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From:   "NeilBrown" <neilb@suse.de>
To:     "Matthew Wilcox" <willy@infradead.org>
Cc:     "Trond Myklebust" <trond.myklebust@hammerspace.com>,
        "Anna Schumaker" <anna.schumaker@netapp.com>,
        "Chuck Lever" <chuck.lever@oracle.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Mel Gorman" <mgorman@suse.de>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/13] Repair SWAP-over-NFS
In-reply-to: <YZMlk2sVjt5viat2@casper.infradead.org>
References: <163702956672.25805.16457749992977493579.stgit@noble.brown>,
 <YZMlk2sVjt5viat2@casper.infradead.org>
Date:   Tue, 16 Nov 2021 14:55:39 +1100
Message-id: <163703493923.13692.11352680016082986967@noble.neil.brown.name>
Precedence: bulk

On Tue, 16 Nov 2021, Matthew Wilcox wrote:
> On Tue, Nov 16, 2021 at 01:44:04PM +1100, NeilBrown wrote:
> > swap-over-NFS currently has a variety of problems.
> > 
> > Due to a newish test in generic_write_checks(), all writes to swap
> > currently fail.
> 
> And by "currently", you mean "for over two years" (August 2019).

That's about the time scale for "enterprise" releases...
Actually, the earliest patches that impacted swap-over-NFS was more like
4 years ago.  I didn't bother tracking Fixes: tags for everything that
was a fix, as I didn't think it would really help and might encourage
people to backport little bits of the series which I wouldn't recommend.

> Does swap-over-NFS (or any other network filesystem) actually have any
> users, and should we fix it or rip it out?
> 
> 
We have at least one user (why else would I be working on this?).  I
think we have more, though they are presumably still on an earlier
release.

I'd prefer "fix it" over "rip it out".

I don't think any other network filesystem supports swap, but it is
not trivial to grep for.. There must be a 'swap_activate' method, and it
must return 0.  There must also be a direct_IO that works.
The only other network filesystem with swap_activate is cifs.  It
returns 0, but direct_IO returns -EINVAL.

NeilBrown