linux-kernel - Re: [PATCH RFC 3/2] fstests: check that we can't write to swap files

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Date:   Thu, 15 Aug 2019 19:13:27 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     hch@...radead.org, tytso@....edu, viro@...iv.linux.org.uk,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        fstests <fstests@...r.kernel.org>
Subject: Re: [PATCH RFC 3/2] fstests: check that we can't write to swap files

On Thu, Aug 15, 2019 at 02:26:03PM -0700, Andrew Morton wrote:
> On Thu, 15 Aug 2019 09:34:34 -0700 "Darrick J. Wong" <darrick.wong@...cle.com> wrote:
> 
> > While active, the media backing a swap file is leased to the kernel.
> > Userspace has no business writing to it.  Make sure we can't do this.
> 
> I don't think this tests the case where a file was already open for
> writing and someone does swapon(that file)?
> 
> And then does swapoff(that file), when writes should start working again?
> 
> Ditto all the above, with s/open/mmap/.

Heh, ok.  I'll start working on a C program to do that.

> Do we handle (and test!) the case where there's unwritten dirty
> pagecache at the time of swapon()? Ditto pte-dirty MAP_SHARED pages?

One of the tests I wrote for iomap_swapfile_activate way back when
checks that.  The iomap version calls vfs_fsync, but AFAICT
generic_swapfile_activate doesn't do that.

--D