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Date:   Thu, 13 Aug 2020 17:21:41 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Josef Bacik' <josef@toxicpanda.com>, "hch@lst.de" <hch@lst.de>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kernel-team@fb.com" <kernel-team@fb.com>
Subject: Re: [PATCH] proc: use vmalloc for our kernel buffer
Message-ID: <20200813162141.GY1236603@ZenIV.linux.org.uk>
References: <20200813145305.805730-1-josef@toxicpanda.com>
 <714c8baabe1a4d0191f8cdaf6e28a32d@AcuMS.aculab.com>
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On Thu, Aug 13, 2020 at 04:19:27PM +0000, David Laight wrote:
> From: Josef Bacik
> > Sent: 13 August 2020 15:53
> > 
> >   sysctl: pass kernel pointers to ->proc_handler
> > 
> > we have been pre-allocating a buffer to copy the data from the proc
> > handlers into, and then copying that to userspace.  The problem is this
> > just blind kmalloc()'s the buffer size passed in from the read, which in
> > the case of our 'cat' binary was 64kib.  Order-4 allocations are not
> > awesome, and since we can potentially allocate up to our maximum order,
> > use vmalloc for these buffers.
> 
> What happens if I run 'dd bs=16M ...' ?

Try it.