Re: [PATCH v2] scsi: st: convert convert get_user_pages() --> pin_user_pages()

From: John Hubbard
Date: Tue May 26 2020 - 21:05:04 EST

For some reason, the "convert convert" subject line is really hard to get rid of
from my scsi st patch. In this case, I'd dropped the patch entirely,
and recreated it with the old subject line somehow. Sorry about that
persistent typo!

I'll send a v3 if necessary, to correct that.

John Hubbard

On 2020-05-26 11:27, John Hubbard wrote:
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":


Cc: "Kai MÃkisara (Kolumbus)" <kai.makisara@xxxxxxxxxxx>
Cc: Bart Van Assche <bvanassche@xxxxxxx>
Cc: James E.J. Bottomley <jejb@xxxxxxxxxxxxx>
Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Cc: linux-scsi@xxxxxxxxxxxxxxx
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>


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.


John Hubbard

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,
/* 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;
if (res > 0) {
- for (j=0; j < res; j++)
- put_page(pages[j]);
+ unpin_user_pages(pages, res);
res = 0;
@@ -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);
- }
STbp->mapped_pages = NULL;