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Date: Wed, 10 Jun 2015 08:36:48 -0700
Message-ID: <CAPcyv4hnd1JRwLYapo_s=vXFYHUA6Cv-ZpJt+Y3_tTCxQsJUPg@mail.gmail.com>
Subject: Re: [PATCH v4 2/9] x86: support kmap_atomic_pfn_t() for persistent memory
From: Dan Williams <dan.j.williams@intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@linux.intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Boaz Harrosh <boaz@plexistor.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
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        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, Tejun Heo <tj@kernel.org>,
        Paul Mackerras <paulus@samba.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>
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On Wed, Jun 10, 2015 at 8:11 AM, Christoph Hellwig <hch@lst.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.