Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
From: John Hubbard <jhubbard@nvidia.com>
To: Souptick Joarder <jrdr.linux@gmail.com>
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>,
	"Jonathan Corbet" <corbet@lwn.net>,
	linux-doc@vger.kernel.org,
	linux-fsdevel <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 <linux-mm@kvack.org>
Subject: Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
Date: Sun, 31 May 2020 22:11:49 -0700	[thread overview]
Message-ID: <b8de5a5e-b53a-81e8-9165-405d203deb33@nvidia.com> (raw)
In-Reply-To: <CAFqt6zaCSngh7-N_qZ6-S3Cj8CHF8DTSPv8anP_oJg5E6UWu9g@mail.gmail.com>

On 2020-05-31 00:11, Souptick Joarder wrote:
...
>> diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
>> index 4675b04e8829..b9f2688a2c67 100644
>> --- a/Documentation/core-api/pin_user_pages.rst
>> +++ b/Documentation/core-api/pin_user_pages.rst
>> @@ -171,6 +171,26 @@ If only struct page data (as opposed to the actual memory contents that a page
>>   is tracking) is affected, then normal GUP calls are sufficient, and neither flag
>>   needs to be set.
>>
>> +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.
> 
> Will it be, *"pin, access page's data, set page dirty, unpin" * ?

Well...the problem can show up with just accessing (writing) the data.
But it is true that this statement is a little different from the
patterns below, which is confusing. I'll delete set_page_dirty() from each
of them, in order to avoid confusing things. (Although each is correct.)
And I'll also change the above to "pin, write to a page's data, upin".

set_page_dirty() interactions are really just extra credit here. :) And
fully read-only situations won't 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()
>> +

I'll send a v2 shortly.

thanks,
-- 
John Hubbard
NVIDIA

  reply	other threads:[~2020-06-01  5:11 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 [this message]
2020-06-12 19:24   ` Matthew Wilcox
2020-06-12 20:03     ` John Hubbard
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=b8de5a5e-b53a-81e8-9165-405d203deb33@nvidia.com \
    --to=jhubbard@nvidia.com \
    --cc=akpm@linux-foundation.org \
    --cc=corbet@lwn.net \
    --cc=david@fromorbit.com \
    --cc=jack@suse.cz \
    --cc=jasowang@redhat.com \
    --cc=jglisse@redhat.com \
    --cc=jrdr.linux@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mst@redhat.com \
    --cc=netdev@vger.kernel.org \
    --cc=vbabka@suse.cz \
    --cc=virtualization@lists.linux-foundation.org \
    --subject='Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).