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From: Jason Gunthorpe <jgg@nvidia.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: akpm@linux-foundation.org, Matthew Wilcox <willy@infradead.org>,
	Jan Kara <jack@suse.cz>, "Darrick J. Wong" <djwong@kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-fsdevel@vger.kernel.org, nvdimm@lists.linux.dev,
	linux-xfs@vger.kernel.org, linux-mm@kvack.org,
	linux-ext4@vger.kernel.org
Subject: Re: [PATCH v2 15/18] devdax: Use dax_insert_entry() + dax_delete_mapping_entry()
Date: Wed, 21 Sep 2022 11:10:36 -0300	[thread overview]
Message-ID: <YysbXPnA3Z6AzWCw@nvidia.com> (raw)
In-Reply-To: <166329939733.2786261.13946962468817639563.stgit@dwillia2-xfh.jf.intel.com>

On Thu, Sep 15, 2022 at 08:36:37PM -0700, Dan Williams wrote:
> Track entries and take pgmap references at mapping insertion time.
> Revoke mappings (dax_zap_mappings()) and drop the associated pgmap
> references at device destruction or inode eviction time. With this in
> place, and the fsdax equivalent already in place, the gup code no longer
> needs to consider PTE_DEVMAP as an indicator to get a pgmap reference
> before taking a page reference.
> 
> In other words, GUP takes additional references on mapped pages. Until
> now, DAX in all its forms was failing to take references at mapping
> time. With that fixed there is no longer a requirement for gup to manage
> @pgmap references. However, that cleanup is saved for a follow-on patch.

A page->pgmap must be valid and stable so long as a page has a
positive refcount. Once we fixed the refcount GUP is automatically
fine. So this explanation seems confusing.

If dax code needs other changes to maintain that invarient it should
be spelled out what those are and why, but the instant we fix the
refcount we can delete the stuff in gup.c and everywhere else.

Jason

  reply	other threads:[~2022-09-21 14:10 UTC|newest]

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-16  3:35 [PATCH v2 00/18] Fix the DAX-gup mistake Dan Williams
2022-09-16  3:35 ` [PATCH v2 01/18] fsdax: Wait on @page not @page->_refcount Dan Williams
2022-09-20 14:30   ` Jason Gunthorpe
2022-09-16  3:35 ` [PATCH v2 02/18] fsdax: Use dax_page_idle() to document DAX busy page checking Dan Williams
2022-09-20 14:31   ` Jason Gunthorpe
2022-09-16  3:35 ` [PATCH v2 03/18] fsdax: Include unmapped inodes for page-idle detection Dan Williams
2022-09-16  3:35 ` [PATCH v2 04/18] ext4: Add ext4_break_layouts() to the inode eviction path Dan Williams
2022-09-16  3:35 ` [PATCH v2 05/18] xfs: Add xfs_break_layouts() " Dan Williams
2022-09-18 22:57   ` Dave Chinner
2022-09-19 16:11     ` Dan Williams
2022-09-19 21:29       ` Dave Chinner
2022-09-20 16:44         ` Dan Williams
2022-09-21 22:14           ` Dave Chinner
2022-09-21 22:28             ` Jason Gunthorpe
2022-09-23  0:18               ` Dave Chinner
2022-09-23  0:41                 ` Dan Williams
2022-09-23  2:10                   ` Dave Chinner
2022-09-23  9:38                     ` Jan Kara
2022-09-23 23:06                       ` Dan Williams
2022-09-25 23:54                       ` Dave Chinner
2022-09-26 14:10                         ` Jan Kara
2022-09-29 23:33                           ` Dan Williams
2022-09-30 13:41                             ` Jan Kara
2022-09-30 17:56                               ` Dan Williams
2022-09-30 18:06                                 ` Jason Gunthorpe
2022-09-30 18:46                                   ` Dan Williams
2022-10-03  7:55                                   ` Jan Kara
2022-09-23 12:39                     ` Jason Gunthorpe
2022-09-26  0:34                       ` Dave Chinner
2022-09-26 13:04                         ` Jason Gunthorpe
2022-09-22  0:02             ` Dan Williams
2022-09-22  0:10               ` Jason Gunthorpe
2022-09-16  3:35 ` [PATCH v2 06/18] fsdax: Rework dax_layout_busy_page() to dax_zap_mappings() Dan Williams
2022-09-16  3:35 ` [PATCH v2 07/18] fsdax: Update dax_insert_entry() calling convention to return an error Dan Williams
2022-09-16  3:35 ` [PATCH v2 08/18] fsdax: Cleanup dax_associate_entry() Dan Williams
2022-09-16  3:36 ` [PATCH v2 09/18] fsdax: Rework dax_insert_entry() calling convention Dan Williams
2022-09-16  3:36 ` [PATCH v2 10/18] fsdax: Manage pgmap references at entry insertion and deletion Dan Williams
2022-09-21 14:03   ` Jason Gunthorpe
2022-09-21 15:18     ` Dan Williams
2022-09-21 21:38       ` Dan Williams
2022-09-21 22:07         ` Jason Gunthorpe
2022-09-22  0:14           ` Dan Williams
2022-09-22  0:25             ` Jason Gunthorpe
2022-09-22  2:17               ` Dan Williams
2022-09-22 17:55                 ` Jason Gunthorpe
2022-09-22 21:54                   ` Dan Williams
2022-09-23  1:36                     ` Dave Chinner
2022-09-23  2:01                       ` Dan Williams
2022-09-23 13:24                     ` Jason Gunthorpe
2022-09-23 16:29                       ` Dan Williams
2022-09-23 17:42                         ` Jason Gunthorpe
2022-09-23 19:03                           ` Dan Williams
2022-09-23 19:23                             ` Jason Gunthorpe
2022-09-27  6:07                             ` Alistair Popple
2022-09-27 12:56                               ` Jason Gunthorpe
2022-09-16  3:36 ` [PATCH v2 11/18] devdax: Minor warning fixups Dan Williams
2022-09-16  3:36 ` [PATCH v2 12/18] devdax: Move address_space helpers to the DAX core Dan Williams
2022-09-27  6:20   ` Alistair Popple
2022-09-29 22:38     ` Dan Williams
2022-09-16  3:36 ` [PATCH v2 13/18] dax: Prep mapping helpers for compound pages Dan Williams
2022-09-21 14:06   ` Jason Gunthorpe
2022-09-21 15:19     ` Dan Williams
2022-09-16  3:36 ` [PATCH v2 14/18] devdax: add PUD support to the DAX mapping infrastructure Dan Williams
2022-09-16  3:36 ` [PATCH v2 15/18] devdax: Use dax_insert_entry() + dax_delete_mapping_entry() Dan Williams
2022-09-21 14:10   ` Jason Gunthorpe [this message]
2022-09-21 15:48     ` Dan Williams
2022-09-21 22:23       ` Jason Gunthorpe
2022-09-22  0:15         ` Dan Williams
2022-09-16  3:36 ` [PATCH v2 16/18] mm/memremap_pages: Support initializing pages to a zero reference count Dan Williams
2022-09-21 15:24   ` Jason Gunthorpe
2022-09-21 23:45     ` Dan Williams
2022-09-22  0:03       ` Alistair Popple
2022-09-22  0:04       ` Jason Gunthorpe
2022-09-22  0:34         ` Dan Williams
2022-09-22  1:36           ` Alistair Popple
2022-09-22  2:34             ` Dan Williams
2022-09-26  6:17               ` Alistair Popple
2022-09-22  0:13       ` John Hubbard
2022-09-16  3:36 ` [PATCH v2 17/18] fsdax: Delete put_devmap_managed_page_refs() Dan Williams
2022-09-16  3:36 ` [PATCH v2 18/18] mm/gup: Drop DAX pgmap accounting Dan Williams
2022-09-20 14:29 ` [PATCH v2 00/18] Fix the DAX-gup mistake Jason Gunthorpe
2022-09-20 16:50   ` Dan Williams
2022-11-09  0:20 ` Andrew Morton
2022-11-09 11:38   ` Jan Kara

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