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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
