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

[Peter Xu]

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@xxxxxxxxxxx>
> Signed-off-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
> Co-developed-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
> Signed-off-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
> Co-developed-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> Signed-off-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@xxxxxxxxxxx>
> ---
>  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.
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.

Thanks,

-- 
Peter Xu