Re: [PATCH] mm, slub: place the trace before freeing memory in kmem_cache_free()

[Matthew Wilcox]

On Sat, Oct 30, 2021 at 08:23:12PM +0800, Muchun Song wrote:
> On Sat, Oct 30, 2021 at 6:12 PM Yunfeng Ye <yeyunfeng@xxxxxxxxxx> wrote:
> >
> > After the memory is freed, it may be allocated by other CPUs and has
> > been recorded by trace. So the timing sequence of the memory tracing is
> > inaccurate.
> >
> > For example, we expect the following timing sequeuce:
> >
> >     CPU 0                 CPU 1
> >
> >   (1) alloc xxxxxx
> >   (2) free  xxxxxx
> >                          (3) alloc xxxxxx
> >                          (4) free  xxxxxx
> >
> > However, the following timing sequence may occur:
> >
> >     CPU 0                 CPU 1
> >
> >   (1) alloc xxxxxx
> >                          (2) alloc xxxxxx
> >   (3) free  xxxxxx
> >                          (4) free  xxxxxx
> >
> > So place the trace before freeing memory in kmem_cache_free().
> 
> Could you tell me what problem you have encountered
> here?

It's confusing to see the memory allocated before it's freed.  If you're
unaware of this problem, you might think it was being used after free
because (1) happened a long time ago, so you see (2) immediately followed
by (3) and then see the memory being used.

The patch makes sense to me.