Re: [PATCH v4] mm/swap: fix race when skipping swapcache - Linux-stable-mirror

20 Feb 2024

On 2024/2/20 13:32, Kairui Song wrote:
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On Tue, Feb 20, 2024 at 12:49 PM Chengming Zhou zhouchengming@bytedance.com
wrote:
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On 2024/2/20 06:10, Barry Song wrote:
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On Mon, Feb 19, 2024 at 9:21 PM Kairui Song ryncsn@gmail.com wrote:
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From: Kairui Song kasong@tencent.com
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).
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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.
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_read_folio() <- read to page A  swap_read_folio() <- read to page
B
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<slow on later locks or interrupt>   <finished swapin first>
..                                  set_pte_at()
                                     swap_free() <- entry is free
                                     <write to page B, now page A
stalled>
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                                 <swap out page B to same swap

entry>
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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!
And besides, for ZRAM, swap_free() allows the swap device to discard
the entry content, so even if page (B) is not modified, if
swap_read_folio() on CPU0 happens later than swap_free() on CPU1,
it may also cause data loss.
To fix this, reuse swapcache_prepare which will pin the swap entry
using
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the cache flag, and allow only one thread to swap it in, also prevent
any parallel code from putting the entry in the cache. Release the pin
after PT unlocked.
Racers just loop and wait since it's a rare and very short event.
A schedule_timeout_uninterruptible(1) call is added to avoid repeated
page faults wasting too much CPU, causing livelock or adding too much
noise to perf statistics. A similar livelock issue was described in
commit 029c4628b2eb ("mm: swap: get rid of livelock in swapin
readahead")
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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!
This reproducer spawns multiple threads sharing the same memory region
using a small swap device. Every two threads updates mapped pages one
by
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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
Cached:     13155355 us (Dropping SWP_SYNCHRONOUS_IO flag)
Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of
synchronous device")
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Link:
https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
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Reported-by: "Huang, Ying" ying.huang@intel.com
Closes:
https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel....
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Signed-off-by: Kairui Song kasong@tencent.com
Cc: stable@vger.kernel.org

V3:
https://lore.kernel.org/all/20240216095105.14502-1-ryncsn@gmail.com/
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Update from V3:

Use schedule_timeout_uninterruptible(1) for now instead of

schedule() to
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prevent the busy faulting task holds CPU and livelocks [Huang, Ying]
V2:
https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@gmail.com/
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Update from V2:

Add a schedule() if raced to prevent repeated page faults wasting CPU
and add noise to perf statistics.
Use a bool to state the special case instead of reusing existing
variables fixing error handling [Minchan Kim].

V1: https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@gmail.com/
Update from V1:

Add some words on ZRAM case, it will discard swap content on

swap_free
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so the race window is a bit different but cure is the same. [Barry
Song]
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Update comments make it cleaner [Huang, Ying]
Add a function place holder to fix CONFIG_SWAP=n built [SeongJae

Park]
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Update the commit message and summary, refer to SWP_SYNCHRONOUS_IO
instead of "direct swapin path" [Yu Zhao]
Update commit message.
Collect Review and Acks.

include/linux/swap.h |  5 +++++
 mm/memory.c          | 20 ++++++++++++++++++++
 mm/swap.h            |  5 +++++
 mm/swapfile.c        | 13 +++++++++++++
 4 files changed, 43 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4db00ddad261..8d28f6091a32 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -549,6 +549,11 @@ static inline int swap_duplicate(swp_entry_t swp)
        return 0;
 }
+static inline int swapcache_prepare(swp_entry_t swp)
+{

  return 0;



+}



static inline void swap_free(swp_entry_t swp)
 {
 }
diff --git a/mm/memory.c b/mm/memory.c
index 7e1f4849463a..a99f5e7be9a5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3799,6 +3799,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
        struct page *page;
        struct swap_info_struct *si = NULL;
        rmap_t rmap_flags = RMAP_NONE;

  bool need_clear_cache = false;
  bool exclusive = false;
  swp_entry_t entry;
  pte_t pte;



@@ -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
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                   * finish swapin first, free the entry, and



swapout
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                   * 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 */
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                          schedule_timeout_uninterruptible(1);



Not a ideal model, imaging two tasks,
task A  - low priority running on a LITTLE core
task B - high priority and have real-time requirements such as audio,
              graphics running on a big core.
The original code will make B win even if it is a bit later than A as
its CPU is
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much faster to finish swap_read_folio for example from zRAM. task B's
swap-in can finish very soon.
With the patch, B will wait a tick and its real-time performance will be
negatively affected from time to time once low priority and high
priority
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tasks fault in the same PTE and high priority tasks are a bit later than
low priority tasks. This is a kind of priority inversion.
When we support large folio swap-in, things can get worse. For example,
to swap-in 16 or even more pages in one do_swap_page, the chance for
task A and task B located in the same range of 16 PTEs will increase
though they are not located in the same PTE.
Please consider this is not a blocker for this patch. But I will put
the problem
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in my list and run some real tests on Android phones later.
Good point. Late for the discussion, I'm wondering why not get an extra
reference
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on the swap entry, instead of swapcache_prepare()? Then the faster thread
will
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succeed, but can't free the swap entry. Later, the slower thread will
find the
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changed pte value and fail, and free the swap entry. Maybe I missed
something?
Hi, Chengming
That was my initial purpose. Then found a lot of problems with it. Increase
swap count here, it may race with another swap free and end up increasing
the swap count of a freed entry.
That can be fixed with audits and new helpers, but there are many other
potential issues too. One major problem is that after count bump, raced
swap threads will fallback to cached swap in. Pages in swapcache can be
swaped out without allocating an entry, making the problem we were trying
to resolve more serious.
Thanks for your clarification! Right, there are many issues I just ignored...

    