Re: Direct IO page bouncing got some garbage?

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jun 12, 2026 at 02:54:22PM +0930, Qu Wenruo wrote:
> 
> 
> 在 2026/6/12 11:11, Qu Wenruo 写道:
> > Hi,
> > 
> > Recently I'm trying to make btrfs utilize IOMAP_DIO_BOUNCE, however I'm
> > experiencing weird data corruption.
> > 
> > During test case generic/708, I'm reliably hitting garbage pages at the
> > last 64KiB, the garbage even contains an ELF header.
> 
> Added ftrace shows that, since btrfs has to disable page fault to avoid
> certain deadlock, bio_iov_iter_bounced() failed with a short copy.
> 
> Initially bio_iov_iter_bounced() got a 1MiB page, but copy_iter_from() only
> copied 64K then failed due to the disabled page fault.
> 
> I don't think it's a coincident that the short 64K exactly matches where the
> garbage is (the last 64K).
> 
> I guess it's in the error path we didn't properly revert the iov iter?

Looks like it.  The better option would probably be not to give up on
a short copy, and just reduce the bio size to fit the short copy
even if that wastes a little memory.