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

[Minchan Kim]

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.

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.

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.