linux-kernel - Re: [PATCH v2] scsi: st: convert convert get_user_pages() --> pin_user

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Date:   Fri, 5 Jun 2020 13:17:58 +0300
From:   "Kai Mäkisara (Kolumbus)" 
        <kai.makisara@...umbus.fi>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        "James E . J . Bottomley" <jejb@...ux.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org
Subject: Re: [PATCH v2] scsi: st: convert convert get_user_pages() -->
 pin_user_pages()



> On 26. May 2020, at 21.27, John Hubbard <jhubbard@...dia.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@...umbus.fi>
> Cc: Bart Van Assche <bvanassche@....org>
> Cc: James E.J. Bottomley <jejb@...ux.ibm.com>
> Cc: Martin K. Petersen <martin.petersen@...cle.com>
> Cc: linux-scsi@...r.kernel.org
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

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