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From: Dan Williams <dan.j.williams@intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>
Cc: <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
	Christoph Hellwig <hch@lst.de>,
	"Darrick J. Wong" <djwong@kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Matthew Wilcox <willy@infradead.org>, <linux-mm@kvack.org>,
	<nvdimm@lists.linux.dev>, <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 00/13] Fix the DAX-gup mistake
Date: Fri, 9 Sep 2022 10:52:18 -0700	[thread overview]
Message-ID: <631b7d5214e77_58016294a2@dwillia2-xfh.jf.intel.com.notmuch> (raw)
In-Reply-To: <YxspQQ7ElQSAN/l3@nvidia.com>

Jason Gunthorpe wrote:
> On Thu, Sep 08, 2022 at 12:27:06PM -0700, Dan Williams wrote:
> >    flag lets the fsdax core track when it has already dropped a page
> >    reference, but still has use for things like memory-failure to
> >    opportunistically use page->mapping on a 0-reference page.
> 
> This is not straightforward, as discussed before the page->mapping is
> allowed to change while the refcount is zero, so there is no generic
> way to safely obtain a pointer to the address space from a 0 reference
> page.

Agree.

> 
> You'd have to pass the 0 reference page into a new pgmap operation
> which could obtain an appropriate internal lock to read page->mapping.

Correct, that's what the memory-failure code does via dax_lock_page().
It pins the pgmap, freezes page->mapping associations via
rcu_read_lock(), speculatively reads page->mapping, takes the Xarray
lock, revalidates page->mapping is the one we read speculatively, and
then finally locks the entry in place until the memory-failure handling
completes.


  reply	other threads:[~2022-09-09 17:52 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-04  2:16 [PATCH 00/13] Fix the DAX-gup mistake Dan Williams
2022-09-04  2:16 ` [PATCH 01/13] fsdax: Rename "busy page" to "pinned page" Dan Williams
2022-09-04  2:16 ` [PATCH 02/13] fsdax: Use page_maybe_dma_pinned() for DAX vs DMA collisions Dan Williams
2022-09-06 12:07   ` Jason Gunthorpe
2022-09-04  2:16 ` [PATCH 03/13] fsdax: Delete put_devmap_managed_page_refs() Dan Williams
2022-09-04  2:16 ` [PATCH 04/13] fsdax: Update dax_insert_entry() calling convention to return an error Dan Williams
2022-09-04  2:16 ` [PATCH 05/13] fsdax: Cleanup dax_associate_entry() Dan Williams
2022-09-04  2:16 ` [PATCH 06/13] fsdax: Rework dax_insert_entry() calling convention Dan Williams
2022-09-04  2:16 ` [PATCH 07/13] fsdax: Manage pgmap references at entry insertion and deletion Dan Williams
2022-09-06 12:30   ` Jason Gunthorpe
2022-09-04  2:16 ` [PATCH 08/13] devdax: Minor warning fixups Dan Williams
2022-09-04  2:16 ` [PATCH 09/13] devdax: Move address_space helpers to the DAX core Dan Williams
2022-09-04  2:16 ` [PATCH 10/13] dax: Prep dax_{associate, disassociate}_entry() for compound pages Dan Williams
2022-09-04  2:17 ` [PATCH 11/13] devdax: add PUD support to the DAX mapping infrastructure Dan Williams
2022-09-04  2:17 ` [PATCH 12/13] devdax: Use dax_insert_entry() + dax_delete_mapping_entry() Dan Williams
2022-09-04  2:17 ` [PATCH 13/13] mm/gup: Drop DAX pgmap accounting Dan Williams
2022-09-06 13:05 ` [PATCH 00/13] Fix the DAX-gup mistake Jason Gunthorpe
2022-09-06 17:23   ` Dan Williams
2022-09-06 17:29     ` Jason Gunthorpe
2022-09-06 18:37       ` Dan Williams
2022-09-06 18:49         ` Jason Gunthorpe
2022-09-06 19:41           ` Dan Williams
2022-09-07  0:54             ` Dan Williams
2022-09-07 12:58               ` Jason Gunthorpe
2022-09-07 17:10                 ` Dan Williams
2022-09-07 18:43                   ` Dan Williams
2022-09-07 19:30                     ` Jason Gunthorpe
2022-09-07 20:45                       ` Dan Williams
2022-09-08 18:49                         ` Jason Gunthorpe
2022-09-08 19:27                           ` Dan Williams
2022-09-09 11:53                             ` Jason Gunthorpe
2022-09-09 17:52                               ` Dan Williams [this message]
2022-09-09 18:11                             ` Matthew Wilcox

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