Re: [GIT PULL] percpu fix for v5.9-rc6

[Arvind Sankar]

On Sat, Sep 19, 2020 at 03:45:56AM +0100, Matthew Wilcox wrote:
> On Fri, Sep 18, 2020 at 06:39:57PM -0400, Arvind Sankar wrote:
> > On Fri, Sep 18, 2020 at 02:18:20PM -0700, Linus Torvalds wrote:
> > > On Fri, Sep 18, 2020 at 2:00 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> > > >
> > > > You could just assert that offsetof(typeof(s),flex) == sizeof(s), no?
> > > 
> > > No, because the whole point is that I want that "sizeof(s)" to *WARN*.
> > 
> > Ouch, offsetof() and sizeof() will give different results in the
> > presence of alignment padding.
> > 
> > https://godbolt.org/z/rqnxTK
> 
> We really should be using offsetof() then.  It's harmless because we're
> currently overallocating, not underallocating.  The test case I did was:
> 

I wonder if there are cases where we know the total size, and are
working out the number of elements in the flexible array by doing
size - sizeof(s).

Would a macro to do the inverse of struct_size(), i.e. get the count
knowing the total size be useful?