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Date: Fri, 15 Jan 2016 00:05:09 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tomeu Vizoso <tomeu@tomeuvizoso.net>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Neil Brown <neilb@suse.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v2 06/11] don't put symlink bodies in pagecache into
 highmem
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On Thu, Jan 14, 2016 at 03:58:22PM -0800, Linus Torvalds wrote:
> On Thu, Jan 14, 2016 at 2:25 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > For one thing, we'll lose RCU ->get_link() for those.
> 
> Why couldn't we just do that in the RCU walker? kmap should be fine..

In map_new_virtual():
                        __set_current_state(TASK_UNINTERRUPTIBLE);
                        add_wait_queue(pkmap_map_wait, &wait);
                        unlock_kmap();
                        schedule();
                        remove_wait_queue(pkmap_map_wait, &wait);
                        lock_kmap();
IOW, not in RCU mode ;-/

> That said, as long as you think it's ok now, I guess I don't care.
> Having some sanity testing in __add_to_page_cache_locked might be a
> good safety net.

That's better as a separate commit, IMO.  The thing I'm not sure about is
whether we want a BUG() in there - VM_WARN_ON(), perhaps?  OTOH, we do
have VM_BUG_ON_PAGE() in the same place already...