linux-kernel - Re: freeze vs freezer

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Date:	Mon, 26 Nov 2007 10:44:20 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	David Chinner <dgc@....com>, xfs-masters@....sgi.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: freeze vs freezer

Rafael J. Wysocki wrote:
> On Thursday, 22 of November 2007, Jeremy Fitzhardinge wrote:
>   
>> It seems that a process blocked in a write to an xfs filesystem due to
>> xfs_freeze cannot be frozen by the freezer.
>>     
>
> The freezer doesn't handle tasks in TASK_UNINTERRUPTIBLE and I don't know how
> to make it handle them without at least partially defeating its purpose.
>   

Well, I guess the question is whether an xfs-frozen writer really needs
to be UNINTERRUPTIBLE from the freezer's perspective (clearly it does
from usermode's perspective - filesystem writes just don't return EINTR).

>From a quick poke around, it looks to me like freezing is actually
implemented in the VFS layer rather than in XFS itself: is that right? 
Could vfs_check_frozen() be changed to something that is freezer-compatible?

>> I see this if I suspend my laptop while doing something xfs-filesystem
>> intensive, like a kernel build.  My suspend scripts freeze the XFS
>> filesystem (as Dave said I should), which presumably blocks some writer,
>> and then the freezer times out and fails to complete.
>>
>> Here's part of the process dump the freezer does when it times out:
>>
>> cc1           D 00000000     0 18138  18137
>>        dd5f1e24 00200082 00000002 00000000 ecdeeb00 ecdeec64 c200f280 00000001 
>>        009c09a0 dd5f1e0c dd5f1e0c 0000000f 00000000 00000000 00000000 dd5f1e74 
>>        c7beb480 dd5f1e88 dd5f1ea8 c0228d97 e8889540 dd5f1e38 c015b75d dd5f1e44 
>> Call Trace:
>>  [<c0228d97>] xfs_write+0xf4/0x6d9
>>  [<c0226038>] xfs_file_aio_write+0x53/0x5b
>>  [<c0171c15>] do_sync_write+0xae/0xec
>>  [<c0172343>] vfs_write+0xa4/0x120
>>  [<c01728d7>] sys_write+0x3b/0x60
>>  [<c0106fae>] sysenter_past_esp+0x6b/0xa1
>>  =======================
>>
>>
>> I haven't looked at how to fix this yet.  I only just worked out why I
>> was getting suspend failures.
>>     
>
> Well, you can add freezer_do_not_count()/freezer_count() annotations to
> xfs_write() (and whatever else is blocked as a result of the XFS being frozen).
>   

What would be the implications of that?  Would that just prevent
freezing while there's something blocked there?

> Generally, that would be risky without the freezing of XFS, however, because it
> might leak us filesystem data to a storage device after creating a hibernation
> image which would result in the filesystem corruption after the resume.
>
> Still, if you only suspend to RAM, that should be safe.
>   

I specifically added it because I was getting data loss due to crashes
during suspend/resume problems.  It's been pretty stable lately, but I
may as well remove the xfs_freeze from my suspend scripts if this is the
solution.

I think the broader issue is that there's no reason in principle why
something blocked due to xfs-freezing (or vfs freezing) should prevent
the freezer from completing.

    J
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