linux-kernel - Re: [PATCH 4/7] ovl: add infrastructure for intercepting file ops

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Date:   Thu, 24 Nov 2016 16:08:52 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] ovl: add infrastructure for intercepting file ops

On Thu, Nov 24, 2016 at 3:51 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Thu, Nov 24, 2016 at 2:12 PM, Amir Goldstein <amir73il@...il.com> wrote:
>> On Thu, Nov 24, 2016 at 2:03 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>>> On Thu, Nov 24, 2016 at 12:52 PM, Amir Goldstein <amir73il@...il.com> wrote:
>>>> On Thu, Nov 24, 2016 at 12:55 PM, Miklos Szeredi <mszeredi@...hat.com> wrote:
>>>
>>>>> +               /*
>>>>> +                * These should be intercepted, but they are very unlikely to be
>>>>> +                * a problem in practice.  Leave them alone for now.
>>>>
>>>> It could also be handled in vfs helpers.
>>>> Since these ops all start with establishing that src and dest are on
>>>> the same sb,
>>>> then the cost of copy up of src is the cost of clone_file_range from
>>>> lower to upper,
>>>> so it is probably worth to copy up src and leave those fops alone.
>>>>
>>>>> +                */
>>>>> +               ofop->fops.copy_file_range = orig->copy_file_range;
>>>>> +               ofop->fops.clone_file_range = orig->clone_file_range;
>>>>> +               ofop->fops.dedupe_file_range = orig->dedupe_file_range;
>>>
>>> Not sure I understand.  Why should we copy up src?  Copy up is the
>>> problem not the solution.
>>>
>>
>> Maybe the idea is ill conceived, but the reasoning is:
>> To avoid the corner case of cloning from a stale lower src,
>> call d_real() in vfs helpers to always copy up src before cloning from it
>> and pass the correct file onwards.
>
> Which correct file?  src is still the wrong one after calling d_real.
> We need to clone-open src, just like we do in ovl_read_iter to get the
> correct file.  But then what's the use of copying it up beforehand?
>
> We could move the whole logic into the vfs, but I don't really see the point.
>
> I left these ops alone because there is some confusion in there about
> getting the f_op from the source or the destination file.

Yes, I saw that. Shouldn't be a problem to always use src->f_ops-> IMO.

Could you please push this work to a topic branch to make it easier for me to
pull and test?

Thanks,
Amir.

>  And while
> it doesn't matter normally (all regular files have the same f_op,
> regardless of open flags)  it does matter for overlayfs intercept,
> because overriding fops in the the dest file would mean additional
> complexity and  resources).  That could easily be fixed in the vfs:
> calling src->f_ops->foo works equally well, but I simply didn't want
> to bother with this.  We can return to it later.
>
> Thanks,
> Miklos