linux-kernel - Re: [PATCH v4 2/9] x86: support kmap_atomic_pfn

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Date:	Wed, 10 Jun 2015 08:36:48 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Matthew Wilcox <willy@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Boaz Harrosh <boaz@...xistor.com>, david <david@...morbit.com>,
	linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Tejun Heo <tj@...nel.org>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 2/9] x86: support kmap_atomic_pfn_t() for persistent memory

On Wed, Jun 10, 2015 at 8:11 AM, Christoph Hellwig <hch@....de> wrote:
> On Wed, Jun 10, 2015 at 11:03:35AM -0400, Matthew Wilcox wrote:
>> On Wed, Jun 10, 2015 at 02:12:02PM +0200, Christoph Hellwig wrote:
>> > Btw, I don't think this actually is safe without refcounting your kmap
>> > structure.
>> >
>> > The driver model ->remove callback can be called at any time, which
>> > will ioremap the memory and remap the kmap structure.  But at this
>> > point a user might still be using it.
>>
>> Won't the device data structures be pinned by the refcount on the bdev?
>
> An open filesystem only keeps a reference on the request_queue.  The
> underlying driver model ->remove method will still be called on
> a surprise removal.

On surprise removal my expectation is that the driver keeps the the
ioremap mapping alive for at least a synchronize_rcu() period.  With
that in place the rcu_read_lock() in kmap_atomic_pfn_t() should keep
the mapping alive for the short duration, or otherwise prevent new
mappings.  However, I missed converting the driver to order its
iounmap() with respect to the pfn range registration via devres, will
fix.
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