Re: [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map pointer

[Michal Hocko]

On Fri 19-01-18 14:21:33, Petr Tesarik wrote:
> On Fri, 19 Jan 2018 13:39:56 +0100
> Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> 
> > On Fri 19-01-18 08:09:08, Petr Tesarik wrote:
> > [...]
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 67f2e3c38939..7522a6987595 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void);
> > >  
> > >  /*
> > >   * We use the lower bits of the mem_map pointer to store
> > > - * a little bit of information.  There should be at least
> > > - * 3 bits here due to 32-bit alignment.
> > > + * a little bit of information.  The pointer is calculated
> > > + * as mem_map - section_nr_to_pfn(pnum).  The result is
> > > + * aligned to the minimum alignment of the two values:
> > > + *   1. All mem_map arrays are page-aligned.
> > > + *   2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
> > > + *      lowest bits.  PFN_SECTION_SHIFT is arch-specific
> > > + *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
> > > + *      worst combination is powerpc with 256k pages,
> > > + *      which results in PFN_SECTION_SHIFT equal 6.
> > > + * To sum it up, at least 6 bits are available.
> > >   */  
> > 
> > This is _much_ better indeed. Do you think we can go one step further
> > and add BUG_ON into the sparse code to guarantee that every mmemap
> > is indeed aligned properly so that SECTION_MAP_LAST_BIT-1 bits are never
> > used?
> 
> This is easy for the section_nr_to_pfn() part. I'd just add:
> 
>   BUILD_BUG_ON(PFN_SECTION_SHIFT < SECTION_MAP_LAST_BIT);
> 
> But for the mem_map arrays... Do you mean adding a run-time BUG_ON into
> all allocation paths?
> 
> Note that mem_map arrays can be allocated by:
> 
>   a) __earlyonly_bootmem_alloc
>   b) memblock_virt_alloc_try_nid
>   c) memblock_virt_alloc_try_nid_raw
>   d) alloc_remap (only arch/tile still has it)
> 
> Some allocation paths are in mm/sparse.c, others are
> mm/sparse-vmemmap.c, so it becomes a bit messy, but since it's
> a single line in each, it may work.

Yeah, it is a mess. So I will leave it up to you. I do not want to block
your comment update which is a nice improvement. So with or without the
runtime check feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
-- 
Michal Hocko
SUSE Labs