Linux Virtualization - Jun 2020 - [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"

This is based on Linux 5.7, plus one prerequisite patch:
   "mm/gup: update pin_user_pages.rst for "case 3" (mmu
notifiers)" [1]

Changes since v1: removed references to set_page_dirty*(), in response to
Souptick Joarder's review (thanks!).

Cover letter for v1, edited/updated slightly:

It recently became clear to me that there are some get_user_pages*()
callers that don't fit neatly into any of the four cases that are so
far listed in pin_user_pages.rst. vhost.c is one of those.

Add a Case 5 to the documentation, and refer to that when converting
vhost.c.

Thanks to Jan Kara for helping me (again) in understanding the
interaction between get_user_pages() and page writeback [2].

Note that I have only compile-tested the vhost.c patch, although that
does also include cross-compiling for a few other arches. Any run-time
testing would be greatly appreciated.

[1] https://lore.kernel.org/r/20200527194953.11130-1-jhubbard at nvidia.com
[2] https://lore.kernel.org/r/20200529070343.GL14550 at quack2.suse.cz

John Hubbard (2):
  docs: mm/gup: pin_user_pages.rst: add a "case 5"
  vhost: convert get_user_pages() --> pin_user_pages()

 Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++
 drivers/vhost/vhost.c                     |  5 ++---
 2 files changed, 20 insertions(+), 3 deletions(-)


base-commit: 3d77e6a8804abcc0504c904bd6e5cdf3a5cf8162
-- 
2.26.2

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 at suse.cz>
Cc: Jan Kara <jack at suse.cz>
Cc: J?r?me Glisse <jglisse at redhat.com>
Cc: Dave Chinner <david at fromorbit.com>
Cc: Jonathan Corbet <corbet at lwn.net>
Cc: linux-doc at vger.kernel.org
Cc: linux-fsdevel at vger.kernel.org
Signed-off-by: John Hubbard <jhubbard at nvidia.com>
---
 Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/core-api/pin_user_pages.rst
b/Documentation/core-api/pin_user_pages.rst
index 4675b04e8829..6068266dd303 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -171,6 +171,24 @@ 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,
+write to a 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()
+    write to the data within the pages
+    unpin_user_pages()
+
+INCORRECT (uses FOLL_GET calls):
+    get_user_pages()
+    write to the data within the pages
+    put_page()
+
 page_maybe_dma_pinned(): the whole point of pinning
 ================================================== 
-- 
2.26.2

This code was using get_user_pages*(), in approximately a "Case 5"
scenario (accessing the data within a page), using the categorization
from [1]. That means that it's time to convert the get_user_pages*() +
put_page() calls to pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Jason Wang <jasowang at redhat.com>
Cc: kvm at vger.kernel.org
Cc: virtualization at lists.linux-foundation.org
Cc: netdev at vger.kernel.org
Signed-off-by: John Hubbard <jhubbard at nvidia.com>
---
 drivers/vhost/vhost.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 21a59b598ed8..596132a96cd5 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1762,15 +1762,14 @@ static int set_bit_to_user(int nr, void __user *addr)
 	int bit = nr + (log % PAGE_SIZE) * 8;
 	int r;
 
-	r = get_user_pages_fast(log, 1, FOLL_WRITE, &page);
+	r = pin_user_pages_fast(log, 1, FOLL_WRITE, &page);
 	if (r < 0)
 		return r;
 	BUG_ON(r != 1);
 	base = kmap_atomic(page);
 	set_bit(bit, base);
 	kunmap_atomic(base);
-	set_page_dirty_lock(page);
-	put_page(page);
+	unpin_user_pages_dirty_lock(&page, 1, true);
 	return 0;
 }
 
-- 
2.26.2

On Sun 31-05-20 22:26:32, John Hubbard wrote:
> 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 at suse.cz>
> Cc: Jan Kara <jack at suse.cz>
> Cc: J?r?me Glisse <jglisse at redhat.com>
> Cc: Dave Chinner <david at fromorbit.com>
> Cc: Jonathan Corbet <corbet at lwn.net>
> Cc: linux-doc at vger.kernel.org
> Cc: linux-fsdevel at vger.kernel.org
> Signed-off-by: John Hubbard <jhubbard at nvidia.com>
Looks good to me. You can add:

Reviewed-by: Jan Kara <jack at suse.cz>

								Honza
> ---
>  Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/Documentation/core-api/pin_user_pages.rst
b/Documentation/core-api/pin_user_pages.rst
> index 4675b04e8829..6068266dd303 100644
> --- a/Documentation/core-api/pin_user_pages.rst
> +++ b/Documentation/core-api/pin_user_pages.rst
> @@ -171,6 +171,24 @@ 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,
> +write to a 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()
> +    write to the data within the pages
> +    unpin_user_pages()
> +
> +INCORRECT (uses FOLL_GET calls):
> +    get_user_pages()
> +    write to the data within the pages
> +    put_page()
> +
>  page_maybe_dma_pinned(): the whole point of pinning
>  ==================================================>  
> -- 
> 2.26.2
> 
-- 
Jan Kara <jack at suse.com>
SUSE Labs, CR