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From:   Giuseppe Scrivano <gscrivan@redhat.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org,
        mingo@kernel.org, dave@stgolabs.net
Subject: Re: [RFC PATCH] ipc, mqueue: lazy call kern_mount_data in new namespaces
References: <20171127125550.15514-1-gscrivan@redhat.com>
        <20171128135316.6d7bba7fe909ba3d90e318db@linux-foundation.org>
        <87wp29l6h3.fsf@redhat.com>
        <20171129141744.5d401ff613116b0bde02cff3@linux-foundation.org>
Date:   Thu, 30 Nov 2017 13:20:11 +0100
In-Reply-To: <20171129141744.5d401ff613116b0bde02cff3@linux-foundation.org>
        (Andrew Morton's message of "Wed, 29 Nov 2017 14:17:44 -0800")
Message-ID: <87o9nkj6v8.fsf@redhat.com>
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Andrew Morton <akpm@linux-foundation.org> writes:

> On Wed, 29 Nov 2017 11:33:28 +0100 Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>
>> Andrew Morton <akpm@linux-foundation.org> writes:
>> 
>> > OK, but this simply moves the expense so it happens later on.  Why is
>> > that better?
>> 
>> the optimization is for new IPC namespaces that don't use mq_open.  In
>> this case there won't be any kern_mount_data cost at all.
>> 
>
> Fair enough.  Please add this paragraph (or similar) to the changelog:
>
> : This is a net saving for new IPC namespaces that don't use mq_open().  In
> : this case there won't be any kern_mount_data() cost at all
>
> And..  the patch calls
> kern_mount_data()->vfs_kern_mount()->...->kmem_cache_zalloc(GFP_KERNEL)
> under spin_lock().  This should have created a might_sleep() warning in
> your testing, but obviously did not.
>
> Could you please find out why?  Do you have
> CONFIG_DEBUG_ATOMIC_SLEEP=n, I hope?  Please peruse
> Documentation/process/submit-checklist.rst, section 12...
>
> I assume a suitable fix would be to create a new mutex (static to
> do_mq_open()) to prevent concurrent mounting.

thanks for the hints.

Indeed, that was a mistake on my side as I didn't use
CONFIG_DEBUG_ATOMIC_SLEEP=y.  The might_sleep() warning is correctly 
raised once I enable CONFIG_DEBUG_ATOMIC_SLEEP (and the other options
suggested in submit-checklist.rst).

Giuseppe

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