On Mon, Jan 06, 2014 at 12:54:22PM -0500, Mikulas Patocka wrote: > Hi > > On Mon, 6 Jan 2014, Joonsoo Kim wrote: > > > Hello, > > > > I'm surprised that this VM_BUG_ON() has not been triggered until now. It was > > introduced in 2007 by commit (b5fab14). Maybe there is no person who test > > with CONFIG_DEBUG_VM. > > Last time I tried it, PS-RISC didn't work with CONFIG_DEBUG_VM at all. > > > There is one more bug report same as this. > > * possible regression on 3.13 when calling flush_dcache_page > > (lkml.org/lkml/2013/12/12/255) > > That link doesn't show anything. > > > As mentioned in the description of commit (b5fab14), slab object may not be > > properly aligned and use of page oriented function to this object can be > > dangerous. I searched the XFS code and found that they only try to allocate > > multiple of 512 bytes, so there is no problem for now. But, IMHO, it is better > > not to use slab objects for this purpose. > > If slab debugging is enabled, kmalloc memory is not aligned. > > In XFS in xfs_buf_allocate_memory they test if the kmalloc memory crosses > page boundary - if it does, they free the kmalloc memory and allocate a > full page. Maybe this approach could still run into problems with some > bus-master adapters that assume alignment in hardware... > > > dm-bufio also does I/O to slab-allocated buffers, but it allocates the > object from slab (not kmalloc) with proper alignment. Hello, Okay. I see. Thanks for good explanation. > > > And I rapidly searched every callsites of page_mapping() and, IMHO, this > > patch would work correctly. But possibly reverting original commit is > > better solution. > > Reverting the original commit wouldn't fix that VM_BUG_ON. Initially, I thought that VM_BUG_ON() isn't wrong and it was better to remove the callsites where do I/O with slab-allocated buffers, because doing I/O with slab-allocated buffers needs a great care. So I didn't fully agreed with your patch and recommended to revert original commit yesterday. After reverting that, I would attempt to remove the callsites. But, now, I change my thought, because of your explanation. There are already some users to do I/O with slab-allocated buffers and they already did it with some cares, so I guess that admitting this usage is more beneficial than forbidding it. Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
