Re: [PATCH resend] slub: Add back check for free nonslab objects

[Matthew Wilcox]

On Mon, Sep 27, 2021 at 03:53:47PM +0800, Kefeng Wang wrote:
> On 2021/9/27 15:22, Vlastimil Babka wrote:
> > On 9/27/21 04:15, Kefeng Wang wrote:
> > > After commit ("f227f0faf63b slub: fix unreclaimable slab stat for bulk
> > > free"), the check for free nonslab page is replaced by VM_BUG_ON_PAGE,
> > > which only check with CONFIG_DEBUG_VM enabled, but this config may
> > > impact performance, so it only for debug.
> > > 
> > > Commit ("0937502af7c9 slub: Add check for kfree() of non slab objects.")
> > > add the ability, which should be needed in any configs to catch the
> > > invalid free, they even could be potential issue, eg, memory corruption,
> > > use after free and double-free, so replace VM_BUG_ON_PAGE to WARN_ON, and
> > > add dump_page() to help use to debug the issue.
> > There are other situations in SLUB (such as with smaller allocations that
> > don't go directly to page allocator) where use after free and double-free
> > are undetected in non-debug configs, and it's expected that anyone debugging
> > them will enable slub_debug or even DEBUG_VM. Why should this special case
> > with nonslab pages be different?
> 
> I want the check back in kfree, this one is used  widely in driver, and the
> probability
> 
> of problem occurred is bigger in driver, especially in some out of tree
> drivers.

Why would we want to improve life for out of tree drivers?  Drivers should
be in-tree.  That's been the Linux Way for thirty years.

I remain sceptical that dump_page() is actually useful for debugging
drivers anyway.  dump_stack(), I could see -- that'll tell you which
driver called kfree() on a bogus pointer.  But how does dump_page() help?