linux-kernel - Re: block: sleeping in atomic warnings

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Date:   Tue, 7 Feb 2023 19:35:54 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Carpenter <error27@...il.com>, linux-block@...r.kernel.org,
        Julia Lawall <julia.lawall@...ia.fr>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Christian Brauner (Microsoft)" <brauner@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maobibo <maobibo@...ngson.cn>,
        Matthew Wilcox <willy@...radead.org>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: block: sleeping in atomic warnings

On Tue, Feb 07, 2023 at 10:57:08AM -0800, Linus Torvalds wrote:
> On Tue, Feb 7, 2023 at 10:36 AM Eric Biggers <ebiggers@...nel.org> wrote:
> >
> > Also note that keys are normally added using an ioctl, which can only be
> > executed after the filesystem was mounted.  The only exception is the key
> > associated with the "test_dummy_encryption" mount option.
> 
> Could we perhaps then replace the
> 
>                 fscrypt_destroy_keyring(s);
> 
> with a more specific
> 
>                 fscrypt_destroy_dummy_keyring(s);
>
> thing, that would only handle the dummy encryption case?

Sure, they would still need to do most of the same things though.

> Or could we just *fix* the dummy encryption test to actually work like
> real encryption cases, so that it doesn't have this bogus case?

We've wanted to do that for a very long time, but there never has been a way to
actually do it.  Especially with the filesystem-level keyring now, if the kernel
doesn't automatically add the key for test_dummy_encryption, then userspace
would have to do it *every time it mounts the filesystem*.

The point of the "test_dummy_encryption" mount option is that you can just add
it to the mount options and run existing tests, such as a full run of xfstests,
and test all the encrypted I/O paths that way.  Which is extremely useful; it
wouldn't really be possible to properly test the encryption feature without it.

So that's why we've gone through some pain to keep "test_dummy_encryption"
working over time.

Now, it's possible that "the kernel automatically adds the key for
test_dummy_encryption" could be implemented a bit differently.  It maybe could
be done at the last minute, when the key is being looked for due to a user
filesystem operation, instead of during the mount itself.  That would eliminate
the need to call fscrypt_destroy_keyring() from __put_super(), which would avoid
the issue being discussed here.  I'll see if there's a good way to do that.

- Eric