linux-kernel - Re: [PATCH v4 2/4] tracing: Have reserve_mem use phys_to

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Message-ID: <20250401184008.77ca9599@gandalf.local.home>
Date: Tue, 1 Apr 2025 18:40:08 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Linus
 Torvalds <torvalds@...ux-foundation.org>, Mark Rutland
 <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Vincent Donnefort
 <vdonnefort@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport
 <rppt@...nel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v4 2/4] tracing: Have reserve_mem use phys_to_virt() and
 separate from memmap buffer

On Tue, 1 Apr 2025 18:16:40 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:


> Note, your patch to free the persistent ring buffer wasn't fully
> functional, as it only did the "vunmap()". That doesn't return the buffer
> back to the buddy allocator. What you saw was just the freeing of all the
> other descriptors that make up a trace instance.
> 
> Before this patch:
> 
>   ~# free
>                  total        used        free      shared  buff/cache   available
>   Mem:         8185908      297404     7825896         916      162288     7888504
>   Swap:        7812092           0     7812092
>   ~# rmdir /sys/kernel/tracing/instances/boot_mapped
>   ~# free
>                  total        used        free      shared  buff/cache   available
>   Mem:         8206384      297956     7845904         916      162260     7908428
>   Swap:        7812092           0     7812092
> 
> Amount freed: 7845904 - 7825896 = 20008 (20M)
> 
> After this patch:
> 
>   ~# free
>                  total        used        free      shared  buff/cache   available
>   Mem:         8185912      301808     7820696         920      162860     7884104
>   Swap:        7812092           0     7812092
>   ~# rmdir /sys/kernel/tracing/instances/boot_mapped
>   ~# free
>                  total        used        free      shared  buff/cache   available
>   Mem:         8226868      295968     7867644         920      162836     7930900
>   Swap:        7812092           0     7812092
> 
> Amount freed: 7867644 - 7820696 = 46948 (46M)!

Bah! My patch is buggy. Yeah, your code released the memory properly, as you had:

	reserve_mem_release_by_name()

which does do free_reserve_area() on the memory mapping.

Which means this patch is buggy and freed the same memory twice.

OK, time for v5 to fix that :-p

-- Steve