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

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Message-ID: <34000b9d-1fc3-fef5-d048-3cb3f8a36f1d@redhat.com>
Date:   Thu, 28 Apr 2022 18:14:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nico Pache <npache@...hat.com>,
        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 22.04.22 16:00, Nico Pache wrote:
> 
> 
> 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 :)

Shared anon in the kernel is really just shmem.  The user space notion
is MAP_ANON|MAP_SHARED, but that really just maps to shmem and the
kernel doesn't really call that thing anonymous memory.

So I agree, renaming this is not appropriate.

-- 
Thanks,

David / dhildenb