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From: NeilBrown <neilb@suse.de>
To: Tejun Heo <tj@kernel.org>
Cc: Milan Broz <mbroz@redhat.com>, Tao Ma <tm@tao.ma>,
	linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	device-mapper development <dm-devel@redhat.com>
Subject: Re: [PATCH][RFC] dm: Do not open log and cow device read-write for read-only mappings
Date: Tue, 15 Feb 2011 10:15:06 +1100	[thread overview]
Message-ID: <20110215101506.43d41ca8@notabene.brown> (raw)
In-Reply-To: <20110214154430.GR18742@htj.dyndns.org>

On Mon, 14 Feb 2011 16:44:30 +0100 Tejun Heo <tj@kernel.org> wrote:

> Hello,
> 
> On Mon, Feb 14, 2011 at 03:23:20PM +0100, Milan Broz wrote:
> > >> I am not sure if this is complete fix... note that:
> > >> - what happens during mirror resync and read-only log?
> > >> - for COW, it there situation we need to update header in read-oly mode? (invalidated snap?)
> > > 
> > > But if the underlying device is marked ro, dm shouldn't update it at
> > > all.  The device should be opened ro and ro policy should be enforced.
> > 
> > Sure. So we need to check these situations I described.
> 
> Yeap, it seems dm folks are gonna take care of dm part.
> 
> > Btw the same pattern is in MD code in lock_rdev() ...
> 
> Indeed, cc'ing Neil.  Hi, the whole thread can be read from the
> following URL.
> 
>   http://thread.gmane.org/gmane.linux.kernel/1099399/focus=1099735
> 
> blkdev_get() now rejects rw open of devices which are marked
> read-only.  I think the right thing to do would be opening the member
> devices ro if the array is assembled for ro access (similar to Milan's
> patch for dm).  How does that sound?
> 
> Thanks.
> 

Sounds sensible ... though it is not all that easy to assemble an
array as 'read-only'....  it is possible though.

When the array is switched to read-write, do I have to call blkdev_get again
asking for rw access, then close the old blkdev, or can I 'upgrade'?

If a device has multiple opens: some read-only and some read-write, can I
find out when the last read-write close is gone?  That would be really useful,
especially if a filesystem down-graded its open to read-only when it is
remounted read-only..

[[And if filesystems could be convinced to open the device read-only when the
  fs is mounted read-only (and just do journal replay to internal data
  structures) that would be really awesome!!]]

NeilBrown


  reply	other threads:[~2011-02-14 23:15 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-02-13  2:02 block device read-only handling regression in 2.6.38-rc4 (bisected) Milan Broz
2011-02-13 10:58 ` [PATCH] loop: clear read-only flag in loop_clr_fd Tao Ma
2011-02-13 14:11   ` Milan Broz
2011-02-13 15:05     ` Tao Ma
2011-02-13 16:44       ` Milan Broz
2011-02-14 10:30         ` Tejun Heo
2011-02-14 11:47           ` Milan Broz
2011-02-14 13:14             ` [PATCH][RFC] dm: Do not open log and cow device read-write for read-only mappings Milan Broz
2011-02-14 14:09               ` Tejun Heo
2011-02-14 14:23                 ` Milan Broz
2011-02-14 15:44                   ` Tejun Heo
2011-02-14 23:15                     ` NeilBrown [this message]
2011-02-15  2:03                       ` [dm-devel] " Alasdair G Kergon
2011-02-15 12:17                         ` Milan Broz
2011-02-15 12:46                           ` Alasdair G Kergon
2011-02-15 15:20                             ` Tejun Heo
2011-02-15 15:46                               ` Alasdair G Kergon
2011-02-15 15:50                                 ` Tejun Heo
2011-02-15 16:05                                   ` Milan Broz
2011-02-15 16:12                                     ` Tejun Heo
2011-02-15 16:36                                       ` Milan Broz
2011-02-15 16:41                                         ` Tejun Heo
2011-02-15 16:56                                           ` Alasdair G Kergon
2011-02-16  8:46                                             ` Tejun Heo
2011-02-15 16:58                                           ` Milan Broz
2011-02-16  8:39                                             ` Tejun Heo
2011-02-15 15:16                           ` [PATCH] Return EROFS if read-only detected on block device Milan Broz
2011-02-14 14:39                 ` [dm-devel] [PATCH][RFC] dm: Do not open log and cow device read-write for read-only mappings Alasdair G Kergon
2011-02-14 14:07             ` [PATCH] loop: clear read-only flag in loop_clr_fd Tejun Heo

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