linux-kernel - Re: [PATCH] support polling of /proc/swaps

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Message-Id: <20101019163837.c2a67d0b.akpm@linux-foundation.org>
Date:	Tue, 19 Oct 2010 16:38:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg KH <greg@...ah.com>
Subject: Re: [PATCH] support polling of /proc/swaps

On Wed, 20 Oct 2010 01:25:47 +0200
Kay Sievers <kay.sievers@...y.org> wrote:

> On Tue, 2010-10-19 at 15:31 -0700, Andrew Morton wrote:
> > On Tue, 19 Oct 2010 11:19:16 +0200
> > Kay Sievers <kay.sievers@...y.org> wrote:
> 
> > It's a bit sad that we have to add quite a pile of infrastructure to
> > make a procfs file pollable.  I wonder if it's possible to provide some
> > core support for this, and reduce the amount of code at each particular
> > handler site.
> 
> You mean something like adding the event counter to the seq_file? There
> is /proc/self/mounts,mountinfo and /proc/swaps so far, I think.

Don't know - I was just waving hands about wondering if we really need
to add 48 new lines of code each time we want to make a procfs file
pollable.

> > Also, I wonder how we are to communicate the existence of this feature
> > to our users.  Nobody will look in Documentation/filesystems/.  Is
> > there a manpage?  Seems not...
> 
> Hmm, 'man 5 proc'?

yes, that mentions /proc/swaps.

> > > +static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
> > > +static int proc_poll_event;
> > 
> > Please pick a lock to protect proc_poll_event.
> 
> An atomic_t should do it too, right?

It does.

> > Then document that
> > locking here, when you also document proc_poll_event ;)
> 
> The actual value has no meaning at all, it just tells that something
> happened if it has changed.
> 

bah.

--- a/mm/swapfile.c~proc-swaps-support-polling-fix
+++ a/mm/swapfile.c
@@ -60,6 +60,7 @@ static struct swap_info_struct *swap_inf
 static DEFINE_MUTEX(swapon_mutex);
 
 static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
+/* Activity counter to indicate that a swapon or swapoff has occurred */
 static atomic_t proc_poll_event = ATOMIC_INIT(0);
 
 static inline unsigned char swap_count(unsigned char ent)
_

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