linux-kernel - Re: [RFC 1/3] mm: change vma_is_anonymous to vma_is_private

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Message-ID: <1f0f3254-31bd-5d09-6520-1897b3c5755b@redhat.com>
Date:   Fri, 22 Apr 2022 10:00:41 -0400
From:   Nico Pache <npache@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC 1/3] mm: change vma_is_anonymous to vma_is_private_anon



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