Re: [linus:master] [mm] d99e3140a4: BUG:KCSAN:data-race_in_folio_remove_rmap_ptes/print_report

[Miaohe Lin]

On 2024/5/28 15:43, David Hildenbrand wrote:
> Am 28.05.24 um 09:11 schrieb kernel test robot:
>>
>>
>> Hello,
>>
>> kernel test robot noticed "BUG:KCSAN:data-race_in_folio_remove_rmap_ptes/print_report" on:
>>
>> commit: d99e3140a4d33e26066183ff727d8f02f56bec64 ("mm: turn folio_test_hugetlb into a PageType")
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>>
>> [test failed on linus/master      c760b3725e52403dc1b28644fb09c47a83cacea6]
>> [test failed on linux-next/master 3689b0ef08b70e4e03b82ebd37730a03a672853a]
>>
>> in testcase: trinity
>> version: trinity-i386-abe9de86-1_20230429
>> with following parameters:
>>
>>     runtime: 300s
>>     group: group-04
>>     nr_groups: 5
>>
>>
>>
>> compiler: gcc-13
>> test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
>>
>> (please refer to attached dmesg/kmsg for entire log/backtrace)
>>
>>
>> we noticed this issue does not always happen. we also noticed there are
>> different random KCSAN issues for both this commit and its parent. but below
>> 4 only happen on this commit with not small rate and keep clean on parent.
>>
> 
> Likely that's just a page_type check racing against concurrent
> mapcount changes.
> 
> In __folio_rmap_sanity_checks() we check
>     VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> 
> To make sure we don't get hugetlb folios in the wrong rmap code path. That
> can easily race with concurrent mapcount changes, just like any other
> page_type checks that end up in folio_test_type/page_has_type e.g., from
> PFN walkers.
> 
> Load tearing in these functions shouldn't really result in false positives
> (what we care about), but READ_ONCE shouldn't hurt or make a difference.
> 
> 
> From b03dc9bf27571442d886d8da624a4e4f737433f2 Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@xxxxxxxxxx>
> Date: Tue, 28 May 2024 09:37:20 +0200
> Subject: [PATCH] mm: read page_type using READ_ONCE
> 
> KCSAN complains about possible data races: while we check for a
> page_type -- for example for sanity checks -- we might concurrently
> modify the mapcount that overlays page_type.
> 
> Let's use READ_ONCE to avoid laod tearing (shouldn't make a difference)
> and to make KCSAN happy.
> 
> Note: nothing should really be broken besides wrong KCSAN complaints.
> 
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202405281431.c46a3be9-lkp@xxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

LGTM. Thanks for fixing.

Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Thanks.
.