Re: [QUESTION] WARNNING after 3d8e2128f26a ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output")

[yangerkun <yangerkun@huawei.com>]

在 2021/4/7 21:21, Joe Perches 写道:
> On Wed, 2021-04-07 at 20:14 +0800, yangerkun wrote:
>>
>> 在 2021/4/2 22:41, Matthew Wilcox 写道:
>>> On Fri, Apr 02, 2021 at 09:45:12AM +0200, Greg KH wrote:
>>>> Why is the buffer alignment considered a "waste" here?  If that change
>>>> is in Linus's tree and newer kernels (it showed up in 5.4 which was
>>>> released quite a while ago), where are the people complaining about it
>>>> there?
>>>>
>>>> I think backporting 59bb47985c1d ("mm, sl[aou]b: guarantee natural
>>>> alignment for kmalloc(power-of-two)") seems like the correct thing to do
>>>> here to bring things into alignment (pun intended) with newer kernels.
>>>
>>> It's only a waste for slabs which need things like redzones (eg we could
>>> get 7 512-byte allocations out of a 4kB page with a 64 byte redzone
>>> and no alignment ; with alignment we can only get four).  Since slub
>>> can enable/disable redzones on a per-slab basis, and redzones aren't
>>> terribly interesting now that we have kasan/kfence, nobody really cares.
>>>
>>> .
>>>
>>
>> Thanks for your explain! The imfluence seems minimal since the "waste"
>> will happen only when we enable slub_debug.
>>
>> One more question for Joe Perches. Patch v2[1] does not add the
>> alignment check for buf and we add it in v3[2]. I don't see the
>> necessity for this check... Can you help to explain that why we need this?
> 
> It's to make sure it's a PAGE_SIZE aligned buffer.
> It's just so it would not be misused/abused in non-sysfs derived cases.
> 
> .
> 

Thanks! It help me to understand the problem better!