linux-ext4 - Re: Lockup in wait_transaction_locked under memory pressure

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Message-ID: <20150701142157.GB6287@dhcp22.suse.cz>
Date:	Wed, 1 Jul 2015 16:21:57 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Dave Chinner <david@...morbit.com>,
	Nikolay Borisov <kernel@...p.com>, linux-ext4@...r.kernel.org,
	Marian Marinov <mm@...com>
Subject: Re: Lockup in wait_transaction_locked under memory pressure

On Wed 01-07-15 07:13:55, Theodore Ts'o wrote:
[...]
> Perhaps we need to make the documentation a bit more explicit?
> All which is stated in include/slab.h:
> 
>  * %GFP_NOIO - Do not do any I/O at all while trying to get memory.
>  *
>  * %GFP_NOFS - Do not make any fs calls while trying to get memory.
> 
> I thought this was obvious, but these flags are used by code which in
> the I/O or FS paths, and so it's always possible that they are trying
> to write back the page which you decide to blocking on when trying to
> do the memory allocation, at which point, *boom*, deadlock.
> 
> So it's just not "do not make any FS or I/O calls", but also "the mm
> layer must not not wait for any FS or I/O operations from completing,
> since the operation you block on might be the one they were in the
> middle of trying to complete --- or they may be holding a lock at the
> time when they were trying to do a memory allocation which blocks the
> I/O or FS operation from completing".

Sure any refinement here would be more than welcome.
-- 
Michal Hocko
SUSE Labs
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