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Date:   Tue, 16 Jul 2019 02:22:16 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     ira.weiny@...el.com, gregkh@...uxfoundation.org,
        Matt.Sickler@...tronics.com, jglisse@...hat.com,
        devel@...verdev.osuosl.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: kpc2000: Convert put_page() to put_user_page*()

On Mon, Jul 15, 2019 at 01:14:13PM -0700, John Hubbard wrote:
> On 7/15/19 12:52 PM, Bharath Vedartham wrote:
> > There have been issues with get_user_pages and filesystem writeback.
> > The issues are better described in [1].
> > 
> > The solution being proposed wants to keep track of gup_pinned pages which will allow to take furthur steps to coordinate between subsystems using gup.
> > 
> > put_user_page() simply calls put_page inside for now. But the implementation will change once all call sites of put_page() are converted.
> > 
> > I currently do not have the driver to test. Could I have some suggestions to test this code? The solution is currently implemented in [2] and
> > it would be great if we could apply the patch on top of [2] and run some tests to check if any regressions occur.
> 
> Hi Bharath,
> 
> Process point: the above paragraph, and other meta-questions (about the patch, rather than part of the patch) should be placed either after the "---", or in a cover letter (git-send-email --cover-letter). That way, the patch itself is in a commit-able state.
> 
> One more below:
Will fix that in the next version. 
> > 
> > [1] https://lwn.net/Articles/753027/
> > [2] https://github.com/johnhubbard/linux/tree/gup_dma_core
> > 
> > Cc: Matt Sickler <Matt.Sickler@...tronics.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Jérôme Glisse <jglisse@...hat.com>
> > Cc: Ira Weiny <ira.weiny@...el.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: linux-mm@...ck.org
> > Cc: devel@...verdev.osuosl.org
> > 
> > Signed-off-by: Bharath Vedartham <linux.bhar@...il.com>
> > ---
> >  drivers/staging/kpc2000/kpc_dma/fileops.c | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/kpc2000/kpc_dma/fileops.c
> > index 6166587..82c70e6 100644
> > --- a/drivers/staging/kpc2000/kpc_dma/fileops.c
> > +++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
> > @@ -198,9 +198,7 @@ int  kpc_dma_transfer(struct dev_private_data *priv, struct kiocb *kcb, unsigned
> >  	sg_free_table(&acd->sgt);
> >   err_dma_map_sg:
> >   err_alloc_sg_table:
> > -	for (i = 0 ; i < acd->page_count ; i++){
> > -		put_page(acd->user_pages[i]);
> > -	}
> > +	put_user_pages(acd->user_pages, acd->page_count);
> >   err_get_user_pages:
> >  	kfree(acd->user_pages);
> >   err_alloc_userpages:
> > @@ -229,9 +227,7 @@ void  transfer_complete_cb(struct aio_cb_data *acd, size_t xfr_count, u32 flags)
> >  	
> >  	dma_unmap_sg(&acd->ldev->pldev->dev, acd->sgt.sgl, acd->sgt.nents, acd->ldev->dir);
> >  	
> > -	for (i = 0 ; i < acd->page_count ; i++){
> > -		put_page(acd->user_pages[i]);
> > -	}
> > +	put_user_pages(acd->user_pages, acd->page_count);
> >  	
> >  	sg_free_table(&acd->sgt);
> >  	
> > 
> 
> Because this is a common pattern, and because the code here doesn't likely need to set page dirty before the dma_unmap_sg call, I think the following would be better (it's untested), instead of the above diff hunk:
>
> diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/kpc2000/kpc_dma/fileops.c
> index 48ca88bc6b0b..d486f9866449 100644
> --- a/drivers/staging/kpc2000/kpc_dma/fileops.c
> +++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
> @@ -211,16 +211,13 @@ void  transfer_complete_cb(struct aio_cb_data *acd, size_t xfr_count, u32 flags)
>         BUG_ON(acd->ldev == NULL);
>         BUG_ON(acd->ldev->pldev == NULL);
>  
> -       for (i = 0 ; i < acd->page_count ; i++) {
> -               if (!PageReserved(acd->user_pages[i])) {
> -                       set_page_dirty(acd->user_pages[i]);
> -               }
> -       }
> -
>         dma_unmap_sg(&acd->ldev->pldev->dev, acd->sgt.sgl, acd->sgt.nents, acd->ldev->dir);
>  
>         for (i = 0 ; i < acd->page_count ; i++) {
> -               put_page(acd->user_pages[i]);
> +               if (!PageReserved(acd->user_pages[i])) {
> +                       put_user_pages_dirty(&acd->user_pages[i], 1);
> +               else
> +                       put_user_page(acd->user_pages[i]);
>         }
>  
>         sg_free_table(&acd->sgt);
I had my doubts on this. This definitley needs to be looked at by the
driver author. 
> Assuming that you make those two changes, you can add:
> 
>     Reviewed-by: John Hubbard <jhubbard@...dia.com>
Great!
> 
> thanks,
> -- 
> John Hubbard
> NVIDIA

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