Re: [PATCH 2/8] mm: memory: extend finish_fault() to support large folio

[David Hildenbrand <david@redhat.com>]

On 08.05.24 05:44, Baolin Wang wrote:
> 
> 
> On 2024/5/7 18:37, Ryan Roberts wrote:
>> On 06/05/2024 09:46, Baolin Wang wrote:
>>> Add large folio mapping establishment support for finish_fault() as a preparation,
>>> to support multi-size THP allocation of anonymous shmem pages in the following
>>> patches.
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>    mm/memory.c | 43 +++++++++++++++++++++++++++++++++----------
>>>    1 file changed, 33 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index eea6e4984eae..936377220b77 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -4747,9 +4747,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>    {
>>>    	struct vm_area_struct *vma = vmf->vma;
>>>    	struct page *page;
>>> +	struct folio *folio;
>>>    	vm_fault_t ret;
>>>    	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>>>    		      !(vma->vm_flags & VM_SHARED);
>>> +	int type, nr_pages, i;
>>> +	unsigned long addr = vmf->address;
>>>    
>>>    	/* Did we COW the page? */
>>>    	if (is_cow)
>>> @@ -4780,24 +4783,44 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>    			return VM_FAULT_OOM;
>>>    	}
>>>    
>>> +	folio = page_folio(page);
>>> +	nr_pages = folio_nr_pages(folio);
>>> +
>>> +	if (unlikely(userfaultfd_armed(vma))) {
>>> +		nr_pages = 1;
>>> +	} else if (nr_pages > 1) {
>>> +		unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>>> +		unsigned long end = start + nr_pages * PAGE_SIZE;
>>> +
>>> +		/* In case the folio size in page cache beyond the VMA limits. */
>>> +		addr = max(start, vma->vm_start);
>>> +		nr_pages = (min(end, vma->vm_end) - addr) >> PAGE_SHIFT;
>>> +
>>> +		page = folio_page(folio, (addr - start) >> PAGE_SHIFT);
>>
>> I still don't really follow the logic in this else if block. Isn't it possible
>> that finish_fault() gets called with a page from a folio that isn't aligned with
>> vmf->address?
>>
>> For example, let's say we have a file who's size is 64K and which is cached in a
>> single large folio in the page cache. But the file is mapped into a process at
>> VA 16K to 80K. Let's say we fault on the first page (VA=16K). You will calculate
> 
> For shmem, this doesn't happen because the VA is aligned with the
> hugepage size in the shmem_get_unmapped_area() function. See patch 7.

Does that cover mremap() and MAP_FIXED as well.

We should try doing this as cleanly as possible, to prepare for the 
future / corner cases.

-- 
Cheers,

David / dhildenb