Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc

Matthew Wilcox
2019-02-26T16:21:01Z

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.