linux-kernel - Re: [PATCH 3/4] configfs: implement committable items

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Message-ID: <CAMpxmJUQF8DwJzZN0960U10tjADh10GiexKmJv9o7AMp=9=caw@mail.gmail.com>
Date:   Thu, 14 Jan 2021 09:40:36 +0100
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>,
        Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH 3/4] configfs: implement committable items

On Thu, Jan 14, 2021 at 12:46 AM Joel Becker <jlbec@...lplan.org> wrote:
>
> On Wed, Nov 25, 2020 at 04:22:46PM +0100, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> >
> > This implements configfs committable items. We mostly follow the
> > documentation except that we extend config_group_ops with uncommit_item()
> > callback for reverting the changes made by commit_item().
>
> Woohoo!  A long time coming, but thank you for working on the
> implementation!
>
> > Each committable group has two sub-directories: pending and live. New
> > items can only be created in pending/. Attributes can only be modified
> > while the item is in pending/. Once it's ready to be committed, it must
> > be moved over to live/ using the rename() system call. This is when the
> > commit_item() function will be called.
>
> The original API intended for live items to still be modifyable.  The
> live/ path forbids mkdir()/rmdir(), but it allows store().  Otherwise,
> items can't be adjusted at all while in use, which is severely limiting.
> Obviously the store() handler must not allow transitions from
> valid-value->invalid-value, but the handler would reject invalid values
> anyway, wouldn't it?
>
> > diff --git a/fs/configfs/file.c b/fs/configfs/file.c
> > index 1f0270229d7b..a20e55fd05e8 100644
> > --- a/fs/configfs/file.c
> > +++ b/fs/configfs/file.c
> > @@ -243,9 +243,17 @@ fill_write_buffer(struct configfs_buffer * buffer, const char __user * buf, size
> >  static int
> >  flush_write_buffer(struct file *file, struct configfs_buffer *buffer, size_t count)
> >  {
> > +     struct config_item *parent_item = buffer->item->ci_parent;
> >       struct configfs_fragment *frag = to_frag(file);
> > +     struct configfs_dirent *sd;
> >       int res = -ENOENT;
> >
> > +     if (parent_item && parent_item->ci_dentry) {
> > +             sd = parent_item->ci_dentry->d_fsdata;
> > +             if (sd->s_type & CONFIGFS_GROUP_LIVE)
> > +                     return -EPERM;
> > +     }
> > +
> >       down_read(&frag->frag_sem);
> >       if (!frag->frag_dead)
> >               res = buffer->attr->store(buffer->item, buffer->page, count);
>
> Basically, I would just leave this hunk out.
>

I would make this configurable per-attribute because for the use-case
I need this for we definitely don't want the items to be modifiable
once they're "live".

Bartosz