linux-kernel - Re: [concept & "good taste" review] persistent store

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Message-ID: <AANLkTikhtWjFBQmA111Q+2JGRudrRNAmegGThMagr4vF@mail.gmail.com>
Date:	Sat, 18 Dec 2010 15:06:57 -0800
From:	Tony Luck <tony.luck@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu,
	greg@...ah.com, akpm@...ux-foundation.org, ying.huang@...el.com,
	Borislav Petkov <bp@...en8.de>,
	David Miller <davem@...emloft.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Kyungmin Park <kmpark@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [concept & "good taste" review] persistent store

On Sat, Dec 18, 2010 at 10:23 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> You want to have a ring of events, and into that ring you also have a
> "this event has been read" pointer. And you _never_ overwrite entries
> that haven't been read yet, because quite frankly, if you get some
> nasty memory corruption, you may end up with a thousand oopses in
> rapid succession, and the latter ones are likely to be just fallout
> from the earlier ones. So you definitely don't want to overwrite the
> earlier ones, because they are more likely to contain the clues about
> the actual original cause.
>
> At the same time, you do want to have the capability of saying "I've
> seen this", and let it be overwritten. For example, if we end up
> teaching syslogd or something like that to use this, syslogd would
> write the oops to disk, do a fdatasync() on the oops file, and after
> it's stable on disk it can mark it "read".
>
> Also, since this is very much about persistent storage, I think any
> events from a previous boot that still exists should be marked "read".
> You still want to be able to read them (so marking something "read"
> does not mean that it goes away), but if a new oops happens, you don't
> want some old entries from long ago to stop it from being written to
> persistent storage. So if you don't have any syslogd or any other tool
> that saves things to disk, you'd still get the new oopses into
> persistent storage.
>
> Doesn't that sound like the best of both worlds?

It sounds like an excellent heuristic for how the platform layer
should manage the persistent store when space is tight. But
I think that I can still keep my /dev/pstore filesystem as a
presentation layer to make the bits available to the user in
a device independent way.

Or perhaps the pstore layer can help with the implementation
of the heuristic. It knows what items are in the pstore, so it
could build & maintain the "ring" and pass the id of the least
wanted item down to the platform layer whenever it wants to
write a record ... with the platform layer giving a status to
say whether it had to delete that item to make space for the
new one?

-Tony
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