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

[Joe Perches <joe@perches.com>]

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.