On 2022/6/20 20:23, Miaohe Lin wrote:
> On 2022/6/20 17:23, Muchun Song wrote:
>> On Mon, Jun 20, 2022 at 05:04:50PM +0800, Miaohe Lin wrote:
>>> On 2022/6/20 15:54, Huang, Ying wrote:
>>>> Miaohe Lin <[email protected]> writes:
>>>>
>>>>> si->inuse_pages could still be accessed concurrently now. The plain reads
>>>>> outside si->lock critical section, i.e. swap_show and si_swapinfo, which
>>>>> results in data races. But these should be ok because they're just used
>>>>> for showing swap info.
>>>>>
>>>>> Signed-off-by: Miaohe Lin <[email protected]>
>>>>> Reviewed-by: David Hildenbrand <[email protected]>
>>>>> ---
>>>>> mm/swapfile.c | 4 ++--
>>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>>>> index d2bead7b8b70..3fa26f6971e9 100644
>>>>> --- a/mm/swapfile.c
>>>>> +++ b/mm/swapfile.c
>>>>> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v)
>>>>> }
>>>>>
>>>>> bytes = si->pages << (PAGE_SHIFT - 10);
>>>>> - inuse = si->inuse_pages << (PAGE_SHIFT - 10);
>>>>> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
>>>>>
>>>>> file = si->swap_file;
>>>>> len = seq_file_path(swap, file, " \t\n\\");
>>>>> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val)
>>>>> struct swap_info_struct *si = swap_info[type];
>>>>>
>>>>> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
>>>>> - nr_to_be_unused += si->inuse_pages;
>>>>> + nr_to_be_unused += READ_ONCE(si->inuse_pages);
>>>>> }
>>>>> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
>>>>> val->totalswap = total_swap_pages + nr_to_be_unused;
>>>>
>>>> READ_ONCE() should be paired with WRITE_ONCE(). So, change the writer
>>>> side too?
>>>
>>> READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here.
>>> The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine
>>
>> I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should
>> also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining.
>
> I tend to agree with you. READ_ONCE() should be paired with WRITE_ONCE() theoretically. But WRITTE_ONCE()
> is ignored while the commit is introduced. Add Qian Cai for helping verify it. It's very kind of @Qian Cai
> if he could tell us whether WRITTE_ONCE() is ignored deliberately.
Update the email address of Qian Cai.
>
> Thanks all of you. :)
>
>>
>>> to see a not-uptodate value of si->inuse_pages because it's just used for showing swap info. So
>>> WRITE_ONCE() is not obligatory. Or am I miss something?
>>>
>>>>
>>>> Best Regards,
>>>> Huang, Ying
>>>
>>> Thanks!
>>>
>>>> .
>>>>
>>>
>>>
>> .
>>
>
On Mon, Jun 20, 2022 at 08:32:27PM +0800, Miaohe Lin wrote:
> >>>>> --- a/mm/swapfile.c
> >>>>> +++ b/mm/swapfile.c
> >>>>> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v)
> >>>>> }
> >>>>>
> >>>>> bytes = si->pages << (PAGE_SHIFT - 10);
> >>>>> - inuse = si->inuse_pages << (PAGE_SHIFT - 10);
> >>>>> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
> >>>>>
> >>>>> file = si->swap_file;
> >>>>> len = seq_file_path(swap, file, " \t\n\\");
> >>>>> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val)
> >>>>> struct swap_info_struct *si = swap_info[type];
> >>>>>
> >>>>> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
> >>>>> - nr_to_be_unused += si->inuse_pages;
> >>>>> + nr_to_be_unused += READ_ONCE(si->inuse_pages);
> >>>>> }
> >>>>> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
> >>>>> val->totalswap = total_swap_pages + nr_to_be_unused;
> >>>>
> >>>> READ_ONCE() should be paired with WRITE_ONCE(). So, change the writer
> >>>> side too?
> >>>
> >>> READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here.
> >>> The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine
> >>
> >> I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should
> >> also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining.
> >
> > I tend to agree with you. READ_ONCE() should be paired with WRITE_ONCE() theoretically. But WRITTE_ONCE()
> > is ignored while the commit is introduced. Add Qian Cai for helping verify it. It's very kind of @Qian Cai
> > if he could tell us whether WRITTE_ONCE() is ignored deliberately.
The write side should be protected by the lock swap_info_struct::lock. Is
that not the case here?
On Mon, Jun 20, 2022 at 09:46:47AM -0400, Qian Cai wrote:
> On Mon, Jun 20, 2022 at 08:32:27PM +0800, Miaohe Lin wrote:
> > >>>>> --- a/mm/swapfile.c
> > >>>>> +++ b/mm/swapfile.c
> > >>>>> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v)
> > >>>>> }
> > >>>>>
> > >>>>> bytes = si->pages << (PAGE_SHIFT - 10);
> > >>>>> - inuse = si->inuse_pages << (PAGE_SHIFT - 10);
> > >>>>> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
> > >>>>>
> > >>>>> file = si->swap_file;
> > >>>>> len = seq_file_path(swap, file, " \t\n\\");
> > >>>>> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val)
> > >>>>> struct swap_info_struct *si = swap_info[type];
> > >>>>>
> > >>>>> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
> > >>>>> - nr_to_be_unused += si->inuse_pages;
> > >>>>> + nr_to_be_unused += READ_ONCE(si->inuse_pages);
> > >>>>> }
> > >>>>> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
> > >>>>> val->totalswap = total_swap_pages + nr_to_be_unused;
> > >>>>
> > >>>> READ_ONCE() should be paired with WRITE_ONCE(). So, change the writer
> > >>>> side too?
> > >>>
> > >>> READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here.
> > >>> The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine
> > >>
> > >> I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should
> > >> also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining.
> > >
> > > I tend to agree with you. READ_ONCE() should be paired with WRITE_ONCE() theoretically. But WRITTE_ONCE()
> > > is ignored while the commit is introduced. Add Qian Cai for helping verify it. It's very kind of @Qian Cai
> > > if he could tell us whether WRITTE_ONCE() is ignored deliberately.
>
> The write side should be protected by the lock swap_info_struct::lock. Is
> that not the case here?
>
The lock does not protect the read sides. So the write side should be
fixed by WRITTE_ONCE().
Thanks.
On Mon, Jun 20, 2022 at 10:20:07PM +0800, Muchun Song wrote:
> The lock does not protect the read sides. So the write side should be
> fixed by WRITTE_ONCE().
https://lwn.net/Articles/816854/
"Unmarked writes (aligned and up to word size) can be treated as if they had
used WRITE_ONCE() by building with
CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y (also selected by default).
Experience has shown that compilers are much less likely to destructively
optimize in-kernel writes than reads. Some developers might therefore
choose to use READ_ONCE() but omit the corresponding WRITE_ONCE(). Other
developers might prefer the documentation benefits and long-term peace of
mind accruing from explicit use of WRITE_ONCE()..."
Qian Cai <[email protected]> writes:
> On Mon, Jun 20, 2022 at 10:20:07PM +0800, Muchun Song wrote:
>> The lock does not protect the read sides. So the write side should be
>> fixed by WRITTE_ONCE().
>
> https://lwn.net/Articles/816854/
>
> "Unmarked writes (aligned and up to word size) can be treated as if they had
> used WRITE_ONCE() by building with
> CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y (also selected by default).
> Experience has shown that compilers are much less likely to destructively
> optimize in-kernel writes than reads. Some developers might therefore
> choose to use READ_ONCE() but omit the corresponding WRITE_ONCE(). Other
> developers might prefer the documentation benefits and long-term peace of
> mind accruing from explicit use of WRITE_ONCE()..."
Thanks for pointing me to this great article. So although not required
by KCSAN strictly, WRITE_ONCE() is still good for documentation, etc.
Just like we have done for swap_info_struct->highest_bit, etc.
Best Regards,
Huang, Ying
On Tue, Jun 21, 2022 at 09:14:00AM +0800, Huang, Ying wrote:
> Qian Cai <[email protected]> writes:
>
> > On Mon, Jun 20, 2022 at 10:20:07PM +0800, Muchun Song wrote:
> >> The lock does not protect the read sides. So the write side should be
> >> fixed by WRITTE_ONCE().
> >
> > https://lwn.net/Articles/816854/
> >
> > "Unmarked writes (aligned and up to word size) can be treated as if they had
> > used WRITE_ONCE() by building with
> > CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y (also selected by default).
All right, CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC help us avoid KCSAN
complaining.
> > Experience has shown that compilers are much less likely to destructively
> > optimize in-kernel writes than reads. Some developers might therefore
> > choose to use READ_ONCE() but omit the corresponding WRITE_ONCE(). Other
> > developers might prefer the documentation benefits and long-term peace of
> > mind accruing from explicit use of WRITE_ONCE()..."
>
> Thanks for pointing me to this great article. So although not required
> by KCSAN strictly, WRITE_ONCE() is still good for documentation, etc.
> Just like we have done for swap_info_struct->highest_bit, etc.
>
+1
> Best Regards,
> Huang, Ying
>
On 2022/6/21 11:39, Muchun Song wrote:
> On Tue, Jun 21, 2022 at 09:14:00AM +0800, Huang, Ying wrote:
>> Qian Cai <[email protected]> writes:
>>
>>> On Mon, Jun 20, 2022 at 10:20:07PM +0800, Muchun Song wrote:
>>>> The lock does not protect the read sides. So the write side should be
>>>> fixed by WRITTE_ONCE().
>>>
>>> https://lwn.net/Articles/816854/
>>>
>>> "Unmarked writes (aligned and up to word size) can be treated as if they had
>>> used WRITE_ONCE() by building with
>>> CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y (also selected by default).
>
> All right, CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC help us avoid KCSAN
> complaining.
>
>>> Experience has shown that compilers are much less likely to destructively
>>> optimize in-kernel writes than reads. Some developers might therefore
>>> choose to use READ_ONCE() but omit the corresponding WRITE_ONCE(). Other
>>> developers might prefer the documentation benefits and long-term peace of
>>> mind accruing from explicit use of WRITE_ONCE()..."
>>
>> Thanks for pointing me to this great article. So although not required
>> by KCSAN strictly, WRITE_ONCE() is still good for documentation, etc.
>> Just like we have done for swap_info_struct->highest_bit, etc.
>>
>
> +1
I tend to agree with Muchun & Huang, Ying. Thanks all of you.
>
>> Best Regards,
>> Huang, Ying
>>
> .
>