linux-kernel - Re: [PATCH v7 5/7] fuse: Simplfiy the posix acl handling logic.

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Date:   Fri, 02 Mar 2018 15:49:56 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alban Crequy <alban@...volk.io>,
        Seth Forshee <seth.forshee@...onical.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Dongsu Park <dongsu@...volk.io>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v7 5/7] fuse: Simplfiy the posix acl handling logic.

Miklos Szeredi <mszeredi@...hat.com> writes:

> On Tue, Feb 27, 2018 at 12:53 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Rename the fuse connection flag posix_acl to cached_posix_acl as that
>> is what it actually means.  That fuse will cache and operate on the
>> cached value of the posix acl.
>>
>> When fc->cached_posix_acl is not set, set ACL_DONT_CACHE on the inode
>> so that get_acl and friends won't cache the acl values even if they
>> are called.
>>
>> Replace forget_all_cached_acls with fuse_forget_cached_acls.  This
>> wrapper only takes effect when cached_posix_acl is true to prevent
>> losing the nocache or noxattr status in when posix acls are not
>> cached.
>
> Shouldn't forget_cached_acl() be taught about ACL_DONT_CACHE?  I think
> it makes sense to generally not clear ACL_DONT_CACHE, since it's not
> an actual acl value that needs forgetting.

After stopping to make certain I understand the issues, I don't think
it makes sense to teach forget_cached_acl about ACL_DONT_CACHE.

If you are fogetting a cached attribute ACL_DONT_CACHE simply doesn't
make sense.

Further it makes sense to cache a negative result for fuse when
!fc->no_getxattr.  Even if you would ordinarily not cache posix acls.

So I think the better plan is to teach the posix acl code how to not
cache results on a case by case basis.  As I did in my rfc patch I
posted a little earlier today.  That works with forget_cached_acl and it
supports local reasoning.  Further while the performance might not be as
good as ACL_DONT_CACHE I don't think that matters as always going to the
fuse server to get acls is almost certainly going to dominate the acl
costs.

Eric