Re: [RFC PATCH 02/02] swapon: add "cluster-discard" support

[Rafael Aquini]

Karel, Motohiro,

Thanks a lot for your time reviewing this patch and providing me with valuable
feedback.

On Tue, May 21, 2013 at 04:17:04PM -0400, KOSAKI Motohiro wrote:
> (5/21/13 6:26 AM), Karel Zak wrote:
> > On Mon, May 20, 2013 at 09:02:43PM -0400, KOSAKI Motohiro wrote:
> >>> -	if (fl_discard)
> >>> +	if (fl_discard) {
> >>>  		flags |= SWAP_FLAG_DISCARD;
> >>> +		if (fl_discard > 1)
> >>> +			flags |= SWAP_FLAG_DISCARD_CLUSTER;
> >>
> >> This is not enough, IMHO. When running this code on old kernel, swapon() return EINVAL.
> >> At that time, we should fall back swapon(0x10000).
> > 
> >  Hmm.. currently we don't use any fallback for any swap flag (e.g.
> >  0x10000) for compatibility with old kernels. Maybe it's better to
> >  keep it simple and stupid and return an error message than introduce
> >  any super-smart semantic to hide incompatible fstab configuration.
> 
> Hm. If so, I'd propose to revert the following change. 
> 
> > .B "\-d, \-\-discard"
> >-Discard freed swap pages before they are reused, if the swap
> >-device supports the discard or trim operation.  This may improve
> >-performance on some Solid State Devices, but often it does not.
> >+Enables swap discards, if the swap device supports that, and performs
> >+a batch discard operation for the swap device at swapon time.
> 
> 
> And instead, I suggest to make --discard-on-swapon like the following.
> (better name idea is welcome) 
> 
> +--discard-on-swapon
> +Enables swap discards, if the swap device supports that, and performs
> +a batch discard operation for the swap device at swapon time.
> 
> I mean, preserving flags semantics removes the reason we need make a fallback.
> 
>

Instead of reverting and renaming --discard, what about making it accept an
optional argument, so we could use --discard (to enable all thing and keep
backward compatibility); --discard=cluster & --discard=batch (or whatever we
think it should be named). I'll try to sort this approach out if you folks think
it's worthwhile. 

-- Rafael

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