Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages

[David Hildenbrand <david@redhat.com>]

On 14.07.21 18:24, David Hildenbrand wrote:
> On 14.07.21 18:08, Peter Xu wrote:
>> On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu wrote:
>>> When a page allocated using the MAP_SHARED flag is swapped out, its pagemap
>>> entry is cleared. In many cases, there is no difference between swapped-out
>>> shared pages and newly allocated, non-dirty pages in the pagemap interface.
>>>
>>> This patch addresses the behaviour and modifies pte_to_pagemap_entry() to
>>> make use of the XArray associated with the virtual memory area struct
>>> passed as an argument. The XArray contains the location of virtual pages
>>> in the page cache, swap cache or on disk. If they are on either of the
>>> caches, then the original implementation still works. If not, then the
>>> missing information will be retrieved from the XArray.
>>>
>>> Co-developed-by: Florian Schmidt <florian.schmidt@nutanix.com>
>>> Signed-off-by: Florian Schmidt <florian.schmidt@nutanix.com>
>>> Co-developed-by: Carl Waldspurger <carl.waldspurger@nutanix.com>
>>> Signed-off-by: Carl Waldspurger <carl.waldspurger@nutanix.com>
>>> Co-developed-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
>>> Signed-off-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
>>> Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@nutanix.com>
>>> ---
>>>    fs/proc/task_mmu.c | 37 +++++++++++++++++++++++++++++--------
>>>    1 file changed, 29 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index eb97468dfe4c..b17c8aedd32e 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -1359,12 +1359,25 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
>>>    	return err;
>>>    }
>>>    
>>> +static void *get_xa_entry_at_vma_addr(struct vm_area_struct *vma,
>>> +		unsigned long addr)
>>> +{
>>> +	struct inode *inode = file_inode(vma->vm_file);
>>> +	struct address_space *mapping = inode->i_mapping;
>>> +	pgoff_t offset = linear_page_index(vma, addr);
>>> +
>>> +	return xa_load(&mapping->i_pages, offset);
>>> +}
>>> +
>>>    static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>>>    		struct vm_area_struct *vma, unsigned long addr, pte_t pte)
>>>    {
>>>    	u64 frame = 0, flags = 0;
>>>    	struct page *page = NULL;
>>>    
>>> +	if (vma->vm_flags & VM_SOFTDIRTY)
>>> +		flags |= PM_SOFT_DIRTY;
>>> +
>>>    	if (pte_present(pte)) {
>>>    		if (pm->show_pfn)
>>>    			frame = pte_pfn(pte);
>>> @@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>>>    			flags |= PM_SOFT_DIRTY;
>>>    		if (pte_uffd_wp(pte))
>>>    			flags |= PM_UFFD_WP;
>>> -	} else if (is_swap_pte(pte)) {
>>> +	} else if (is_swap_pte(pte) || shmem_file(vma->vm_file)) {
>>>    		swp_entry_t entry;
>>> -		if (pte_swp_soft_dirty(pte))
>>> -			flags |= PM_SOFT_DIRTY;
>>> -		if (pte_swp_uffd_wp(pte))
>>> -			flags |= PM_UFFD_WP;
>>> -		entry = pte_to_swp_entry(pte);
>>> +		if (is_swap_pte(pte)) {
>>> +			entry = pte_to_swp_entry(pte);
>>> +			if (pte_swp_soft_dirty(pte))
>>> +				flags |= PM_SOFT_DIRTY;
>>> +			if (pte_swp_uffd_wp(pte))
>>> +				flags |= PM_UFFD_WP;
>>> +		} else {
>>> +			void *xa_entry = get_xa_entry_at_vma_addr(vma, addr);
>>> +
>>> +			if (xa_is_value(xa_entry))
>>> +				entry = radix_to_swp_entry(xa_entry);
>>> +			else
>>> +				goto out;
>>> +		}
>>>    		if (pm->show_pfn)
>>>    			frame = swp_type(entry) |
>>>    				(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
>>> @@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>>>    		flags |= PM_FILE;
>>>    	if (page && page_mapcount(page) == 1)
>>>    		flags |= PM_MMAP_EXCLUSIVE;
>>> -	if (vma->vm_flags & VM_SOFTDIRTY)
>>> -		flags |= PM_SOFT_DIRTY;
>>
>> IMHO moving this to the entry will only work for the initial iteration, however
>> it won't really help anything, as soft-dirty should always be used in pair with
>> clear_refs written with value "4" first otherwise all pages will be marked
>> soft-dirty then the pagemap data is meaningless.
>>
>> After the "write 4" op VM_SOFTDIRTY will be cleared and I expect the test case
>> to see all zeros again even with the patch.
>>
>> I think one way to fix this is to do something similar to uffd-wp: we leave a
>> marker in pte showing that this is soft-dirtied pte even if swapped out.
> 
> How exactly does such a pte look like? Simply pte_none() with another
> bit set?
> 
>> However we don't have a mechanism for that yet in current linux, and the
>> uffd-wp series is the first one trying to introduce something like that.
> 
> Can you give me a pointer? I'm very interested in learning how to
> identify this case.
> 

I assume it's 
https://lore.kernel.org/lkml/20210527202117.30689-1-peterx@redhat.com/

-- 
Thanks,

David / dhildenb