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

Ming Lei
2019-02-27T01:43:37Z

On Tue, Feb 26, 2019 at 08:19:12AM -0800, Matthew Wilcox wrote:
> 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.

I have mentioned several times that it is triggered on xfs over
loop/dio, however it may be addressed by falling back to buffered IO
in case unaligned buffer.

Please see lo_rw_aio().

Thanks,
Ming