linux-kernel - Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper

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Message-Id: <201203230523.q2N5Nhn4068469@www262.sakura.ne.jp>
Date:	Fri, 23 Mar 2012 14:23:43 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	bharrosh@...asas.com
Cc:	oleg@...hat.com, akpm@...ux-foundation.org, rjw@...k.pl,
	keyrings@...ux-nfs.org, linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org, Trond.Myklebust@...app.com,
	sbhamare@...asas.com, dhowells@...hat.com, eparis@...hat.com,
	srivatsa.bhat@...ux.vnet.ibm.com, kay.sievers@...y.org,
	jmorris@...ei.org, ebiederm@...ssion.com,
	gregkh@...uxfoundation.org, rusty@...tcorp.com.au, tj@...nel.org,
	rientjes@...gle.com
Subject: Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper_timeout()API

Boaz Harrosh wrote:
> That's fine. I will fail totally gracefully, and nothing will happen. I like this
> example if the system is under heavy load and there is no memory and the
> iscsi auto-login takes more then 15 second (Settable by module param) then I'd
> rather fail the login and revert to plain NFS-MDS IO, instead of the direct osd-target
> IO. Believe me.

So, race with timeout causes no inconsistency to the kernel.
Then, it will be no problem to have timeout-able version.

> >> I don't want any chance of D stating IO operations.
> >> (My code is in the IO path, either fsync or write-back. There is not
> >>  always a killable target)
> > 
> > Then, isn't UMH_NO_WAIT better than UMH_WAIT_PROC?
> > 
> No I need to wait for the application to finish the iscsi login before
> I can continue IO to the target. Otherwise what's the point.

Your code which launches usermodehelper is in the I/O path, isn't it?
Then, don't you need to use GFP_NOFS rather than GFP_KERNEL?
http://lists.infradead.org/pipermail/linux-mtd-cvs/2007-October/005937.html

I'm not familiar with GFP_* flags usage. Can somebody clarify when we need to
use GFP_NOFS rather than GFP_KERNEL by enumerating the name of function (e.g.
xxxfs_writepage()) and/or the name of locks (e.g. xxx_mutex)? I've asked this
in the past http://www.spinics.net/lists/linux-fsdevel/msg30248.html but
explanation with actual function/variable names is easier to understand.
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