Re: [PATCH] Possible fix to recent data corruption on HEAD since USB2

On Thu, Apr 16, 2009 at 01:36:18PM -0400, John Baldwin wrote:
> Due to some good sleuthing by avg_at_,
> there is a patch that might fix the recent 
> reports of data corruption on current.  It would explain some of the recent 
> reports where a file that was read would have missing gaps of bytes.  The 
> problem is with the BUS_DMA_KEEP_PG_OFFSET changes to bus_dma.  When a bounce 
> page was used by USB2, the changes to bus_dma would actually change the 
> starting virtual and physical addresses of the bounce page.  When the bounce 
> page was no longer needed it was left in this bogus state.  Later if another 
> device used the same bounce page for DMA it would use the wrong offset and 
> address.  The issue there is if the second device was doing a full page of 
> I/O.  In that case the DMA from the device would actually spill over into the 
> next page which could in theory be used by another DMA request.  It could 
> also break alignment assumptions (since the previous PG_OFFSET may not be 
> aligned and the bus_dma code assumes bounce pages for the !PG_OFFSET case are 
> page aligned).  The quick fix is to always restore the bounce page to the 
> normal state when a PG_OFFSET DMA request is finished.   I'd actually prefer 
> not ever touching the page's starting addresses, but those changes would be 
> more invasive I believe.
> 
> http://www.FreeBSD.org/~jhb/patches/dma_sg.patch
> 
Am I right that hardware prerequisite in order to observe these problems
is amd64 + 4Gb or more of RAM?
Is it possible to fabricate some (artificial) test case to stress this
particular situation (interleaved use of bounce pages by USB and some other
device (?HDD?))?
Asking because as I understand the data corruption is silent
and affected consumer (of bounce pages) should have some mechanism
of detecting this (e.g. zfs' CRCs).
In my case stess testing unpatched system till UFS filesystems are dead
is no fun...

Alexey.