Re: [PATCH v9 2/4] ring-buffer: Flush and stop persistent ring buffer on panic

[Google]

On Wed, 18 Mar 2026 10:53:26 -0400
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> On 2026-03-18 10:19, Masami Hiramatsu (Google) wrote:
> > On Wed, 11 Mar 2026 10:32:29 +0900
> > "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:
> > 
> >> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> >>
> >> On real hardware, panic and machine reboot may not flush hardware cache
> >> to memory. This means the persistent ring buffer, which relies on a
> >> coherent state of memory, may not have its events written to the buffer
> >> and they may be lost. Moreover, there may be inconsistency with the
> >> counters which are used for validation of the integrity of the
> >> persistent ring buffer which may cause all data to be discarded.
> >>
> >> To avoid this issue, stop recording of the ring buffer on panic and
> >> flush the cache of the ring buffer's memory.
> > 
> > Hmm, on some architectures, flush_cache_vmap() is implemented using
> > on_each_cpu() which waits IPI. But that does not safe in panic notifier
> > because it is called after smp_send_stop().
> > 
> > Since this cache flush issue is currently only confirmed on arm64,
> > I would like to make it doing nothing (do { } while (0)) by default.
> 
> FWIW, I've sent a related series a while ago about flushing pmem
> areas to memory on panic:
> 
> https://lore.kernel.org/lkml/20240618154157.334602-3-mathieu.desnoyers@xxxxxxxxxxxx/
> 

Ah, nice!

> When reading your patch, I feel like I'm missing something, so please bear with
> me for a few questions:
> 
> - What exactly are you trying to flush ? By "flush" do you mean
>    evince cache lines or write back cache lines ? (I expect you aim
>    at the second option)

Yes, I need to write back cache lines, at least it can be
read after hot reboot. (not evict cache)

> 
> - AFAIU, you are not trying to evince cache lines after creation
>    of a new virtual mapping (which is the documented intent of
>    flush_cache_vmap).

Ah, OK. That's a good point!
(anyway I will replace it with do { } while (0) in the next version.)

>    
> - AFAIU flush_cache_vmap maps to no-code on arm64 (asm-generic), what am
>    I missing ? It makes sense to be a no-op because AFAIR arm64 does not
>    have to deal with virtually aliasing caches.

Yeah, so my patch also introduces arm64 specific implementation.

> 
> see commit 8690bbcf3b7 ("Introduce cpu_dcache_is_aliasing() across all architectures")

OK, let me check.

> 
> The arch_wb_cache_pmem is specific to pmem, which is not exactly what you want
> to use, but on arm64 it's implemented as:
> 
>          /* Ensure order against any prior non-cacheable writes */
>          dmb(osh);
>          dcache_clean_pop((unsigned long)addr, (unsigned long)addr + size);
> 
> Which I think has the writeback semantic you are looking for, and AFAIU should no
> require IPIs (at least on arm64) to flush cache lines across the entire system.

Yes, that's what I need.

Thank you!


> 
> Cheers,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>