The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
had memcg->memsw->failcnt and ->watermark could be 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
__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
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 the failcnt and watermark are tolerant of some inaccuracy, a data
race will not be harmful, thus mark them as intentional data races with
the data_race() macro.
Signed-off-by: Qian Cai <[email protected]>
---
mm/page_counter.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/mm/page_counter.c b/mm/page_counter.c
index de31470655f6..13934636eafd 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 (data_race(new > c->watermark))
+ data_race(c->watermark = new);
}
}
@@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
* This is racy, but we can live with some
* inaccuracy in the failcnt.
*/
- c->failcnt++;
+ data_race(c->failcnt++);
*fail = c;
goto failed;
}
@@ -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 (data_race(new > c->watermark))
+ data_race(c->watermark = new);
}
return true;
--
2.21.0 (Apple Git-122.2)
On Tue 28-01-20 23:20:19, Qian Cai wrote:
> The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> had memcg->memsw->failcnt and ->watermark could be 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
> __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
> 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 the failcnt and watermark are tolerant of some inaccuracy, a data
> race will not be harmful, thus mark them as intentional data races with
> the data_race() macro.
I am not familiar with KCSAN and git grep for data_race on the current
linux-next doesn't really show any users of this macro. Is there a
general consensus that data_race is going to be used to silence all
KCSAN false positives?
> Signed-off-by: Qian Cai <[email protected]>
> ---
> mm/page_counter.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index de31470655f6..13934636eafd 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 (data_race(new > c->watermark))
> + data_race(c->watermark = new);
> }
> }
>
> @@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
> * This is racy, but we can live with some
> * inaccuracy in the failcnt.
> */
> - c->failcnt++;
> + data_race(c->failcnt++);
> *fail = c;
> goto failed;
> }
> @@ -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 (data_race(new > c->watermark))
> + data_race(c->watermark = new);
> }
> return true;
>
> --
> 2.21.0 (Apple Git-122.2)
--
Michal Hocko
SUSE Labs
On Wed, 29 Jan 2020 at 09:51, Michal Hocko <[email protected]> wrote:
>
> On Tue 28-01-20 23:20:19, Qian Cai wrote:
> > The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> > had memcg->memsw->failcnt and ->watermark could be 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
Why are the line numbers for the remaining symbols missing? Doesn't
scripts/decode_stacktrace.sh give you all line numbers?
[ As an aside: if you want to use what syzbot uses to put line numbers
on symbols, which is a bit faster:
https://github.com/google/syzkaller/tree/master/tools/syz-symbolize
https://github.com/google/syzkaller/blob/master/docs/linux/setup.md
then 'go build tools/syz-symbolize'. ]
> > __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
> > 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 the failcnt and watermark are tolerant of some inaccuracy, a data
> > race will not be harmful, thus mark them as intentional data races with
> > the data_race() macro.
>
> I am not familiar with KCSAN and git grep for data_race on the current
> linux-next doesn't really show any users of this macro. Is there a
> general consensus that data_race is going to be used to silence all
> KCSAN false positives?
It was discussed here:
https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/
If they're intentional data races that should remain, data_race() is
one option. There are 4 options (other than address the data race) to
deal with 'false positives':
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/dev-tools/kcsan.rst#n101
That being said, every use of data_race() needs to be justified, and
not just applied without understanding the issue. See below.
> > Signed-off-by: Qian Cai <[email protected]>
> > ---
> > mm/page_counter.c | 10 +++++-----
> > 1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/page_counter.c b/mm/page_counter.c
> > index de31470655f6..13934636eafd 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 (data_race(new > c->watermark))
> > + data_race(c->watermark = new);
These should be using 'READ_ONCE' and 'WRITE_ONCE' for c->watermark.
Store or load tearing would change the logic here, since the
comparison might see garbage.
> > }
> > }
> >
> > @@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
> > * This is racy, but we can live with some
> > * inaccuracy in the failcnt.
> > */
> > - c->failcnt++;
> > + data_race(c->failcnt++);
This is probably fine.
> > *fail = c;
> > goto failed;
> > }
> > @@ -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 (data_race(new > c->watermark))
> > + data_race(c->watermark = new);
This should be READ_ONCE / WRITE_ONCE.
> > }
> > return true;
> >
> > --
> > 2.21.0 (Apple Git-122.2)
>
> --
> Michal Hocko
> SUSE Labs
> On Jan 29, 2020, at 4:06 AM, Marco Elver <[email protected]> wrote:
>
> Why are the line numbers for the remaining symbols missing? Doesn't
> scripts/decode_stacktrace.sh give you all line numbers?
I used scripts/faddr2line and never used decode_stacktrace.sh before. I did not insert other line numbers because it can be easily determined by looking at the function itself through an code browser.
>
> [ As an aside: if you want to use what syzbot uses to put line numbers
> on symbols, which is a bit faster:
> https://github.com/google/syzkaller/tree/master/tools/syz-symbolize
> https://github.com/google/syzkaller/blob/master/docs/linux/setup.md
> then 'go build tools/syz-symbolize'. ]
That is good to know.
> On Jan 29, 2020, at 4:06 AM, Marco Elver <[email protected]> wrote:
>
> These should be using 'READ_ONCE' and 'WRITE_ONCE' for c->watermark.
> Store or load tearing would change the logic here, since the
> comparison might see garbage.
I originally thought that it probably does not matter because it is racy there by doing lockless access anyway. Another thread could change the value at anytime.
Now, I agree set it to a garbage due to a data race could be quite unpleasant there.