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SubjectRe: [PATCH] support polling of /proc/swaps
On Tue, 19 Oct 2010 11:19:16 +0200
Kay Sievers <kay.sievers@vrfy.org> wrote:

> From: Kay Sievers <kay.sievers@vrfy.org>
> Subject: support polling of /proc/swaps
>
> System management wants to subscribe to changes in swap
> configuration. Make /proc/swaps pollable like /proc/mounts.
>

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.

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...

> ---
> 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 int proc_poll_event;

Please pick a lock to protect proc_poll_event. Then document that
locking here, when you also document proc_poll_event ;)

> 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;
> + proc_poll_event++;

Then fix this race.

> + wake_up_interruptible(&proc_poll_wait);
>
> out_dput:
> filp_close(victim, NULL);



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