* [PATCH v2 0/2] vhost, docs: convert to pin_user_pages(), new "case 5"
@ 2020-06-01 5:26 John Hubbard
2020-06-01 5:26 ` [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a " John Hubbard
2020-06-01 5:26 ` [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages() John Hubbard
0 siblings, 2 replies; 5+ messages in thread
From: John Hubbard @ 2020-06-01 5:26 UTC (permalink / raw)
To: Andrew Morton
Cc: Michael S . Tsirkin, Jason Wang, Vlastimil Babka,
Jérôme Glisse, Jan Kara, Dave Chinner,
Souptick Joarder, Jonathan Corbet, linux-doc, linux-fsdevel, kvm,
virtualization, netdev, LKML, linux-mm, John Hubbard
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@nvidia.com
[2] https://lore.kernel.org/r/20200529070343.GL14550@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
^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
2020-06-01 5:26 [PATCH v2 0/2] vhost, docs: convert to pin_user_pages(), new "case 5" John Hubbard
@ 2020-06-01 5:26 ` John Hubbard
2020-06-01 11:34 ` Jan Kara
2020-06-01 5:26 ` [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages() John Hubbard
1 sibling, 1 reply; 5+ messages in thread
From: John Hubbard @ 2020-06-01 5:26 UTC (permalink / raw)
To: Andrew Morton
Cc: Michael S . Tsirkin, Jason Wang, Vlastimil Babka,
Jérôme Glisse, Jan Kara, Dave Chinner,
Souptick Joarder, Jonathan Corbet, linux-doc, linux-fsdevel, kvm,
virtualization, netdev, LKML, linux-mm, John Hubbard
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 | 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
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages()
2020-06-01 5:26 [PATCH v2 0/2] vhost, docs: convert to pin_user_pages(), new "case 5" John Hubbard
2020-06-01 5:26 ` [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a " John Hubbard
@ 2020-06-01 5:26 ` John Hubbard
2020-06-02 6:08 ` Pankaj Gupta
1 sibling, 1 reply; 5+ messages in thread
From: John Hubbard @ 2020-06-01 5:26 UTC (permalink / raw)
To: Andrew Morton
Cc: Michael S . Tsirkin, Jason Wang, Vlastimil Babka,
Jérôme Glisse, Jan Kara, Dave Chinner,
Souptick Joarder, Jonathan Corbet, linux-doc, linux-fsdevel, kvm,
virtualization, netdev, LKML, linux-mm, John Hubbard
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@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: netdev@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@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
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
2020-06-01 5:26 ` [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a " John Hubbard
@ 2020-06-01 11:34 ` Jan Kara
0 siblings, 0 replies; 5+ messages in thread
From: Jan Kara @ 2020-06-01 11:34 UTC (permalink / raw)
To: John Hubbard
Cc: Andrew Morton, Michael S . Tsirkin, Jason Wang, Vlastimil Babka,
Jérôme Glisse, Jan Kara, Dave Chinner,
Souptick Joarder, Jonathan Corbet, linux-doc, linux-fsdevel, kvm,
virtualization, netdev, LKML, linux-mm
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@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>
Looks good to me. You can add:
Reviewed-by: Jan Kara <jack@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@suse.com>
SUSE Labs, CR
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages()
2020-06-01 5:26 ` [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages() John Hubbard
@ 2020-06-02 6:08 ` Pankaj Gupta
0 siblings, 0 replies; 5+ messages in thread
From: Pankaj Gupta @ 2020-06-02 6:08 UTC (permalink / raw)
To: John Hubbard
Cc: Andrew Morton, Michael S . Tsirkin, Jason Wang, Vlastimil Babka,
Jérôme Glisse, Jan Kara, Dave Chinner,
Souptick Joarder, Jonathan Corbet, linux-doc, linux-fsdevel, kvm,
virtualization, netdev, LKML, Linux MM
> 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@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: kvm@vger.kernel.org
> Cc: virtualization@lists.linux-foundation.org
> Cc: netdev@vger.kernel.org
> Signed-off-by: John Hubbard <jhubbard@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;
> }
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
^ permalink raw reply [flat|nested] 5+ messages in thread
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-- links below jump to the message on this page --
2020-06-01 5:26 [PATCH v2 0/2] vhost, docs: convert to pin_user_pages(), new "case 5" John Hubbard
2020-06-01 5:26 ` [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a " John Hubbard
2020-06-01 11:34 ` Jan Kara
2020-06-01 5:26 ` [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages() John Hubbard
2020-06-02 6:08 ` Pankaj Gupta
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