Re: [regression] Re: [PATCH v6 06/12] mm/gup: track FOLL_PIN pages

[Alex Williamson]

On Fri, 24 Apr 2020 12:20:03 -0700
John Hubbard <jhubbard@xxxxxxxxxx> wrote:

> On 2020-04-24 11:18, Alex Williamson wrote:
> ...
> > Hi John,
> > 
> > I'm seeing a regression bisected back to this commit (3faa52c03f44
> > mm/gup: track FOLL_PIN pages).  I've attached some vfio-pci test code
> > that reproduces this by mmap'ing a page of MMIO space of a device and
> > then tries to map that through the IOMMU, so this should be attempting
> > a gup/pin of a PFNMAP page.  Previously this failed gracefully (-EFAULT),
> > but now results in:  
> 
> 
> Hi Alex,
> 
> Thanks for this report, and especially for source code to test it, 
> seeing as how I can't immediately spot the problem just from the crash
> data so far.  I'll get set up and attempt a repro.
> 
> Actually this looks like it should be relatively easier than the usual 
> sort of "oops, we leaked a pin_user_pages() or unpin_user_pages() call,
> good luck finding which one" report that I fear the most. :) This one 
> looks more like a crash that happens directly, when calling into the 
> pin_user_pages_remote() code. Which should be a lot easier to solve...
> 
> btw, if you are set up for it, it would be nice to know what source file 
> and line number corresponds to the RIP (get_pfnblock_flags_mask+0x22) 
> below. But if not, no problem, because I've likely got to do the repro 
> in any case.

Hey John,

TBH I'm feeling a lot less confident about this bisect.  This was
readily reproducible to me on a clean tree a bit ago, but now it
eludes me.  Let me go back and figure out what's going on before you
spend any more time on it.  Thanks,

Alex