linux-kernel - Re: [PATCH 1/4] fs/notify: fdinfo can report unsupported file handles.

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Date:   Mon, 11 Dec 2017 16:08:50 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Pavel Emelyanov <xemul@...tuozzo.com>
Cc:     NeilBrown <neilb@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH 1/4] fs/notify: fdinfo can report unsupported file handles.

On Mon, Dec 11, 2017 at 3:46 PM, Pavel Emelyanov <xemul@...tuozzo.com> wrote:
> On 12/11/2017 10:05 AM, Amir Goldstein wrote:
>> On Mon, Dec 11, 2017 at 8:41 AM, Amir Goldstein <amir73il@...il.com> wrote:
>>> On Mon, Dec 11, 2017 at 8:04 AM, NeilBrown <neilb@...e.com> wrote:
>>>> If a filesystem does not set sb->s_export_op, then it
>>>> does not support filehandles and export_fs_encode_fh()
>>>> and exportfs_encode_inode_fh() should not be called.
>>>> They will use export_encode_fh() is which is a default
>>>> that uses inode number generation number, but in general
>>>> they may not be stable.
>>>>
>>>> So change exportfs_encode_inode_fh() to return FILEID_INVALID
>>>> if called on an unsupported Filesystem.  Currently only
>>>> notify/fdinfo can do that.
>>>>
>>>
>>> I wish you would leave this check to the caller, maybe add a helper
>>> exportfs_can_decode_fh() for callers to use.
>>>
>>> Although there are no current uses for it in-tree, there is value in
>>> being able to encode a unique file handle even when it cannot be
>>> decoded back to an open file.
>>>
>>> I am using this property in my fanotify super block watch patches,
>>> where the object identifier on the event is an encoded file handle
>>> of the object, which delegates tracking filesystem objects to
>>> userspace and prevents fanotify from keeping elevated refcounts
>>> on inodes and dentries.
>>>
>>> There are quite a few userspace tools out there that are checking
>>> that st_ino hasn't changed on a file between non atomic operations.
>>> Those tools (or others) could benefit from a unique file handle if
>>> we ever decide to provide a relaxed version of name_to_handle_at().
>>>
>>
>> And this change need a clause about not breaking userspace.
>>
>> Pavel,
>>
>> Will this break any version of criu in the wild?
>
> If there's no fliehandle in the output, it will make dump fail, but we're
> already prepared for the fact, that there's no handle at hands. In the
> worst case criu will exit with error.
>
> I also agree that it should only happen when current is OOM killed, and in
> case of CRIU this means killing criu process itself.
>

But this patch [1/4] changes behavior so you cannot dump fsnotify
state if watched file system does not support *decoding* file handles.
This means that criu anyway won't be able to restore the fsnotify state.
Is it OK that criu dump state will fail in that case?

Amir.