Re: [RFC 1/3] mm: change vma_is_anonymous to vma_is_private_anon

From: Nico Pache <npache@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Hari Bathini <hbathini@linux.ibm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Yang Shi <shy828301@gmail.com>, Miaohe Lin <linmiaohe@huawei.com>,
	William Kucharski <william.kucharski@oracle.com>,
	Hugh Dickins <hughd@google.com>
Subject: Re: [RFC 1/3] mm: change vma_is_anonymous to vma_is_private_anon
Date: Fri, 22 Apr 2022 10:00:41 -0400	[thread overview]
Message-ID: <1f0f3254-31bd-5d09-6520-1897b3c5755b@redhat.com> (raw)
In-Reply-To: <YmGwYKtz3sFkJlcv@casper.infradead.org>

On 4/21/22 15:28, Matthew Wilcox wrote:
> On Thu, Apr 21, 2022 at 03:05:31PM -0400, Nico Pache wrote:
>> The vma_is_anonymous function isn't fully indicative of what it checks.
>>
>> Without having full knowledge of the mmap process, one may incorrectly
>> assume this covers all types of anonymous memory; which is not the case.
> 
> Is your complaint that anonymous memory can also be found in file VMAs
> that were mapped with MAP_PRIVATE?  ie COWed pages?
I should have been more descriptive in my commit msg about how I came to this
conclusion.

From my understanding of the mmap process, a vma->vm_ops field is only NULL when
mmapped as !file and !shared:

	if (file){
		...
	} else if (vm_flags & VM_SHARED) { 	//ANON SHARED
		error = shmem_zero_setup(vma);
	        if (error)
        		goto free_vma;
	} else { 				//ANON PRIVATE
		vma_set_anonymous(vma);		//set vma->vm_ops= NULL
	}

To me this means that the VMA is PRIVATE ANON memory. The vma_is_anonymous
function returns true when vm_ops == NULL. So my intentions were to more
accurately describe what we are checking for. I could be wrong though thats why
I started with an RFC :)

There could be some aspect of COW that I dont fully understand. It is not
something I've looked into much.

Cheers,
-- Nico