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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb