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Subject[PATCH v2] scsi: st: convert convert get_user_pages() --> pin_user_pages()
Date
This code was using get_user_pages*(), in a "Case 1" scenario
(Direct IO), 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.

Note that this effectively changes the code's behavior as well: it now
ultimately calls set_page_dirty_lock(), instead of SetPageDirty().This
is probably more accurate.

As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
dealing with a file backed page where we have reference on the inode it
hangs off." [3]

Also, this deletes one of the two FIXME comments (about refcounting),
because there is nothing wrong with the refcounting at this point.

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

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

[3] https://lore.kernel.org/r/20190723153640.GB720@lst.de

Cc: "Kai Mäkisara (Kolumbus)" <kai.makisara@kolumbus.fi>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: James E.J. Bottomley <jejb@linux.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---

Hi,

As mentioned in the v1 review thread, we probably still want/need
this. Or so I claim. :) Please see what you think...

Changes since v1: changed the commit log, to refer to Direct IO
(Case 1), instead of DMA/RDMA (Case 2). And added Bart to Cc.

v1:
https://lore.kernel.org/linux-scsi/20200519045525.2446851-1-jhubbard@nvidia.com/

thanks,
John Hubbard
NVIDIA


drivers/scsi/st.c | 20 +++++---------------
1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index c5f9b348b438..1e3eda9fa231 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -4922,7 +4922,7 @@ static int sgl_map_user_pages(struct st_buffer *STbp,
unsigned long end = (uaddr + count + PAGE_SIZE - 1) >> PAGE_SHIFT;
unsigned long start = uaddr >> PAGE_SHIFT;
const int nr_pages = end - start;
- int res, i, j;
+ int res, i;
struct page **pages;
struct rq_map_data *mdata = &STbp->map_data;

@@ -4944,7 +4944,7 @@ static int sgl_map_user_pages(struct st_buffer *STbp,

/* Try to fault in all of the necessary pages */
/* rw==READ means read from drive, write into memory area */
- res = get_user_pages_fast(uaddr, nr_pages, rw == READ ? FOLL_WRITE : 0,
+ res = pin_user_pages_fast(uaddr, nr_pages, rw == READ ? FOLL_WRITE : 0,
pages);

/* Errors and no page mapped should return here */
@@ -4964,8 +4964,7 @@ static int sgl_map_user_pages(struct st_buffer *STbp,
return nr_pages;
out_unmap:
if (res > 0) {
- for (j=0; j < res; j++)
- put_page(pages[j]);
+ unpin_user_pages(pages, res);
res = 0;
}
kfree(pages);
@@ -4977,18 +4976,9 @@ static int sgl_map_user_pages(struct st_buffer *STbp,
static int sgl_unmap_user_pages(struct st_buffer *STbp,
const unsigned int nr_pages, int dirtied)
{
- int i;
-
- for (i=0; i < nr_pages; i++) {
- struct page *page = STbp->mapped_pages[i];
+ /* FIXME: cache flush missing for rw==READ */
+ unpin_user_pages_dirty_lock(STbp->mapped_pages, nr_pages, dirtied);

- if (dirtied)
- SetPageDirty(page);
- /* FIXME: cache flush missing for rw==READ
- * FIXME: call the correct reference counting function
- */
- put_page(page);
- }
kfree(STbp->mapped_pages);
STbp->mapped_pages = NULL;

--
2.26.2
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