On Tue, Feb 26, 2013 at 01:45:12PM -0800, Alex Elsayed wrote:
> Hey, I was looking at the wiki 'unclaimed projects' list and I thought I'd
> take a look at what might be needed to use the swap-over-n{fs,bd}
> functionality in order to implement swapfile support. It *looks* like it's
> surprisingly uncomplicated - to the point where I suspect I'm missing
> something, and would like some sanity checking.
>
> My primary references for this are the patch[1] implementing swap-over-NFS
> and HEAD of cmason's for-linus branch.
The patch [1] is not enough to cook swap support for btrfs, have look at
the rest of the series. There's some explanation how the interfaces look
like. The read/write mechanism is done via direct_IO, but the filesystem
has to add the swap_activate and swap_deactivate operations and take
care of preallocating the swap file extents.
Details:
Analogy of these patches has to be done in btrfs as well:
d56b4ddf7781e nfs: teach the NFS client how to treat PG_swapcache pages
29418aa4bd487 nfs: disable data cache revalidation for swapfiles
The patch you've analyzed in detail adds some nfs-specific bits, not the
core work (and thus I don't disagree that it looks surprisingly
uncomplicated :)
The generic swap support is demonstrated in
a509bc1a9e48 mm: swap: implement generic handler for swap_activate
and in case of btrfs it has to avoid using bmap.
Thanks for tackling this project, I think it's not an easy one. There
are implications and API requirements that need to be taken into account
wrt snapshots and stable block ranges during the file lifetime. I have
a few commetns about that from Mel and can share them if you still want
to proceed with this project.
david
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