2020-08-18 04:25:05

by Souptick Joarder

Subject: [RFC PATCH] xen/gntdev.c: Convert get_user_pages*() to pin_user_pages*()

In 2019, we introduced pin_user_pages*() and now we are converting
get_user_pages*() to the new API as appropriate. [1] & [2] could
be referred for more information. This is case 5 as per document [1].

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

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

Signed-off-by: Souptick Joarder <[email protected]>
Cc: John Hubbard <[email protected]>
---
drivers/xen/gntdev.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 64a9025a..e480509 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -730,7 +730,7 @@ static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user *virt,
unsigned long xen_pfn;
int ret;

- ret = get_user_pages_fast(addr, 1, writeable ? FOLL_WRITE : 0, &page);
+ ret = pin_user_pages_fast(addr, 1, writeable ? FOLL_WRITE : 0, &page);
if (ret < 0)
return ret;

@@ -744,10 +744,7 @@ static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user *virt,

static void gntdev_put_pages(struct gntdev_copy_batch *batch)
{
- unsigned int i;
-
- for (i = 0; i < batch->nr_pages; i++)
- put_page(batch->pages[i]);
+ unpin_user_pages(batch->pages, batch->nr_pages);
batch->nr_pages = 0;
}

--
1.9.1

2020-08-20 00:30:05

by Boris Ostrovsky

Subject: Re: [RFC PATCH] xen/gntdev.c: Convert get_user_pages*() to pin_user_pages*()

On 8/18/20 12:32 AM, Souptick Joarder wrote:
> In 2019, we introduced pin_user_pages*() and now we are converting
> get_user_pages*() to the new API as appropriate. [1] & [2] could
> be referred for more information. This is case 5 as per document [1].
>
> [1] Documentation/core-api/pin_user_pages.rst
>
> [2] "Explicit pinning of user-space pages":
> https://lwn.net/Articles/807108/
>
> Signed-off-by: Souptick Joarder <[email protected]>
> Cc: John Hubbard <[email protected]>
> ---
> drivers/xen/gntdev.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 64a9025a..e480509 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -730,7 +730,7 @@ static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user *virt,
> unsigned long xen_pfn;
> int ret;
>
> - ret = get_user_pages_fast(addr, 1, writeable ? FOLL_WRITE : 0, &page);
> + ret = pin_user_pages_fast(addr, 1, writeable ? FOLL_WRITE : 0, &page);
> if (ret < 0)
> return ret;
>
> @@ -744,10 +744,7 @@ static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user *virt,
>
> static void gntdev_put_pages(struct gntdev_copy_batch *batch)
> {
> - unsigned int i;
> -
> - for (i = 0; i < batch->nr_pages; i++)
> - put_page(batch->pages[i]);
> + unpin_user_pages(batch->pages, batch->nr_pages);


There seems to be a bug in the original code (just like there was in
privcmd.c that you fixed earlier) --- when gntdev_get_page() is called
with writeable=true then the page needs to be marked dirty before being
put.


You can add a `bool writeable` to gntdev_copy_batch and set it in
gntdev_grant_copy_seg() (and drop `writeable` argument to
gntdev_get_page()) and then, based on batch->writeable, use either
unpin_user_pages() or unpin_user_pages_dirty_lock().


(But to fix this in stable branches please make first patch call
set_page_dirty_lock() for each page like you did in previous series)


-boris


> batch->nr_pages = 0;
> }
>