linux-kernel - Re: [RFC v1 1/2] mm/gup: fixup for 9947ea2c1e608e32 "mm/gup: track FOLL

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Date:   Fri, 28 Feb 2020 15:08:35 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        <linux-next@...r.kernel.org>, <akpm@...ux-foundation.org>
CC:     <borntraeger@...ibm.com>, <david@...hat.com>,
        <aarcange@...hat.com>, <linux-mm@...ck.org>,
        <frankja@...ux.ibm.com>, <sfr@...b.auug.org.au>,
        <linux-kernel@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [RFC v1 1/2] mm/gup: fixup for 9947ea2c1e608e32 "mm/gup: track
 FOLL_PIN pages"

On 2/28/20 7:43 AM, Claudio Imbrenda wrote:
> In case pin fails, we need to unpin, a simple put_page will not be enough
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> ---
>  mm/gup.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index f589299b0d4a..0b9a806898f3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2134,7 +2134,10 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  			goto pte_unmap;
>  
>  		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> -			put_page(head);
> +			if (flags & FOLL_GET)
> +				put_page(head);
> +			else if (flags & FOLL_PIN)
> +				unpin_user_page(head);

Hi Claudio,

Instead, I think that should actually be:

		put_compound_head(page, 1, flags);

which does a bit more (bug checks and /proc/vmstat instrumentation) than your diff,
but has the same basic idea: call the right "put" function.  

...oh, actually, I see you have the commit hash in the subject line. Instead, it should 
be in the commit description. Let's maybe change the subject line to approx:

    mm/gup: Fix a missing put_compound_head() call in gup_pte_range()

And the write up...how about something like this, if you like it:

try_grab_compound_head() must be undone via put_compound_head(), not put_page().
This was missed in the original implementation of the gup/dma tracking patch, so
fix it now.

Fixes: 0ea2781c3de4 ("mm/gup: track FOLL_PIN pages")

(Aside: I'm using the linux-next commit hash. How does one get the correct hash before
it goes to mainline? I guess maintainer scripts fix all those up?)

It's also good to Cc some reviewers in case I'm overlooking something, so I'm adding
Jan and Kirill.

thanks,
-- 
John Hubbard
NVIDIA
>  			goto pte_unmap;
>  		}
>  
>