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > >       if (!folio) {
> > >               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> > >                   __swap_count(entry) == 1) {
> > > +                     /*
> > > +                      * Prevent parallel swapin from proceeding with
> > > +                      * the cache flag. Otherwise, another thread may
> > > +                      * finish swapin first, free the entry, and swapout
> > > +                      * reusing the same entry. It's undetectable as
> > > +                      * pte_same() returns true due to entry reuse.
> > > +                      */
> > > +                     if (swapcache_prepare(entry)) {
> > > +                             /* Relax a bit to prevent rapid repeated page faults */
> > > +                             schedule_timeout_uninterruptible(1);
> >
> > Well this is unpleasant.  How often can we expect this to occur?
> >
>
> The chance is very low, using the current mainline kernel and ZRAM,
> even with threads set to race on purpose using the reproducer I
> provides, for 647132 page faults it occured 1528 times (~0.2%).
>
> If I run MySQL and sysbench with 128 threads and 16G buffer pool, with
> 6G cgroup limit and 32G ZRAM, it occured 1372 times for 40 min,
> 109930201 page faults in total (~0.001%).

it might not be a problem for throughput. but for real-time and tail latency,
this hurts. For example, this might increase dropping frames of UI which
is an important parameter to evaluate performance :-)

BTW, I wonder if ying's previous proposal - moving swapcache_prepare()
after swap_read_folio() will further help decrease the number?

Thanks
Barry