linux-ext4 - Re: [PATCH 01/19] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

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Message-ID: <CAPcyv4gY9fU-T0V0kJBOU9RzCVaagF5NnrDbkyLUJ9iuMdfuOg@mail.gmail.com>
Date:   Tue, 17 Oct 2017 12:38:14 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "tytso@....edu" <tytso@....edu>, "arnd@...db.de" <arnd@...db.de>
Subject: Re: [PATCH 01/19] mm: introduce MAP_SHARED_VALIDATE, a mechanism to
 safely define new mmap flags

On Tue, Oct 17, 2017 at 4:50 AM, Jan Kara <jack@...e.cz> wrote:
> On Mon 16-10-17 00:45:04, Christoph Hellwig wrote:
>> > How about the following incremental update? It allows ->mmap_validate()
>> > to be used as a full replacement for ->mmap() and it limits the error
>> > code freedom to a centralized mmap_status_errno() routine:
>>
>> Nah - my earlier comment was simply misinformed because I didn't
>> read the whole patch and the _validate name mislead me.
>>
>> So I think the current calling conventions are ok, I'd just like a
>> better name (mmap_flags maybe?) and avoid the need the file system
>> also has to implement ->mmap.
>
> OK, I can do that. But I had just realized that if MAP_DIRECT isn't going
> to end up using mmap(2) interface but something else (and I'm not sure
> where discussions on this matter ended), we don't need flags argument for
> ->mmap at all. MAP_SYNC uses a VMA flag anyway and thus it is fine with the
> current ->mmap interface. We still need some opt-in mechanism for
> MAP_SHARED_VALIDATE though (probably supported mmap flags as Dan had in one
> version of his patch). Thoughts on which way to go for now?

The "supported mmap flags" approach also solves the problem you raised
about MAP_SYNC being silently accepted by an ->mmap() handler that
does not know about the new flag. I.e. leading userpace to potentially
assume an invalid data consistency model. I'll revive that approach
now that the MAP_DIRECT problem is going to be solved via a different
interface.