From: Christoph Hellwig <hch@infradead.org>
To: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: David Howells <dhowells@redhat.com>,
Christoph Hellwig <hch@infradead.org>,
David Hildenbrand <david@redhat.com>,
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>,
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,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()
Date: Fri, 26 May 2023 01:17:53 -0700 [thread overview]
Message-ID: <ZHBrMTkKiqhxBo5w@infradead.org> (raw)
In-Reply-To: <89c7f535-8fc5-4480-845f-de94f335d332@lucifer.local>
On Fri, May 26, 2023 at 09:10:33AM +0100, Lorenzo Stoakes wrote:
> On Thu, May 25, 2023 at 11:39:51PM +0100, David Howells wrote:
> > Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
> > to it from the page tables and make unpin_user_page*() correspondingly
> > ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a
> > zero page's refcount as we're only allowed ~2 million pins on it -
> > something that userspace can conceivably trigger.
>
> I guess we're not quite as concerned about FOLL_GET because FOLL_GET should
> be ephemeral and FOLL_PIN (horrifically) adds GUP_PIN_COUNTING_BIAS each
> time?
I think FOLL_GET would be just as useful. But given that we have
a few places that release pins while gets just do a put_page it would
be a lot more effort to audit all of them. Maybe it's better do only
do this once we've converted all the places that should do pin and
have very few FOLL_GET users left.
next prev parent reply other threads:[~2023-05-26 8:18 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-25 22:39 [RFC PATCH v2 0/3] block: Make old dio use iov_iter_extract_pages() and page pinning David Howells
2023-05-25 22:39 ` [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() David Howells
2023-05-26 8:10 ` Lorenzo Stoakes
2023-05-26 8:17 ` Christoph Hellwig [this message]
2023-05-26 8:22 ` Christoph Hellwig
2023-05-26 8:27 ` Lorenzo Stoakes
2023-05-26 8:29 ` David Howells
2023-05-26 8:43 ` David Howells
2023-05-26 8:58 ` Lorenzo Stoakes
2023-05-26 9:15 ` David Howells
2023-05-26 9:23 ` Lorenzo Stoakes
2023-05-26 9:24 ` David Hildenbrand
2023-05-25 22:39 ` [RFC PATCH v2 2/3] mm: Provide a function to get an additional pin on a page David Howells
2023-05-26 2:29 ` Linus Torvalds
2023-05-26 5:50 ` David Howells
2023-05-26 8:20 ` Christoph Hellwig
2023-05-26 8:31 ` David Howells
2023-05-25 22:39 ` [RFC PATCH v2 3/3] block: Use iov_iter_extract_pages() and page pinning in direct-io.c David Howells
2023-05-26 8:26 ` Christoph Hellwig
2023-05-26 8:33 ` David Howells
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