Re: [BUG] SLOB's krealloc() seems bust

[Nick Piggin]

On Wednesday 08 October 2008 10:08, Matt Mackall wrote:
> On Tue, 2008-10-07 at 20:31 +0300, Pekka Enberg wrote:
> > Hi Matt,
> >
> > On Tue, Oct 7, 2008 at 8:13 PM, Matt Mackall <mpm@xxxxxxxxxxx> wrote:
> > >> > @@ -515,7 +515,7 @@
> > >> >
> > >> >        sp = (struct slob_page *)virt_to_page(block);
> > >> >        if (slob_page(sp))
> > >> > -               return ((slob_t *)block - 1)->units + SLOB_UNIT;
> > >> > +               return (((slob_t *)block - 1)->units - 1) *
> > >> > SLOB_UNIT;
> > >>
> > >> Hmm. I don't understand why we do the "minus one" thing here. Aren't
> > >> we underestimating the size now?
> > >
> > > The first -1 takes us to the object header in front of the object
> > > pointer. The second -1 subtracts out the size of the header.
> > >
> > > But it's entirely possible I'm off by one, so I'll double-check. Nick?
> >
> > Yeah, I was referring to the second subtraction. Looking at
> > slob_page_alloc(), for example, we compare the return value of
> > slob_units() to SLOB_UNITS(size), so I don't think we count the header
> > in ->units. I mean, we ought to be seeing the subtraction elsewhere in
> > the code as well, no?
>
> Ok, I've looked a bit closer at it and I think we need a different fix.
>
> The underlying allocator, slob_alloc, takes a size in bytes and returns
> an object of that size, with the first word containing the number of
> slob_t units.
>
> kmalloc calls slob_alloc after adding on some space for header and
> architecture padding. This space is not necessarily 1 slob unit:
>
>         unsigned int *m;
>         int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> ...
>                 m = slob_alloc(size + align, gfp, align, node);
>                 *m = size;
>  	        return (void *)m + align;
>
> Note that we overwrite the header with our own size -in bytes-.
> kfree does the reverse:

Right.

>                 int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> 		unsigned int *m = (unsigned int *)(block - align);
>                 slob_free(m, *m + align);
>
> That second line is locating the kmalloc header. All looks good.
>
> The MINALIGN business was introduced by Nick with:
>
>  slob: improved alignment handling
>
> but seems to have missed ksize, which should now be doing the following
> to match:
>
> diff -r 5e32b09a1b2b mm/slob.c
> --- a/mm/slob.c	Fri Oct 03 14:04:43 2008 -0500
> +++ b/mm/slob.c	Tue Oct 07 18:05:15 2008 -0500
> @@ -514,9 +514,11 @@
>  		return 0;
>
>  	sp = (struct slob_page *)virt_to_page(block);
> -	if (slob_page(sp))
> -		return ((slob_t *)block - 1)->units + SLOB_UNIT;
> -	else
> +	if (slob_page(sp)) {
> +		int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> +		unsigned int *m = (unsigned int *)(block - align);
> +		return SLOB_UNITS(*m); /* round up */
> +	} else
>  		return sp->page.private;
>  }

Yes, I came up with nearly the same patch before reading this

--- linux-2.6/mm/slob.c 2008-10-08 14:43:17.000000000 +1100
+++ suth/mm/slob.c      2008-10-08 15:11:06.000000000 +1100
@@ -514,9 +514,11 @@ size_t ksize(const void *block)
                return 0;

        sp = (struct slob_page *)virt_to_page(block);
-       if (slob_page(sp))
-               return (((slob_t *)block - 1)->units - 1) * SLOB_UNIT;
-       else
+       if (slob_page(sp)) {
+               int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
+               unsigned int *m = (unsigned int *)(block - align);
+               return *m + align;
+       } else
                return sp->page.private;
 }

However, mine is lifted directly from kfree, wheras you do something a
bit different. Hmm, ksize arguably could be used to find the underlying
allocated slab size in order to use a little bit more than we'd asked
for. So probably we should really just `return *m` (don't round up or
add any padding).


> That leaves the question of why this morning's patch worked at all,
> given that it was based on how SLOB worked before Nick's patch. But I
> haven't finished working through that. Peter, can I get you to test the
> above?

I didn't have ksize in my slob user test harness, but added a couple of
tests in there, and indeed ksize was returning complete garbage both
before and after the latest patch to slob. I'd say it was simply luck.
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