linux-kernel - Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

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Date:   Fri, 3 Mar 2017 08:50:03 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     'Peter Zijlstra' <peterz@...radead.org>, <mingo@...nel.org>,
        <tglx@...utronix.de>, <walken@...gle.com>, <boqun.feng@...il.com>,
        <kirill@...temov.name>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <iamjoonsoo.kim@....com>,
        <akpm@...ux-foundation.org>, <npiggin@...il.com>,
        <kernel-team@....com>
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

On Thu, Mar 02, 2017 at 06:39:49AM -0800, Matthew Wilcox wrote:
> On Thu, Mar 02, 2017 at 01:45:35PM +0900, byungchul.park wrote:
> > From: Matthew Wilcox [mailto:willy@...radead.org]
> > > On Tue, Feb 28, 2017 at 07:15:47PM +0100, Peter Zijlstra wrote:
> > > > (And we should not be returning to userspace with locks held anyway --
> > > > lockdep already has a check for that).
> > > 
> > > Don't we return to userspace with page locks held, eg during async
> > > directio?
> > 
> > Hello,
> > 
> > I think that the check when returning to user with crosslocks held
> > should be an exception. Don't you think so?
> 
> Oh yes.  We have to keep the pages locked during reads, and we have to
> return to userspace before I/O is complete, therefore we have to return
> to userspace with pages locked.  They'll be unlocked by the interrupt
> handler in page_endio().

Agree.

> Speaking of which ... this feature is far too heavy for use in production
> on pages.  You're almost trebling the size of struct page.  Can we
> do something like make all struct pages share the same lockdep_map?
> We'd have to not complain about holding one crossdep lock and acquiring
> another one of the same type, but with millions of pages in the system,
> it must surely be creating a gargantuan graph right now?

Um.. I will try it for page locks to work with one lockmap. That is also
what Peterz pointed out and what I worried about when implementing..

Thanks for your opinion.