On 29.01.20 11:52, Qian Cai wrote:
> The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") could
> had memcg->memsw->watermark been accessed concurrently as reported by
> KCSAN,
>
> Reported by Kernel Concurrency Sanitizer on:
> BUG: KCSAN: data-race in page_counter_try_charge / page_counter_try_charge
>
> read to 0xffff8fb18c4cd190 of 8 bytes by task 1081 on cpu 59:
> page_counter_try_charge+0x4d/0x150 mm/page_counter.c:138
> try_charge+0x131/0xd50 mm/memcontrol.c:2405
> __memcg_kmem_charge_memcg+0x58/0x140
> __memcg_kmem_charge+0xcc/0x280
> __alloc_pages_nodemask+0x1e1/0x450
> alloc_pages_current+0xa6/0x120
> pte_alloc_one+0x17/0xd0
> __pte_alloc+0x3a/0x1f0
> copy_p4d_range+0xc36/0x1990
> copy_page_range+0x21d/0x360
> dup_mmap+0x5f5/0x7a0
> dup_mm+0xa2/0x240
> copy_process+0x1b3f/0x3460
> _do_fork+0xaa/0xa20
> __x64_sys_clone+0x13b/0x170
> do_syscall_64+0x91/0xb47
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> write to 0xffff8fb18c4cd190 of 8 bytes by task 1153 on cpu 120:
> page_counter_try_charge+0x5b/0x150 mm/page_counter.c:139
> try_charge+0x131/0xd50 mm/memcontrol.c:2405
> mem_cgroup_try_charge+0x159/0x460
> mem_cgroup_try_charge_delay+0x3d/0xa0
> wp_page_copy+0x14d/0x930
> do_wp_page+0x107/0x7b0
> __handle_mm_fault+0xce6/0xd40
> handle_mm_fault+0xfc/0x2f0
> do_page_fault+0x263/0x6f9
> page_fault+0x34/0x40
>
> Since watermark could be compared or set to garbage due to load or
> store tearing which would change the code logic, fix it by adding a pair
> of READ_ONCE() and WRITE_ONCE() in those places.
>
> Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
> mm/page_counter.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index de31470655f6..a17841150906 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -82,8 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
> * This is indeed racy, but we can live with some
> * inaccuracy in the watermark.
> */
> - if (new > c->watermark)
> - c->watermark = new;
> + if (new > READ_ONCE(c->watermark))
> + WRITE_ONCE(c->watermark, new);
> }
> }
>
> @@ -135,8 +135,8 @@ bool page_counter_try_charge(struct page_counter *counter,
> * Just like with failcnt, we can live with some
> * inaccuracy in the watermark.
> */
> - if (new > c->watermark)
> - c->watermark = new;
> + if (new > READ_ONCE(c->watermark))
> + WRITE_ONCE(c->watermark, new);
So, if this is racy, isn't it a problem that that "new" could suddenly
be < c->watermark (concurrent writer). So you would use the "higher"
watermark.
--
Thanks,
David / dhildenb
