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SubjectRe: [PATCH v2] scsi: st: convert convert get_user_pages() --> pin_user_pages()
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> On 26. May 2020, at 21.27, John Hubbard <jhubbard@nvidia.com> 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":
> 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>

Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>

> ---
>
> 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

I am not a memory management expert, but looks correct and necessary to me
with the current gup implementation. (You can changed Acked-by to
Reviewed-by if it is necessary and you accept this as a review.)

Thanks,
Kai

>
> 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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