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

[Sergey Senozhatsky]

On (04/17/17 10:20), 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.

sure, but at the same time it's not completely uncommon and unseen thing

~/_next$ git grep kmalloc | grep PAGE_SIZE | wc -l
75

not all, if any, of those pages get into copy_page(), of course. may be... hopefully.
so may be a warning would make sense and save time some day. but up to MM
people to decide.


p.s. Christoph, FYI, gmail automatically marked your message
     as a spam message, for some reason.

	-ss