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From: Christoph Hellwig <hch@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>, Al Viro <viro@zeniv.linux.org.uk>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Hillf Danton <hdanton@sina.com>,
	Christian Brauner <brauner@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: Extending page pinning into fs/direct-io.c
Date: Tue, 23 May 2023 22:55:28 -0700	[thread overview]
Message-ID: <ZG2m0PGztI2BZEn9@infradead.org> (raw)
In-Reply-To: <3068545.1684872971@warthog.procyon.org.uk>

On Tue, May 23, 2023 at 09:16:11PM +0100, David Howells wrote:
> I've been poking at it this afternoon, but it doesn't look like it's going to
> be straightforward, unfortunately.  The mm folks have been withdrawing access
> to the pinning API behind the ramparts of the mm/ dir.  Further, the dio code
> will (I think), under some circumstances, arbitrarily insert the zero_page
> into a list of things that are maybe pinned or maybe unpinned, but I can (I
> think) also be given a pinned zero_page from the GUP code if the page tables
> point to one and a DIO-write is requested - so just doing if page == zero_page
> isn't sufficient.

Yes.  I think the proper workaround is to add a MM helper that just
pins a single page and make it available to direct-io.c.  It should not
be exported and clearly marked to not be used in new code.  

> What I'd like to do is to make the GUP code not take a ref on the zero_page
> if, say, FOLL_DONT_PIN_ZEROPAGE is passed in, and then make the bio cleanup
> code always ignore the zero_page.

I don't think that'll work, as we can't mix different pin vs get types
in a bio.  And that's really a good thing.

> Something that I noticed is that the dio code seems to wangle to page bits on
> the target pages for a DIO-read, which seems odd, but I'm not sure I fully
> understand the code yet.

I don't understand this sentence.

  reply	other threads:[~2023-05-24  5:55 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-22 20:57 [PATCH v21 0/6] block: Use page pinning David Howells
2023-05-22 20:57 ` [PATCH v21 1/6] iomap: Don't get an reference on ZERO_PAGE for direct I/O block zeroing David Howells
2023-05-23  8:07   ` Jan Kara
2023-05-23 12:35   ` Christian Brauner
2023-05-22 20:57 ` [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it David Howells
2023-05-23  8:07   ` Jan Kara
2023-05-23 12:37   ` Christian Brauner
2023-05-22 20:57 ` [PATCH v21 3/6] block: Replace BIO_NO_PAGE_REF with BIO_PAGE_REFFED with inverted logic David Howells
2023-05-23  8:07   ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 4/6] block: Add BIO_PAGE_PINNED and associated infrastructure David Howells
2023-05-23  8:08   ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 5/6] block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages David Howells
2023-05-23  8:15   ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 6/6] block: convert bio_map_user_iov " David Howells
2023-05-23  8:14   ` Jan Kara
2023-05-23  6:39 ` [PATCH v21 0/6] block: Use page pinning Christoph Hellwig
2023-05-23 20:16 ` Extending page pinning into fs/direct-io.c David Howells
2023-05-24  5:55   ` Christoph Hellwig [this message]
2023-05-24  7:06   ` David Hildenbrand
2023-05-24  8:47   ` David Howells
2023-05-25  9:51     ` Christoph Hellwig
2023-05-25 16:31     ` Linus Torvalds
2023-05-25 16:45       ` David Hildenbrand
2023-05-25 17:04         ` Linus Torvalds
2023-05-25 17:15         ` David Howells
2023-05-25 17:25           ` Linus Torvalds
2023-05-25 17:07       ` David Howells
2023-05-25 17:17         ` Linus Torvalds
2023-05-25 17:00     ` David Howells
2023-05-25 17:13       ` Linus Torvalds
2023-05-23 21:38 ` [PATCH v21 0/6] block: Use page pinning Jens Axboe
2023-05-24  5:52   ` Christoph Hellwig
2023-05-24 14:43     ` Jens Axboe
2023-05-24  7:35   ` David Howells

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