From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Darrick J. Wong" <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of
 blocks
Date: Thu, 3 Mar 2016 10:12:40 -0800
Message-ID: <20160303181240.GB8782@birch.djwong.org>
References: <20160302040932.16685.62789.stgit@birch.djwong.org>
 <20160302040947.16685.42926.stgit@birch.djwong.org>
 <CA+55aFx_NPn0VYk=+Ad5S_r=D6J1xFmWmf7JzQ7RmkwKmdkYOg@mail.gmail.com>
 <20160302225601.GB21890@birch.djwong.org>
 <CA+55aFyNPnjJ-Nsu3bMr+HuLQnkj1B8FMddNnXW7HJqxdUzJmQ@mail.gmail.com>
 <yq1bn6v8mqg.fsf@sermon.lab.mkp.net>
 <20160303180924.GA4116@infradead.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Content-Disposition: inline
In-Reply-To: <20160303180924.GA4116-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
To: Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Cc: "Martin K. Petersen" <martin.petersen-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Linux Kernel Mailing List <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, shane.seymour-ZPxbGqLxI0U@public.gmane.org, Bruce Fields <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>, linux-fsdevel <linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Jeff Layton <jlayton-vpEMnDpepFuMZCB2o+C8xQ@public.gmane.org>
List-Id: linux-api@vger.kernel.org

On Thu, Mar 03, 2016 at 10:09:24AM -0800, Christoph Hellwig wrote:
> On Thu, Mar 03, 2016 at 01:01:11PM -0500, Martin K. Petersen wrote:
> > That's not entirely true. Writing the blocks may cause them to be
> > allocated on the storage device (depending on which flags we feed it in
> > WRITE SAME).
> > 
> > The filesystems people were wanted the following semantics:
> > 
> >  - deallocate, don't care about contents for future reads (discard)
> >  - deallocate, guarantee zeroes on future reads (zeroout)
> >  - (re)allocate, guarantee zeroes on future reads (zeroout)
> > 
> > Maybe we just need a better naming scheme...
> 
> In filesystem terms we have two and three:
> 
>  - FALLOC_FL_PUNCH_HOLE assures zeroes are returned, but space is
>    deallocated as much as possible
>  - FALLOC_FL_ZERO_RANGE assures zeroes are returned, AND blocks are
>    actually allocated
> 
> Returning stale blocks in a file system is a nasty security risk, so
> we don't do that, and so shouldn't storage that offers any kind
> of multi tenancy, and if it's just VMs using multiple partitions on it.

Any particular reason why we can't just implement those two fallocate
flags for block devices?

--D