Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page refcount

From: Vlastimil Babka <vbabka@suse.cz>
To: Ajay Kaher <akaher@vmware.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
Cc: "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"punit.agrawal@arm.com" <punit.agrawal@arm.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	"willy@infradead.org" <willy@infradead.org>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"mszeredi@redhat.com" <mszeredi@redhat.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Srivatsa Bhat <srivatsab@vmware.com>,
	"srivatsa@csail.mit.edu" <srivatsa@csail.mit.edu>,
	Alexey Makhalov <amakhalov@vmware.com>,
	Srinidhi Rao <srinidhir@vmware.com>,
	Vikash Bansal <bvikas@vmware.com>,
	Anish Swaminathan <anishs@vmware.com>,
	Vasavi Sirnapalli <vsirnapalli@vmware.com>,
	Steven Rostedt <srostedt@vmware.com>,
	"stable@kernel.org" <stable@kernel.org>,
	Ben Hutchings <ben@decadent.org.uk>
Subject: Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page refcount
Date: Wed, 6 Nov 2019 09:55:30 +0100	[thread overview]
Message-ID: <35d74931-2c18-00ff-7622-522a79be9103@suse.cz> (raw)
In-Reply-To: <0E5175FB-7058-4211-9AA4-9D5E2F6A30B9@vmware.com>

On 10/25/19 8:18 AM, Ajay Kaher wrote:
> On 17/10/19, 9:58 PM, "Ajay Kaher" <akaher@vmware.com> wrote:
>     
>> > This seems to have the same issue as the 4.9 stable version [1], in not
>> > touching the arch-specific gup.c variants.
>> >    
>> > [1]
>> > https://lore.kernel.org/lkml/6650323f-dbc9-f069-000b-f6b0f941a065@suse.cz/
>>    
>> Thanks Vlastimil for highlighting this here.
>> 
>> Yes, arch-specific gup.c variants also need to handle not only for 4.4.y,
>> however it should be handled till 4.19.y. I believe it's better to start
>> from 4.19.y and then backport those changes till 4.4.y.
>>    
>> Affected areas of gup.c (where page->count have been used) are:
>> #1: get_page() used in these files and this is safe as
>>        it's defined in mm.h (here it's already taken care of)
>> #2: get_head_page_multiple() has following:
>>               VM_BUG_ON_PAGE(page_count(page) == 0, page);
>>          Need to change this to:
>>               VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
>> #3: Some of the files have used page_cache_get_speculative(),
>>        page_cache_add_speculative() with combination of compound_head(),
>>        this scenario needs to be handled as it was handled here:
>>            https://lore.kernel.org/stable/1570581863-12090-7-git-send-email-akaher@vmware.com/
>>    
>> Please share with me any suggestions or patches if you have already  
>> worked on this.
>>    
>> Could we handle arch-specific gup.c in different patch sets and 
>> let these patches to merge to 4.4.y?
>   
> Vlastimil, please suggest if it's fine to merge these patches to 4.4.y

I'm not sure if it makes much sense to merge them without the arch-specific gup
support, when we're aware that it's missing.

> and handle arch-specific gup.c in different patch sets starts from 4.19.y,

Actually arch-specific gup.c were removed in 4.13, so it's enough to start from
4.9.y, which I'm going to finally look into.

> then backport all the way to 4.4.y. 
> 
> Greg, any suggestion from your side.
> 
>>    - Ajay
>     
>     
>     
> 