linux-kernel - Re: [PATCH v4 0/3] FUSE: Implement atomic lookup + open/create

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Date:   Wed, 11 May 2022 11:59:45 +0200
From:   Bernd Schubert <bschubert@....com>
To:     Miklos Szeredi <miklos@...redi.hu>, Vivek Goyal <vgoyal@...hat.com>
Cc:     Dharmendra Hans <dharamhans87@...il.com>,
        linux-fsdevel@...r.kernel.org,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/3] FUSE: Implement atomic lookup + open/create

On 5/11/22 11:40, Miklos Szeredi wrote:
> On Thu, 5 May 2022 at 21:59, Vivek Goyal <vgoyal@...hat.com> wrote:
> 
>> Oh, I have no issues with the intent. I will like to see cut in network
>> traffic too (if we can do this without introducing problems). My primary
>> interest is that this kind of change should benefit virtiofs as well.
> 
> One issue with that appears to be checking permissions.   AFAIU this
> patchset only enables the optimization if default_permissions is
> turned off (i.e. all permission checking is done by the server).  But
> virtiofs uses the default_permissions model.
> 
> I'm not quite sure about this limitation, guessing that it's related
> to the fact that the permissions may be stale at the time of checking?

Yes exactly, I had actually pointed this out in one of the mails.

<quote myself from 2022-04-05 >
Adding in a vfs ->open_revalidate might have the advantage that we could 
also support 'default_permissions' - ->open_revalidate  needs to 
additionally check the retrieved file permissions and and needs to call 
into generic_permissions for that. Right now that is not easily 
feasible, without adding some code dup to convert flags in MAY_* flags - 
a vfs change would be needed here to pass the right flags.
</quote>

Avoiding lookup for create should work without default_permissions, though.

With the current patches this comment should be added in 
fuse_dentry_revalidate() to clarify things (somehow it got lost in the 
(internal) review process).

/* Default permissions actually also would not need revalidate, if 
atomic_open() could verify attributes after opening the file. But the 
MAY_ flags are missing and the vfs build_open_flags() to recreate these 
flags not exported. Thus, default_permissions() cannot be called from 
here - vfs updates would be required */

Thanks,
Bernd