On Tue, Feb 26, 2019 at 08:14:33AM -0800, Darrick J. Wong wrote: > On Tue, Feb 26, 2019 at 06:04:40AM -0800, Matthew Wilcox wrote: > > On Tue, Feb 26, 2019 at 09:42:48PM +0800, Ming Lei wrote: > > > On Tue, Feb 26, 2019 at 05:02:30AM -0800, Matthew Wilcox wrote: > > > > Wait, we're imposing a ridiculous amount of complexity on XFS for no > > > > reason at all? We should just change this to 512-byte alignment. Tying > > > > it to the blocksize of the device never made any sense. > > > > > > OK, that is fine since we can fallback to buffered IO for loop in case of > > > unaligned dio. > > > > > > Then something like the following patch should work for all fs, could > > > anyone comment on this approach? > > > > That's not even close to what I meant. > > > > diff --git a/fs/direct-io.c b/fs/direct-io.c > > index ec2fb6fe6d37..dee1fc47a7fc 100644 > > --- a/fs/direct-io.c > > +++ b/fs/direct-io.c > > @@ -1185,18 +1185,20 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode, > > Wait a minute, are you all saying that /directio/ is broken on XFS too?? > XFS doesn't use blockdev_direct_IO anymore. > > I thought we were talking about alignment of XFS metadata buffers > (xfs_buf.c), which is a very different topic. > > As I understand the problem, in non-debug mode the slab caches give > xfs_buf chunks of memory that are aligned well enough to work, but in > debug mode the slabs allocate slightly more bytes to carry debug > information which pushes the returned address up slightly, thus breaking > the alignment requirements. > > So why can't we just move the debug info to the end of the object? If > our 512 byte allocation turns into a (512 + a few more) bytes we'll end > up using 1024 bytes on the allocation regardless, so it shouldn't matter > to put the debug info at offset 512. If the reason is fear that kernel > code will scribble off the end of the object, then return (*obj + 512). > Maybe you all have already covered this, though? I don't know _what_ Ming Lei is saying. I thought the problem was with slab redzones, which need to be before and after each object, but apparently the problem is with KASAN as well.
