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Date: Wed, 20 Oct 2010 01:25:47 +0200
From: Kay Sievers <kay.sievers@...y.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
Greg KH <greg@...ah.com>
Subject: Re: [PATCH] support polling of /proc/swaps
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.
> 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'?
> > +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?
> 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.
From: Kay Sievers <kay.sievers@...y.org>
Subject: support polling of /proc/swaps
System management wants to subscribe to changes in swap
configuration. Make /proc/swaps pollable like /proc/mounts.
Signed-Off-By: Kay Sievers <kay.sievers@...y.org>
---
mm/swapfile.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 47 insertions(+), 1 deletion(-)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -30,6 +30,7 @@
#include <linux/capability.h>
#include <linux/syscalls.h>
#include <linux/memcontrol.h>
+#include <linux/poll.h>
#include <asm/pgtable.h>
#include <asm/tlbflush.h>
@@ -58,6 +59,9 @@ static struct swap_info_struct *swap_inf
static DEFINE_MUTEX(swapon_mutex);
+static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
+static atomic_t proc_poll_event = ATOMIC_INIT(0);
+
static inline unsigned char swap_count(unsigned char ent)
{
return ent & ~SWAP_HAS_CACHE; /* may include SWAP_HAS_CONT flag */
@@ -1680,6 +1684,8 @@ SYSCALL_DEFINE1(swapoff, const char __us
}
filp_close(swap_file, NULL);
err = 0;
+ atomic_inc(&proc_poll_event);
+ wake_up_interruptible(&proc_poll_wait);
out_dput:
filp_close(victim, NULL);
@@ -1688,6 +1694,25 @@ out:
}
#ifdef CONFIG_PROC_FS
+struct proc_swaps {
+ struct seq_file seq;
+ int event;
+};
+
+static unsigned swaps_poll(struct file *file, poll_table *wait)
+{
+ struct proc_swaps *s = file->private_data;
+
+ poll_wait(file, &proc_poll_wait, wait);
+
+ if (s->event != atomic_read(&proc_poll_event)) {
+ s->event = atomic_read(&proc_poll_event);
+ return POLLIN | POLLRDNORM | POLLERR | POLLPRI;
+ }
+
+ return POLLIN | POLLRDNORM;
+}
+
/* iterator */
static void *swap_start(struct seq_file *swap, loff_t *pos)
{
@@ -1771,7 +1796,24 @@ static const struct seq_operations swaps
static int swaps_open(struct inode *inode, struct file *file)
{
- return seq_open(file, &swaps_op);
+ struct proc_swaps *s;
+ int ret;
+
+ s = kmalloc(sizeof(struct proc_swaps), GFP_KERNEL);
+ if (!s)
+ return -ENOMEM;
+
+ file->private_data = s;
+
+ ret = seq_open(file, &swaps_op);
+ if (ret) {
+ kfree(s);
+ return ret;
+ }
+
+ s->seq.private = s;
+ s->event = atomic_read(&proc_poll_event);
+ return ret;
}
static const struct file_operations proc_swaps_operations = {
@@ -1779,6 +1821,7 @@ static const struct file_operations proc
.read = seq_read,
.llseek = seq_lseek,
.release = seq_release,
+ .poll = swaps_poll,
};
static int __init procswaps_init(void)
@@ -2084,6 +2127,9 @@ SYSCALL_DEFINE2(swapon, const char __use
swap_info[prev]->next = type;
spin_unlock(&swap_lock);
mutex_unlock(&swapon_mutex);
+ atomic_inc(&proc_poll_event);
+ wake_up_interruptible(&proc_poll_wait);
+
error = 0;
goto out;
bad_swap:
--
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