Re: [PATCH v10] mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS

From: Jan Kara <jack@suse.cz>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-nvdimm@lists.01.org,
	"Martin Schwidefsky" <schwidefsky@de.ibm.com>,
	"Heiko Carstens" <heiko.carstens@de.ibm.com>,
	"Michal Hocko" <mhocko@suse.com>,
	"kbuild test robot" <lkp@intel.com>,
	"Thomas Meyer" <thomas@m3y3r.de>,
	"Dave Jiang" <dave.jiang@intel.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Jérôme Glisse" <jglisse@redhat.com>, "Jan Kara" <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v10] mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS
Date: Fri, 18 May 2018 11:00:55 +0200	[thread overview]
Message-ID: <20180518090055.o2q5wrawk67v6ppr@quack2.suse.cz> (raw)
In-Reply-To: <152658753673.26786.16458605771414761966.stgit@dwillia2-desk3.amr.corp.intel.com>

On Thu 17-05-18 13:06:54, Dan Williams wrote:
> In preparation for fixing dax-dma-vs-unmap issues, filesystems need to
> be able to rely on the fact that they will get wakeups on dev_pagemap
> page-idle events. Introduce MEMORY_DEVICE_FS_DAX and
> generic_dax_page_free() as common indicator / infrastructure for dax
> filesytems to require. With this change there are no users of the
> MEMORY_DEVICE_HOST designation, so remove it.
> 
> The HMM sub-system extended dev_pagemap to arrange a callback when a
> dev_pagemap managed page is freed. Since a dev_pagemap page is free /
> idle when its reference count is 1 it requires an additional branch to
> check the page-type at put_page() time. Given put_page() is a hot-path
> we do not want to incur that check if HMM is not in use, so a static
> branch is used to avoid that overhead when not necessary.
> 
> Now, the FS_DAX implementation wants to reuse this mechanism for
> receiving dev_pagemap ->page_free() callbacks. Rework the HMM-specific
> static-key into a generic mechanism that either HMM or FS_DAX code paths
> can enable.
> 
> For ARCH=um builds, and any other arch that lacks ZONE_DEVICE support,
> care must be taken to compile out the DEV_PAGEMAP_OPS infrastructure.
> However, we still need to support FS_DAX in the FS_DAX_LIMITED case
> implemented by the s390/dcssblk driver.
> 
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Reported-by: kbuild test robot <lkp@intel.com>
> Reported-by: Thomas Meyer <thomas@m3y3r.de>
> Reported-by: Dave Jiang <dave.jiang@intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: "Jï¿½rï¿½me Glisse" <jglisse@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Yeah, it looks simpler than original patches and it looks OK to me. You can
add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR