Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "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, "John Hubbard" <jhubbard@nvidia.com>
Subject: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
Date: Fri, 29 May 2020 16:43:08 -0700	[thread overview]
Message-ID: <20200529234309.484480-2-jhubbard@nvidia.com> (raw)
In-Reply-To: <20200529234309.484480-1-jhubbard@nvidia.com>

There are four cases listed in pin_user_pages.rst. These are
intended to help developers figure out whether to use
get_user_pages*(), or pin_user_pages*(). However, the four cases
do not cover all the situations. For example, drivers/vhost/vhost.c
has a "pin, write to page, set page dirty, unpin" case.

Add a fifth case, to help explain that there is a general pattern
that requires pin_user_pages*() API calls.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 Documentation/core-api/pin_user_pages.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

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. 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()
+
 page_maybe_dma_pinned(): the whole point of pinning
 ===================================================
 
-- 
2.26.2


  reply	other threads:[~2020-05-29 23:43 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 ` John Hubbard [this message]
2020-05-31  7:11   ` [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a " Souptick Joarder
2020-06-01  5:11     ` John Hubbard
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=20200529234309.484480-2-jhubbard@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).