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From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	"Jason Wang" <jasowang@redhat.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Jérôme Glisse" <jglisse@redhat.com>, "Jan Kara" <jack@suse.cz>,
	"Dave Chinner" <david@fromorbit.com>,
	"Souptick Joarder" <jrdr.linux@gmail.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
Date: Fri, 12 Jun 2020 13:03:07 -0700	[thread overview]
Message-ID: <d717e596-4ac7-9350-8734-379852c151d2@nvidia.com> (raw)
In-Reply-To: <20200612192426.GK8681@bombadil.infradead.org>

On 2020-06-12 12:24, Matthew Wilcox wrote:
> On Fri, May 29, 2020 at 04:43:08PM -0700, John Hubbard wrote:
>> +CASE 5: Pinning in order to write to the data within the page
>> +-------------------------------------------------------------
>> +Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
>> +access page's data, unpin" can cause a problem. Case 5 may be considered a
>> +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
>> +other words, if the code is neither Case 1 nor Case 2, it may still require
>> +FOLL_PIN, for patterns like this:
>> +
>> +Correct (uses FOLL_PIN calls):
>> +    pin_user_pages()
>> +    access the data within the pages
>> +    set_page_dirty_lock()
>> +    unpin_user_pages()
>> +
>> +INCORRECT (uses FOLL_GET calls):
>> +    get_user_pages()
>> +    access the data within the pages
>> +    set_page_dirty_lock()
>> +    put_page()
> 
> Why does this case need to pin?  Why can't it just do ...
> 
> 	get_user_pages()
> 	lock_page(page);
> 	... modify the data ...
> 	set_page_dirty(page);
> 	unlock_page(page);
> 

Yes, it could do that. And that would also make a good additional "correct"
example. Especially for the case of just dealing with a single page,
lock_page() has the benefit of completely fixing the problem *today*,
without waiting for the pin_user_pages*() handling improvements to get
implemented.

And it's also another (probably better) way to fix the vhost.c problem, than
commit 690623e1b496 ("vhost: convert get_user_pages() --> pin_user_pages()").

I'm inclined to leave vhost.c alone for now, unless someone really prefers
it to be changed, but to update the Case 5 documentation with your point
above. Sound about right?


thanks,
-- 
John Hubbard
NVIDIA

  reply	other threads:[~2020-06-12 20:03 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-29 23:43 [PATCH 0/2] vhost, docs: convert to pin_user_pages(), new " John Hubbard
2020-05-29 23:43 ` [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a " John Hubbard
2020-05-31  7:11   ` Souptick Joarder
2020-06-01  5:11     ` John Hubbard
2020-06-12 19:24   ` Matthew Wilcox
2020-06-12 20:03     ` John Hubbard [this message]
2020-05-29 23:43 ` [PATCH 2/2] vhost: convert get_user_pages() --> pin_user_pages() John Hubbard
2020-06-01 11:30   ` Jan Kara
2020-06-02  4:22   ` Michael S. Tsirkin

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