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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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