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

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Date:   Fri, 1 Sep 2017 07:15:41 -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 Fri, 1 Sep 2017 10:16:21 +0200
Vlastimil Babka <vbabka@...e.cz> wrote:
 
> > Right, but that should work with the latest trace-cmd. Does it?  
> 
> Hmm, by "sparse memory model without vmemmap" I don't mean there's a
> number instead of "vmemmap_base". I mean CONFIG_SPARSEMEM=y
> 
> Then __pfn_to_page() looks like this:
> 
> #define __page_to_pfn(pg)                                       \
> ({      const struct page *__pg = (pg);                         \
>         int __sec = page_to_section(__pg);                      \
>         (unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \
> })
> 
> Then the part of format file looks like this:
> 
> REC->pfn != -1UL ? ({ unsigned long __pfn = (REC->pfn); struct mem_section *__sec = __pfn_to_section(__pfn); __section_mem_map_addr(__sec) + __pfn; }) : ((void *)0)

Ouch.

> 
> The section things involve some array lookups, so I don't see how we
> could pass it to tracing userspace. Would we want to special-case
> this config to store both pfn and struct page in the trace frame? And
> make sure the simpler ones work despite all the exsisting gotchas?
> I'd rather say we should either store both pfn and page pointer, or
> just throw away the page pointer as the pfn is enough to e.g. match
> alloc and free, and also much more deterministic.

Write up a patch and we'll take a look.

-- Steve