Message-ID: <ece87ac3-71e2-4c43-a144-659d19b1e75d@redhat.com>
Date: Thu, 7 Nov 2024 18:59:19 +0100
From: Hanna Czenczek <hreitz@...hat.com>
To: Miklos Szeredi <miklos@...redi.hu>
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 07.11.24 13:18, Miklos Szeredi wrote:
> 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.

Sure!

>>>> +       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.

Ah, right…  I must have forgotten that at some point.

> 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!

Hanna

>
> Thanks,
> Miklos
>

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