linux-kernel - Re: [PATCH V2 1/2] mm/damon/dbgfs: Modify Damon dbfs interface dependency in Kconfig

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Date:   Tue, 23 Nov 2021 19:25:52 +0800
From:   Xin Hao <xhao@...ux.alibaba.com>
To:     SeongJae Park <sj@...nel.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/2] mm/damon/dbgfs: Modify Damon dbfs interface
 dependency in Kconfig

Hi Park:

On 11/23/21 6:38 PM, SeongJae Park wrote:
> On Sun, 21 Nov 2021 22:07:04 +0800 Xin Hao <xhao@...ux.alibaba.com> wrote:
>
>> If you want to support "DAMON_DBGFS" in config file, it only depends on
>> any one of "DAMON_VADDR" and "DAMON_PADDR", and sometimes we just want to
>> use damon virtual address function, but it is unreasonable to include "DAMON_PADDR"
>> in config file which cause the damon/paddr.c be compiled, so there fix it.
> Seems the above lines are not well wrapped[1].
>
> [1] https://docs.kernel.org/process/submitting-patches.html?highlight=75#the-canonical-patch-format
>
>> Signed-off-by: Xin Hao <xhao@...ux.alibaba.com>
>> ---
>>   include/linux/damon.h | 12 ++++++++++++
>>   mm/damon/Kconfig      |  2 +-
>>   2 files changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/damon.h b/include/linux/damon.h
>> index 8a73e825e0d5..00ad96f2ec10 100644
>> --- a/include/linux/damon.h
>> +++ b/include/linux/damon.h
>> @@ -463,11 +463,23 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
>>   #ifdef CONFIG_DAMON_VADDR
>>   void damon_va_set_primitives(struct damon_ctx *ctx);
>>   bool damon_va_target_valid(void *t);
>> +#else
>> +static inline void damon_va_set_primitives(struct damon_ctx *ctx) {}
>> +static inline bool damon_va_target_valid(void *t)
>> +{
>> +	return false;
>> +}
>>   #endif	/* CONFIG_DAMON_VADDR */
>>   
>>   #ifdef CONFIG_DAMON_PADDR
>>   void damon_pa_set_primitives(struct damon_ctx *ctx);
>>   bool damon_pa_target_valid(void *t);
>> +#else
>> +static inline void damon_pa_set_primitives(struct damon_ctx *ctx) {}
>> +static inline bool damon_pa_target_valid(void *t)
>> +{
>> +	return false;
>> +}
>>   #endif	/* CONFIG_DAMON_PADDR */
>>   
>>   #endif	/* _DAMON_H */
>> diff --git a/mm/damon/Kconfig b/mm/damon/Kconfig
>> index 5bcf05851ad0..971ffc496596 100644
>> --- a/mm/damon/Kconfig
>> +++ b/mm/damon/Kconfig
>> @@ -54,7 +54,7 @@ config DAMON_VADDR_KUNIT_TEST
>>   
>>   config DAMON_DBGFS
>>   	bool "DAMON debugfs interface"
>> -	depends on DAMON_VADDR && DAMON_PADDR && DEBUG_FS
>> +	depends on DAMON_VADDR || DAMON_PADDR && DEBUG_FS
> Due to the lack of parentheses, this allows enabling DAMON_DBGFS while DEBUG_FS
> is disabled.
>
> Sorry to say this but this makes me unsure if this patch is sufficiently
> tested.  I am also unsure if this patch is carefully handling all possible
> corner cases in appropriate ways.  For example, on kernels that doesn't having
> 'CONFIG_DAMON_PADDR=y', this patch would still make
> 'echo paddr > $debugfs/damon/target_ids' success.  Is that what you are
> intending?

Yes, you are all right, I have not tested as above, sorry, i did not 
fully consider the impact on the code after doing this,

Please ignore this patch.

> And, have you confirmed that will never make any issue, both in
> kernel space and the user space?  Could the behavioral change make the user
> space confused?
>
> Basically, reducing the dependency is a good idea.  But, IMHO, the benefit
> seems not so big.  After all, efficiency was not a first goal of DAMON debugfs
> interface.  That's why it is implemented on debugfs rather than its own
> specialized/optimized file system.  Better way for the efficiency of the user
> space interface might be inventing such DAMON's own efficient file system, like
> tracepoints had its interface on debugfs but now uses its own file system,
> tracefs.
>
> So, unless you make me believe the benefit is big enough and all possible
> corner cases are well handled and sufficiently tested, I'm sorry but I would
> not be convinced with this change.

I think it makes sense to separate PADDR & VADDR, users can freely 
choose dbfs + vaddr or dbfs + paddr;

Doing this can make the code look like it doesn’t mix together，i will 
continue to try to separate this part

of the code later.

>
>
> Thanks,
> SJ
>
>>   	help
>>   	  This builds the debugfs interface for DAMON.  The user space admins
>>   	  can use the interface for arbitrary data access monitoring.
>> -- 
>> 2.31.0

-- 
Best Regards!
Xin Hao