Re: [PATCH 1/6] mm/gup_benchmark: Time put_page

[Keith Busch]

On Wed, Oct 10, 2018 at 03:26:55PM -0700, Andrew Morton wrote:
> On Wed, 10 Oct 2018 13:56:00 -0600 Keith Busch <keith.busch@xxxxxxxxx> wrote:
> 
> > We'd like to measure time to unpin user pages, so this adds a second
> > benchmark timer on put_page, separate from get_page.
> > 
> > Adding the field will breaks this ioctl ABI, but should be okay since
> > this an in-tree kernel selftest.
> > 
> > ...
> >
> > --- a/mm/gup_benchmark.c
> > +++ b/mm/gup_benchmark.c
> > @@ -8,7 +8,8 @@
> >  #define GUP_FAST_BENCHMARK	_IOWR('g', 1, struct gup_benchmark)
> >  
> >  struct gup_benchmark {
> > -	__u64 delta_usec;
> > +	__u64 get_delta_usec;
> > +	__u64 put_delta_usec;
> >  	__u64 addr;
> >  	__u64 size;
> >  	__u32 nr_pages_per_call;
> 
> If we move put_delta_usec to the end of this struct, the ABI remains
> back-compatible?

If the kernel writes to a new value appended to the end of the struct,
and the application allocated the older sized struct, wouldn't that
corrupt the user memory?