Re: [PATCHv2 0/5] fix direct io device mapper errors

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On Mon, Nov 14, 2022 at 06:31:36AM -0500, Mikulas Patocka wrote:
> 
> 
> On Fri, 11 Nov 2022, Keith Busch wrote:
> 
> > > There are other DM targets that override logical_block_size in their
> > > .io_hints hook (writecache, ebs, zoned). Have you reasoned through why
> > > those do _not_ need updating too?
> > 
> > Yeah, that's a good question. The ones that have a problem all make
> > assumptions about a bio's bv_offset being logical block size aligned,
> > and each of those is accounted for here. Everything else looks fine with
> > respect to handling offsets.
> 
> Unaligned bv_offset should work - because XFS is sending such bios. If you 
> compile the kernel with memory debugging, kmalloc returns unaligned 
> memory. XFS will allocate a buffer with kmalloc, test if it crosses a page 
> boundary, if not, use the buffer, if yes, free the buffer and allocate a 
> full page.
> 
> There have been device mapper problems about unaligned bv_offset in the 
> past and I have fixed them.
> 
> Unaligned bv_length is a problem for the affected targets.

kmalloc is physically contiguous, and bio's support multi-page bvecs for
these, so the bv_len is always aligned if the kmalloc is sized
correctly. The unaligned offsets become a problem with virtually
contiguous buffers since individual bv lengths might not be block size
aligned when bv offsets exist.

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