Message-ID: <CAMgjq7DeJ9SEzMU36Crbzd7d8n2bfsdR+8tNtMBokkD6-mZQEA@mail.gmail.com>
Date: Mon, 5 Feb 2024 22:16:36 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	"Huang, Ying" <ying.huang@...el.com>, Chris Li <chrisl@...nel.org>, 
	Minchan Kim <minchan@...nel.org>, Hugh Dickins <hughd@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, David Hildenbrand <david@...hat.com>, Yu Zhao <yuzhao@...gle.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swap: fix race condition in direct swapin path

On Mon, Feb 5, 2024 at 8:25 PM Barry Song <21cnbao@...il.com> wrote:
>

Hi, Barry

Thanks for the comments.

> On Mon, Feb 5, 2024 at 7:10 PM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > In the direct swapin path, when two or more threads swapin the same entry
> > at the same time, they get different pages (A, B) because swap cache is
> > skipped. 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 modify and swap-out the page again, using the
>
> Even if T0's  swap_read_folio is later than T1, problems can still happen.
> after T1 swaps in and sets ptes, then frees the swap entry. T0 reads zRAM
> later. it will get zero as zRAM will fill zero for freed slot,
>
> static int zram_read_from_zspool(struct zram *zram, struct page *page,
>                                  u32 index)
> {
>        ...
>
>
>                  value = handle ? zram_get_element(zram, index) : 0;
>                 mem = kmap_local_page(page);
>                zram_fill_page(mem, PAGE_SIZE, value);
>                  kunmap_local(mem);
>                 return 0;
>          }
>  }
>
> Even though nobody modifies the data before the page is swapped out to the
> same swap offset as before tT0's orig_pte, T0's pte_same check is still true
> and T0 will map filled zeroed page to pte.
>
> so there is more than one risk besides modified data losses.

Thanks for the complement, I think this is true, and it shares the
same problem of the entry reuse, so this patch also covered this
potential race. I can add more words later to cover this case as well.

>
> > same entry. It break the pte_same check because PTE value is unchanged,
> > causing ABA problem. Then thread (T0) will then install the stalled page
> > (A) into the PTE so new data in page (B) is lost, one possible callstack
> > is like this:
> >
> > CPU0                                CPU1
> > ----                                ----
> > do_swap_page()                      do_swap_page() with same entry
> > <direct swapin path>                <direct swapin path>
> > <alloc page A>                      <alloc page B>
> > swap_readpage() <- read to page A   swap_readpage() <- read to page B
> > <slow on later locks or interrupt>  <finished swapin first>
> > ..                                 set_pte_at()
> >                                     swap_free() <- Now the entry is freed.
> >                                     <write to page B, now page A stalled>
> >                                     <swap out page B using same swap entry>
> > pte_same() <- Check pass, PTE seems
> >               unchanged, but page A
> >               is stalled!
> > swap_free() <- page B content lost!
> > set_pte_at() <- staled page A installed!
> >
> > To fix this, reuse swapcache_prepare which will pin the swap entry using
> > the cache flag, and allow only one thread to pin it. Release the pin
> > after PT unlocked. Racers will simply busy wait since it's a rare
> > and very short event.
> >
> > Other methods like increasing the swap count don't seem to be a good
> > idea after some tests, that will cause racers to fall back to the
> > cached swapin path, two swapin path being used at the same time
> > leads to a much more complex scenario.
> >
> > Reproducer:
> >
> > This race issue can be triggered easily using a well constructed
> > reproducer and patched brd (with a delay in read path) [1]:
> >
> > With latest 6.8 mainline, race caused data loss can be observed easily:
> > $ gcc -g -lpthread test-thread-swap-race.c && ./a.out
> >   Polulating 32MB of memory region...
> >   Keep swapping out...
> >   Starting round 0...
> >   Spawning 65536 workers...
> >   32746 workers spawned, wait for done...
> >   Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss!
> >   Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss!
> >   Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss!
> >   Round 0 Failed, 15 data loss!
>
> i am also reading these codes recently. It is quite unbelievable this
> is really happening
> now. as freeing swaps is returning slot to slots_ret, but allocating
> swap is from slots.
> so if swapfile is large, the chance that the newly allocated swap was
> a recently freed swap
> is close to 0%. but yes, the code does have the risk.

Indeed, for reproducing I used a 32M swap device, and the data being
swapped in/out is large enough to make full use of it. So the
reproduce rate is increased by a lot. It's not a completely fictional
test as some low end device do have smaller swaps, and real world race
could happen in many strange ways.

> >
> > This reproducer spawns multiple threads sharing the same memory region
> > using a small swap device. Every two threads updates mapped pages one by
> > one in opposite direction trying to create a race, with one dedicated
> > thread keep swapping out the data out using madvise.
> >
> > The reproducer created a reproduce rate of about once every 5 minutes,
> > so the race should be totally possible in production.
> >
> > After this patch, I ran the reproducer for over a few hundred rounds
> > and no data loss observed.
> >
> > Performance overhead is minimal, microbenchmark swapin 10G from 32G
> > zram:
> >
> > Before:     10934698 us
> > After:      11157121 us
> > Non-direct: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag)
> >
> > Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device")
> > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
> > Signed-off-by: Kairui Song <kasong@...cent.com>
>
> I will also run your patch on my problem I reported today[1]. will update
> the result to you this week.
>
> [1] https://lore.kernel.org/linux-mm/d4f602db-403b-4b1f-a3de-affeb40bc499@arm.com/T/#m41701d0c0e127cdae636e97a13ab521364a810f4
>

Thanks!

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