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Date: Tue, 18 Nov 2014 15:01:38 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Seth Jennings <sjennings@variantweb.net>
Cc: Minchan Kim <minchan@kernel.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Jerome Marchand <jmarchan@redhat.com>
Subject: Re: [PATCH] zsmalloc: correct fragile [kmap|kunmap]_atomic use
Message-Id: <20141118150138.668c81fda55c3ce39d7b2aac@linux-foundation.org>
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On Fri, 14 Nov 2014 09:07:32 -0600 Seth Jennings <sjennings@variantweb.net> wrote:

> On Fri, Nov 14, 2014 at 10:11:01AM +0900, Minchan Kim wrote:
> > The kunmap_atomic should use virtual address getting by kmap_atomic.
> > However, some pieces of code in zsmalloc uses modified address,
> > not the one got by kmap_atomic for kunmap_atomic.
> > 
> > It's okay for working because zsmalloc modifies the address
> > inner PAGE_SIZE bounday so it works with current kmap_atomic's
> > implementation. But it's still fragile with potential changing
> > of kmap_atomic so let's correct it.

It is a bit alarming, but I've seen code elsewhere in which a modified
pointer is passed to kunmap_atomic().  So the kunmap_atomic() interface
is "kvaddr should point somewhere into the page" and that won't be
changing without a big effort.

> Seems like you could just use PAGE_MASK to get the base page address
> from link like this:

I think Minchan's approach is better: it explicitly retains the
kmap_atomic() return value for passing to kunmap_atomic().  That's
nicer than modifying it and then setting it back again.

I mean, a cleaner way of implementing your suggestion would be

void kunmap_atomic_unaligned(void *p)
{
	kunmap_atomic(void *)((unsigned long)p & PAGE_MASK);
}

but then one looks at

void __kunmap_atomic(void *kvaddr)
{
	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;

and asks "what the heck".


So I dunno.  We could leave the code as-is.  I have no strong feelings
either way.  Minchan's patch has no effect on zsmalloc.o section sizes
with my compiler.