Re: [PATCH] dma-debug: New interfaces to debug dma mapping errors

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Sep 18, 2012 at 01:42:49PM -0600, Shuah Khan wrote:
> On Tue, 2012-09-18 at 15:34 +0200, Joerg Roedel wrote:
> > On Mon, Sep 17, 2012 at 04:45:15PM -0600, Shuah Khan wrote:
> > > Yeah. I will firm up my ideas a bit and summarize in a day or two. Would
> > > like to hear your ideas as well at that time, so we can pick the one
> > > that works the best.
> > 
> > I think the best approach for this functionality is to add a flag to
> > 'struct dma_debug_entry' which tells whether the address has been
> > checked with dma_mapping error or not. On unmap or driver unload you can
> > then check for that flag and print a warning when an unchecked address
> > is detected.
> 
> Was hoping to get comments from you as well. You are original author for
> this dam-debug module.
> 
> Are you ok with the system wide and per device error counts I added? Any
> comments on the overall approach?
> 
> The approach you suggested will cover the cases where drivers fail to
> check good map cases. We won't able to catch failed maps that get used
> without checks. Are you not concerned about these cases? These could
> cause a silent error with wild writes or could bring the system down. Or
> are you recommending changing the infrastructure to track failed maps as
> well?
> 
> I am still pursuing a way to track failed map cases. I combined the flag
> idea with one of the ideas I am looking into. Details below: (if this
> sounds like a reasonable approach, I can do v2 patch and we can discuss
> the code)
> 
> . Add new fields dma_map_errors, dma_map_errors_not_checked,
> dma_unmap_errors, iotlb_overflow_cnt, and flag to struct
> dma_debug_entry. Maybe flag is not even needed if
> dma_map_errors_not_checked can double as status.

Not sure if you need the iotlb_overflow_cnt anymore. Just having
dma_map_errors_not_checked and the dma_map_errors
(which you can increment/decrement) would suffice. Unless you
were thinking to check that dma_map_errors == dma_unmap_errors and
if they != then produce a warning?
> 
> . Enhance dma_debug_init() to create a second table to track failed maps
> with PREALLOC_DMA_DEBUG_ENTRIES/64 = 64. 64 devices probably is good
> enough.
> 
> . Entries added to this new table when debug_dma_map_page() detects
> error when mapping error is detected for the first time. Subsequent
> errors, will increment dma_map_errors, dma_map_errors_not_checked for
> that the device that is tracked by this entry. Note: paddr field could
> work as an index into this table (existing table uses dma_addr)

> 
> . Decrement dma_map_errors_not_checked from debug_dma_mapping_error(),
> clear the flag.
> 
> . check_unmap() when it detects mapping error, checks flag (status) and
> prints warn message.

<nods>
> 
> -- Shuah