linux-kernel - Re: [PATCH] virtio-fs: Query rootmode during mount

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Message-ID: <CAJfpegvfYhL4-U-4=sSkcne3MSNZk3P3jqBAPYWp5b5o4Ryk6w@mail.gmail.com>
Date: Thu, 7 Nov 2024 13:18:46 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Hanna Czenczek <hreitz@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	virtualization@...ts.linux.dev, Miklos Szeredi <mszeredi@...hat.com>, 
	German Maglione <gmaglione@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] virtio-fs: Query rootmode during mount

On Thu, 7 Nov 2024 at 11:00, Hanna Czenczek <hreitz@...hat.com> wrote:
> It isn’t much, but I believe it’s most of fuse_fill_super_common()
> (without restructuring the code so flags returned by INIT are put into a
> separate structure and then re-joined into sb and fc later).

Probably not worth it.

> fuse_send_init() reads sb->s_bdi->ra_pages, process_init_reply() writes
> it and sb->s_time_gran, ->s_flags, ->s_stack_depth, ->s_export_op, and
> ->s_iflags.  In addition, process_init_reply() depends on several flags
> and objects in fc being set up (among those are fc->dax and
> fc->default_permissions), which is done by fuse_fill_super_common().

Okay, got it.

> So I think what we need from fuse_fill_super_common() is:
> - fuse_sb_defaults() (so these values can then be overwritten by
> process_init_reply()),
> - fuse_dax_conn_alloc(),
> - fuse_bdi_init(),
> - fc->default_permissions at least, but I’d just take the fc->[flag]
> setting block as a whole then.
>
> I assume we’ll also want the SB_MANDLOCK check then, and
> rcu_assign_pointer().  Then we might as well also set the block sizes
> and the subtype.
>
> The problem is that I don’t know the order things in
> fuse_fill_super_common() need to be in, and fuse_dev_alloc_install() is
> called before fuse_bdi_init(), so I didn’t want to move that.
>
> So what I understand is that calling fuse_dev_alloc_install() there
> isn’t necessary?  I’m happy to move that to part 2, as you suggest, but

Hmm, fuse_dev_install() chains the fud onto fc->devices.  This is used
by fuse_resend() and fuse_abort_conn().  Resending isn't really
interesting at this point, but aborting should work from the start, so
this should not be moved after sending requests.

> I’m not sure we can really omit much from part 1 without changing how
> process_init_reply() operates.  We could in theory delay
> process_init_reply() until after GETATTR (and thus after setting
> s_root), but that seems kind of wrong, and would still require setting
> up BDI and DAX for fuse_send_init().

Agree, let's keep the split as is, but store the fud temporarily in
fuse_fs_context and leave setting *ctx->fudptr to part2.

> >> +       if (sb->s_root || (fm->fc && fm->fc->initialized && !fm->submount)) {
> > How could fm->submount be set if sb->s_root isn't?
>
> fuse_get_tree_submount(), specifically fuse_fill_super_submount() whose
> error path leads to deactivate_locked_super(), can fail before
> sb->s_root is set.

Right.

> Still, the idea was rather to make it clear that this condition (INIT
> sent but s_root not set) is unique to non-submounts, so as not to mess
> with the submount code unintentionally.
>
> > Or sb->s_root set
> > and fc->initialized isn't?
>
> That would be the non-virtio-fs non-submount case (fuse_fill_super()),
> where s_root is set first and INIT sent after.

But this is virtiofs specific code.

Regardless, something smells here: fuse_mount_remove() is only called
if sb->s_root is set (both plain fuse and virtiofs).  The top level
fuse_mount is added to fc->mounts in fuse_conn_init(), way before
sb->s_root is set...

Will look into this.

Thanks,
Miklos