linux-kernel - Re: [PATCH 1/5] tracing, mm: Record pfn instead of pointer to struct page

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Date:   Thu, 31 Aug 2017 10:44:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org
Subject: Re: [PATCH 1/5] tracing, mm: Record pfn instead of pointer to
 struct page

On Thu, 31 Aug 2017 16:31:36 +0200
Vlastimil Babka <vbabka@...e.cz> wrote:


> > Which version of trace-cmd failed? It parses for me. Hmm, the
> > vmemmap_base isn't in the event format file. It's the actually address.
> > That's probably what failed to parse.  
> 
> Mine says 2.6. With 4.13-rc6 I get FAILED TO PARSE.

Right, but you have the vmemmap_base in the event format, which can't
be parsed by userspace because it has no idea what the value of the
vmemmap_base is.

> 
> >   
> >>
> >> I'm quite sure it's due to the "page=%p" part, which uses pfn_to_page().
> >> The events/kmem/mm_page_alloc/format file contains this for page:
> >>
> >> REC->pfn != -1UL ? (((struct page *)vmemmap_base) + (REC->pfn)) : ((void *)0)  
> > 


> >> On older 4.4-based kernel:
> >>
> >> REC->pfn != -1UL ? (((struct page *)(0xffffea0000000000UL)) + (REC->pfn)) : ((void *)0)  
> > 
> > This is what I have on 4.13-rc7
> >   
> >>
> >> This also fails to parse, so it must be the struct page part?  
> > 
> > Again, what version of trace-cmd do you have?  
> 
> On the older distro it was 2.0.4

Right. That's probably why it failed to parse here. If you installed
the latest trace-cmd from the git repo, it probably will parse fine.

> 
> >   
> >>
> >> I think the problem is, even if ve solve this with some more
> >> preprocessor trickery to make the format file contain only constant
> >> numbers, pfn_to_page() on e.g. sparse memory model without vmmemap is
> >> more complicated than simple arithmetic, and can't be exported in the
> >> format file.
> >>
> >> I'm afraid that to support userspace parsing of the trace data, we will
> >> have to store both struct page and pfn... or perhaps give up on reporting
> >> the struct page pointer completely. Thoughts?  
> > 
> > Had some thoughts up above.  
> 
> Yeah, it could be made to work for some configurations, but see the part
> about "sparse memory model without vmemmap" above.

Right, but that should work with the latest trace-cmd. Does it?

-- Steve