Re: [PATCH v3] mm/swap: fix race when skipping swapcache

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On 18.02.24 08:59, Huang, Ying wrote:
David Hildenbrand <david@xxxxxxxxxx> writes:

On 16.02.24 10:51, Kairui Song 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.
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
<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>
                                       <swap out page B to 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!
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
the cache flag, and allow only one thread to pin it. Release the pin
after PT unlocked. Racers will simply wait since it's a rare and very
short event. A schedule() call is added to avoid wasting too much CPU
or adding too much noise to perf statistics
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 use the
swap cache again. Parallel swapin using different methods 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!
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]
Reported-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
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].
V2:
https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@xxxxxxxxx/
Update from V1:
- Add some words on ZRAM case, it will discard swap content on swap_free so the race window is a bit different but cure is the same. [Barry Song]
- Update comments make it cleaner [Huang, Ying]
- Add a function place holder to fix CONFIG_SWAP=n built [SeongJae Park]
- 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.
V1:
https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@xxxxxxxxx/
   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..7059230d0a54 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
+			 * 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();
+				goto out;
+			}
+			need_clear_cache = true;
+

I took a closer look at __read_swap_cache_async() and it essentially
does something similar.

Instead of returning, it keeps retrying until it finds that
swapcache_prepare() fails for another reason than -EEXISTS (e.g.,
freed concurrently) or it finds the entry in the swapcache.

So if you would succeed here on a freed+reused swap entry,
__read_swap_cache_async() would simply retry.

It spells that out:

		/*
		 * We might race against __delete_from_swap_cache(), and
		 * stumble across a swap_map entry whose SWAP_HAS_CACHE
		 * has not yet been cleared.  Or race against another
		 * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
		 * in swap_map, but not yet added its folio to swap cache.
		 */

Whereby we could not race against this code here as well where we
speculatively set SWAP_HAS_CACHE and might never add something to the swap
cache.


I'd probably avoid the wrong returns and do something even closer to
__read_swap_cache_async().

while (true) {
	/*
	 * Fake that we are trying to insert a page into the swapcache, to
	 * serialize against concurrent threads wanting to do the same.
	 * [more from your description]
	 */
	ret = swapcache_prepare(entry);
	if (likely(!ret)
		/*
		 * Move forward with swapin, we'll recheck if the PTE hasn't
		 * changed later.
		 */
		break;
	else if (ret != -EEXIST)
		goto out;

The swap entry may be kept in swap cache for long time.  For example, it
may be read into swap cache via MADV_WILLNEED.

Right, we'd have to check for the swapcache.

I briefly thought about just factoring out what we have in __read_swap_cache_async() and reusing here. Similar problem to solve, and quite a lot of duplicate code.

But not worth the churn in a simple fix. We could explore that option
as a cleanup on top.

--
Cheers,

David / dhildenb





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