linux-kernel - Re: Dirtiable inode bdi default != sb bdi btrfs

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Message-ID: <1285767616.2437.142.camel@localhost>
Date:	Wed, 29 Sep 2010 16:40:16 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Cesar Eduardo Barros <cesarb@...arb.net>,
	Andrew Morton <akpm@...ux-foundation.org>, hch@....de,
	linux-kernel@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>,
	linux-btrfs@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, stable@...nel.org,
	Jens Axboe <axboe@...nel.dk>,
	Michał Piotrowski <mkkp4x4@...il.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	kernel@...ts.fedoraproject.org
Subject: Re: Dirtiable inode bdi default != sb bdi btrfs

On Wed, 2010-09-29 at 15:00 +0200, Jan Kara wrote:
> On Tue 28-09-10 10:05:49, Artem Bityutskiy wrote:
> > On Mon, 2010-09-27 at 18:54 -0400, Chris Mason wrote:
> > > On Tue, Sep 28, 2010 at 12:25:48AM +0200, Jan Kara wrote:
> > > > [Added CCs for similar ecryptfs warning]
> > > > On Thu 23-09-10 12:38:49, Andrew Morton wrote:
> > > > > > This started appearing for me on v2.6.36-rc5-49-gc79bd89; it did not 
> > > > > > happen on v2.6.36-rc5-33-g1ce1e41, probably because it does not have 
> > > > > > commit 692ebd17c2905313fff3c504c249c6a0faad16ec which introduces the 
> > > > > > warning.
> > > > > > [...]
> > > > > > device fsid 44d595920ddedfa-3ece6b56e80f689e devid 1 transid 22342 
> > > > > > /dev/mapper/vg_cesarbinspiro-lv_home
> > > > > > SELinux: initialized (dev dm-3, type btrfs), uses xattr
> > > > > > ------------[ cut here ]------------
> > > > > > WARNING: at fs/fs-writeback.c:87 inode_to_bdi+0x62/0x6d()
> > > > > > Hardware name: Inspiron N4010
> > > > > > Dirtiable inode bdi default != sb bdi btrfs
> > > > > > Modules linked in: ipv6 kvm_intel kvm uinput arc4 ecb 
> > > > > > snd_hda_codec_intelhdmi snd_hda_codec_realtek iwlagn snd_hda_intel 
> > > > > > iwlcore snd_hda_codec uvcvideo snd_hwdep mac80211 videodev snd_seq 
> > > > > > snd_seq_device v4l1_compat snd_pcm atl1c v4l2_compat_ioctl32 btusb 
> > > > > > cfg80211 snd_timer i2c_i801 bluetooth iTCO_wdt dell_wmi dell_laptop snd 
> > > > > > pcspkr wmi dcdbas shpchp iTCO_vendor_support soundcore snd_page_alloc 
> > > > > > rfkill joydev microcode btrfs zlib_deflate libcrc32c cryptd aes_x86_64 
> > > > > > aes_generic xts gf128mul dm_crypt usb_storage i915 drm_kms_helper drm 
> > > > > > i2c_algo_bit i2c_core video output [last unloaded: scsi_wait_scan]
> > > > > > Pid: 1073, comm: find Not tainted 2.6.36-rc5+ #8
> > > > > > Call Trace:
> > > > > >   [<ffffffff8104d0e4>] warn_slowpath_common+0x85/0x9d
> > > > > >   [<ffffffff8104d19f>] warn_slowpath_fmt+0x46/0x48
> > > > > >   [<ffffffff811308b7>] inode_to_bdi+0x62/0x6d
> > > > > >   [<ffffffff81131b48>] __mark_inode_dirty+0xd0/0x177
> > > > > >   [<ffffffff81127168>] touch_atime+0x107/0x12a
> > > > > >   [<ffffffff81122384>] ? filldir+0x0/0xd0
> > > > > >   [<ffffffff8112259b>] vfs_readdir+0x8d/0xb4
> > > > > >   [<ffffffff8112270b>] sys_getdents+0x81/0xd1
> > > > > >   [<ffffffff81009c72>] system_call_fastpath+0x16/0x1b
> > > >   Thanks for the report. These bdi pointers are a mess. As Chris pointed
> > > > out, btrfs forgets to properly initialize inode->i_mapping.backing_dev_info
> > > > for directories and special inodes and thus these were previously attached
> > > > to default_backing_dev_info which probably isn't what Chris would like to
> > > > see.
> > > 
> > > There's no actual writeback for these, so it works fine for btrfs either
> > > way.
> > 
> > Side note: every time inode is marked as dirty, we wake up a bdi thread
> > or the default bdi thread. So if we have inodes which do not need
> > write-back, we should never mark them as dirty.
>   Are you sure? I think we wake up the thread only when it's the first
> dirty inode for the bdi...

Err, right. If no one ever marks it as clean then we won't wake-up the
thread. But I thought that marking it as dirty even once is bad because
this causes bdi thread creation, which consumes resources.

Sorry for my ignorance, I did not really follow the conversation, I just
remember that when I looked at bdi stuff, I noticed that during boot the
kernel created many bdi threads which were never used then. They
eventually exited. But I thought that creating useless bdi threads it
about concuming resources and slowing down the boot.

As I remember, the reason was touch_atime() for some of the threads.

But really, I did not dig this, I just noticed this conversation and
wanted to let you know about the issue I noticed this summer.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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