linux-kernel - Re: [PATCH v1 1/2] mm/memory: cleanly support zeropage in vm_insert_page*(), vm_map_pages*() and vmf_insert

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Date: Tue, 21 May 2024 10:25:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Dan Williams <dan.j.williams@...el.com>, rostedt@...dmis.org
Subject: Re: [PATCH v1 1/2] mm/memory: cleanly support zeropage in
 vm_insert_page*(), vm_map_pages*() and vmf_insert_mixed()

On 17.05.24 17:07, Vincent Donnefort wrote:
> Hi David,
> 
> [...]
> 
>> -static int validate_page_before_insert(struct page *page)
>> +static bool vm_mixed_zeropage_allowed(struct vm_area_struct *vma)
>> +{
>> +	VM_WARN_ON_ONCE(vma->vm_flags & VM_PFNMAP);
>> +	/*
>> +	 * Whoever wants to forbid the zeropage after some zeropages
>> +	 * might already have been mapped has to scan the page tables and
>> +	 * bail out on any zeropages. Zeropages in COW mappings can
>> +	 * be unshared using FAULT_FLAG_UNSHARE faults.
>> +	 */
>> +	if (mm_forbids_zeropage(vma->vm_mm))
>> +		return false;
>> +	/* zeropages in COW mappings are common and unproblematic. */
>> +	if (is_cow_mapping(vma->vm_flags))
>> +		return true;
>> +	/* Mappings that do not allow for writable PTEs are unproblematic. */
>> +	if (!(vma->vm_flags & (VM_WRITE | VM_MAYWRITE)))
>> +		return false;
> 
> Shouldn't we return true here?

Indeed, thanks! I wish we would have user in the tree already that could 
exercise that code path.

[...]

>> @@ -2043,7 +2085,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte,
>>   
>>   	if (!page_count(page))
>>   		return -EINVAL;
> 
> This test here prevents inserting the zero-page.

You mean the existing page_count() check? or the (wrong) vma->vm_flags 
check in vm_mixed_zeropage_allowed() ?

-- 
Cheers,

David / dhildenb