Re: copy_page() on a kmalloc-ed page with DEBUG_SLAB enabled (was "zram: do not use copy_page with non-page alinged address")

[Michal Hocko]

On Tue 18-04-17 09:03:19, Minchan Kim wrote:
> On Mon, Apr 17, 2017 at 10:20:42AM -0500, Christoph Lameter wrote:
> > On Mon, 17 Apr 2017, Sergey Senozhatsky wrote:
> > 
> > > Minchan reported that doing copy_page() on a kmalloc(PAGE_SIZE) page
> > > with DEBUG_SLAB enabled can cause a memory corruption (See below or
> > > lkml.kernel.org/r/1492042622-12074-2-git-send-email-minchan@xxxxxxxxxx )
> > 
> > Yes the alignment guarantees do not require alignment on a page boundary.
> > 
> > The alignment for kmalloc allocations is controlled by KMALLOC_MIN_ALIGN.
> > Usually this is either double word aligned or cache line aligned.
> > 
> > > that's an interesting problem. arm64 copy_page(), for instance, wants src
> > > and dst to be page aligned, which is reasonable, while generic copy_page(),
> > > on the contrary, simply does memcpy(). there are, probably, other callpaths
> > > that do copy_page() on kmalloc-ed pages and I'm wondering if there is some
> > > sort of a generic fix to the problem.
> > 
> > Simple solution is to not allocate pages via the slab allocator but use
> > the page allocator for this. The page allocator provides proper alignment.
> > 
> > There is a reason it is called the page allocator because if you want a
> > page you use the proper allocator for it.

Agreed. Using the slab allocator for page sized object is just wasting
cycles and additional metadata.

> It would be better if the APIs works with struct page, not address but
> I can imagine there are many cases where don't have struct page itself
> and redundant for kmap/kunmap.

I do not follow. Why would you need kmap for something that is already
in the kernel space?

> Another approach is the API does normal thing for non-aligned prefix and
> tail space and fast thing for aligned space.
> Otherwise, it would be happy if the API has WARN_ON non-page SIZE aligned
> address.

copy_page is a performance sensitive function and I believe that we do
those tricks exactly for this purpose. Why would we want to add an
overhead for the alignment check or WARN_ON when using unaligned
pointers? I do see that debugging a subtle memory corruption is PITA
but that doesn't imply we should clobber the hot path IMHO.

A big fat warning for copy_page would be definitely helpful though.

-- 
Michal Hocko
SUSE Labs