linux-kernel - Re: [PATCH 7/8] pstore: Remove needless lock during console writes

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Date:   Fri, 2 Nov 2018 14:50:33 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 7/8] pstore: Remove needless lock during console writes

On Fri, Nov 02, 2018 at 01:40:06PM -0700, Kees Cook wrote:
> On Fri, Nov 2, 2018 at 11:32 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
> > On Thu, Nov 01, 2018 at 04:51:59PM -0700, Kees Cook wrote:
> >> Since commit 70ad35db3321 ("pstore: Convert console write to use
> >> ->write_buf"), the console writer does not use the preallocated crash
> >> dump buffer any more, so there is no reason to perform locking around it.
> >
> > Out of curiosity, what was the reason for having this preallocated crash
> > buffer in the first place? I thought the 'console' type only did regular
> > kernel console logging, not crash dumps.
> 
> The primary reason is that the dumper needs to write to somewhere and
> we don't know the state of the system (memory allocation may not work
> for example).
> 
> The other frontends tend to run at "sane" locations in the kernel. The
> dumper, however, is quite fragile.

Makes sense. thanks.

> > Also I wonder if Namhyung is still poking around that virtio pstore driver he
> > mentioned in the commit mentioned above. :)
> 
> Did that never land? I thought it mostly had to happen at the qemu end?
> 
> With nvdimm emulation, we can just use ramoops. :)
> 

Yes it seems like it never landed: https://lwn.net/Articles/694742/

One of the nice thing about his virtio set though, vs the nvdimm way is that
he actually gets a directory on the host instead of a backing memory file.
Then he can just list this directory and see all the pstore files as he shows
in his example.

I guess it should not be too hard to write a tool to post-process the nvdimm
images and convert it to files anyway :)

thanks,

 - Joel