Messages in this thread | | | Date | Thu, 24 Sep 2009 08:40:56 +0530 | From | Nitin Gupta <> | Subject | Re: [PATCH RFC 1/2] Add notifiers for various swap events |
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On 09/24/2009 07:17 AM, KAMEZAWA Hiroyuki wrote: > On Mon, 21 Sep 2009 19:03:59 +0530 > Nitin Gupta <ngupta@vflare.org> wrote: > >> Add notifiers for following swap events: >> - Swapon >> - Swapoff >> - When a swap slot is freed >> >> This is required for ramzswap module which implements RAM based block >> devices to be used as swap disks. These devices require a notification >> on these events to function properly (as shown in patch 2/2). >> >> Currently, I'm not sure if any of these event notifiers have any other >> users. However, adding ramzswap specific hooks instead of this generic >> approach resulted in a bad/hacky code. >> > Hmm ? if it's not necessary to make ramzswap as module, for-ramzswap-only > code is much easier to read.. >
The patches posted earlier (v3 patches) inserts special hooks for swap slot free event only. In this version, the callback is set when we get first R/W request. Actually ramzswap needs callback for swapon/swapoff too but I just didn't do it.
Then Pekka posted test patch that allows setting this callback during swapon itself. Looking that all these patches, I realized its already too messy even if we just make everything ramzswap specific. Just FYI, Pekka's test patch: http://patchwork.kernel.org/patch/48472/
Then I added this generic notifier interface which, compared to earlier version, looks much cleaner. The code to add these notifiers is also very small.
> > >> For SWAP_EVENT_SLOT_FREE, callbacks are made under swap_lock. Currently, this >> is not a problem since ramzswap is the only user and the callback it registers >> can be safely made under this lock. However, if this event finds more users, >> we might have to work on reducing contention on this lock. >> >> Signed-off-by: Nitin Gupta <ngupta@vflare.org> >> > > In general, notifier chain codes allowed to return NOTIFY_BAD. > But this patch just assumes all chains should return NOTIFY_OK or > just ignore return code. > > That's not good as generic interface, I think.
What action we can take here if the notifier_call_chain() returns an error (apart from maybe printing an error)? Perhaps we can add a warning in case of swapon/off events but not in case of swap slot free event which is called under swap_lock.
Thanks, Nitin
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