Subject: Re: linux-next: manual merge of the akpm-current tree with the kvms390 tree

From: Christian Borntraeger <[email-masked]>
Date: Thu, 27 Feb 2020 09:02:29 +0100

On 27.02.20 06:58, John Hubbard wrote:
> On 2/26/20 7:11 PM, Stephen Rothwell wrote:
>> Hi all,
>>
>> Today's linux-next merge of the akpm-current tree got a conflict in:
>>
>>   mm/gup.c
>>
>> between commit:
>>
>>   732b80e677b8 ("mm/gup/writeback: add callbacks for inaccessible pages")
>>
>> from the kvms390 tree and commit:
>>
>>   9947ea2c1e60 ("mm/gup: track FOLL_PIN pages")
>>
>> from the akpm-current tree.
>>
>> I fixed it up (see below - maybe not optimally) and can carry the fix as
>> necessary. This is now fixed as far as linux-next is concerned, but any
>> non trivial conflicts should be mentioned to your upstream maintainer
>> when your tree is submitted for merging.  You may also want to consider
>> cooperating with the maintainer of the conflicting tree to minimise any
>> particularly complex conflicts.
>>
> 
> Yes. Changes to mm/gup.c really should normally go through linux-mm and 
> Andrew's tree, if at all possible. This would have been caught, and figured out
> on linux-mm, had that been done--instead of leaving the linux-next maintainer
> trying to guess at how to resolve the conflict.

Yes. This patch should go via Andrew. Claudio is going to provide a fixed up
version that takes care of the new semantics.

This patch was posted several times on linux-mm (also before rc1) and I will 
drop it from my tree due to the conflict.

> 
> +Cc David Hildenbrand, who I see looked at the kvms390 proposed patch a bit.
> Maybe he has some opinions, especially about my questions below.
> 
> The fix-up below may (or may not) need some changes:

> 
> 
> diff --cc mm/gup.c
> index 354bcfbd844b,f589299b0d4a..000000000000
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@@ -269,18 -470,11 +468,19 @@@ retry
>   		goto retry;
>   	}
>   
> + 	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
> + 	if (unlikely(!try_grab_page(page, flags))) {
> + 		page = ERR_PTR(-ENOMEM);
> + 		goto out;
> + 	}
>  +	if (flags & FOLL_GET) {
> 
> 
> If I'm reading the diff correctly, I believe that line should *maybe* be changed to:
> 
> 	if (flags & (FOLL_GET | FOLL_PIN)) {
> 
> ...because each of those flags has a similar effect: pinned pages for DMA or RDMA
> use. So either flag will require a call to arch_make_page_accessible()...except that
> I'm not sure that's what you want. Would the absence of a call to 
> arch_make_page_accessible() cause things like pin_user_pages() to not work correctly?
> Seems like it would, to me.
> 
> (I'm pretty unhappy that we have to ask this at the linux-next level.)
> 
> Also below...
> 
> 
> - 		if (unlikely(!try_get_page(page))) {
> - 			page = ERR_PTR(-ENOMEM);
> - 			goto out;
> - 		}
>  +		ret = arch_make_page_accessible(page);
>  +		if (ret) {
>  +			put_page(page);
> 
> 
> put_page() only works with FOLL_GET. So if we do allow to get here via either FOLL_GET or
> FOLL_PIN, the we need to do an unpin_user_page(), like this:
> 
> 		if (flags & FOLL_PIN)
> 			unpin_user_page(page);
> 		else
> 			put_page(page);
> 
> 
> 
>  +			page = ERR_PTR(ret);
>  +			goto out;
>  +		}
>  +	}
>   	if (flags & FOLL_TOUCH) {
>   		if ((flags & FOLL_WRITE) &&
>   		    !pte_dirty(pte) && !PageDirty(page))
> 
> thanks,
>