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

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