Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a constructor

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 11, 2018 at 04:11:17PM -0500, Christopher Lameter wrote:
> On Wed, 11 Apr 2018, Matthew Wilcox wrote:
> > > Please put this in a code path that is enabled by specifying
> > >
> > > slub_debug
> > >
> > > on the kernel command line.
> >
> > I don't understand.  First, I had:
> >
> > 	if (unlikely(gfpflags & __GFP_ZERO) && object && !WARN_ON_ONCE(s->ctor))
> >
> > and you didn't like that because it was putting checking into a (semi)fast
> > path.  Now you want me to add a check for slub_debug somewhere?  I dont
> > see an existing one I can leverage that will hit on every allocation.
> > Perhaps I'm missing something.
> 
> The WARN_ON is only enabled when you configure and build the kernel with
> debugging enabled (CONFIG_VM_DEBUG). That is a compile time debugging
> feature like supported by SLAB.

Yes.  I want to have an option to check *every single* allocation.

> "slub_debug" enables kmem_cache->flags & SLAB_DEBUG and that forces all
> fastpath processing to be disabled. Thus you can check reliably in the
> slow path only for the GFP_ZERO problem.
> 
> Add the check to the other debug stuff already there. F.e. in
> alloc_debug_processing() or after
> 
> if (kmem_cache_debug(s) ...
> 
> in ____slab_alloc()

I don't see how that works ... can you explain a little more?

I see ___slab_alloc() is called from __slab_alloc().  And I see
slab_alloc_node does this:

        object = c->freelist;
        page = c->page;
        if (unlikely(!object || !node_match(page, node))) {
                object = __slab_alloc(s, gfpflags, node, addr, c);
                stat(s, ALLOC_SLOWPATH);

But I don't see how slub_debug leads to c->freelist always being NULL.
It looks like it gets repopulated from page->freelist in ___slab_alloc()
at the load_freelist label.