Re: [PATCH] fs/proc/task_mmu: report SOFT_DIRTY bits through the PAGEMAP_SCAN ioctl

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 11/2/23 10:33 PM, Andrei Vagin wrote:
> The PAGEMAP_SCAN ioctl returns information regarding page table entries.
> It is more efficient compared to reading pagemap files. CRIU can start
> to utilize this ioctl, but it needs info about soft-dirty bits to track
> memory changes.
> 
> We are aware of a new method for tracking memory changes implemented in
> the PAGEMAP_SCAN ioctl. For CRIU, the primary advantage of this method
> is its usability by unprivileged users. However, it is not feasible to
> transparently replace the soft-dirty tracker with the new one. The main
> problem here is userfault descriptors that have to be preserved between
> pre-dump iterations.  It means criu continues supporting the soft-dirty
> method to avoid breakage for current users. The new method will be
> implemented as a separate feature.
> 
> Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
> Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> Signed-off-by: Andrei Vagin <avagin@google.com>
> ---
>  fs/proc/task_mmu.c      | 13 ++++++++++++-
>  include/uapi/linux/fs.h |  1 +
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index ef2eb12906da..cefa676bd33b 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1761,7 +1761,7 @@ static int pagemap_release(struct inode *inode, struct file *file)
>  #define PM_SCAN_CATEGORIES	(PAGE_IS_WPALLOWED | PAGE_IS_WRITTEN |	\
>  				 PAGE_IS_FILE |	PAGE_IS_PRESENT |	\
>  				 PAGE_IS_SWAPPED | PAGE_IS_PFNZERO |	\
> -				 PAGE_IS_HUGE)
> +				 PAGE_IS_HUGE | PAGE_IS_SOFT_DIRTY)
>  #define PM_SCAN_FLAGS		(PM_SCAN_WP_MATCHING | PM_SCAN_CHECK_WPASYNC)
>  
>  struct pagemap_scan_private {
> @@ -1853,12 +1853,16 @@ static unsigned long pagemap_thp_category(struct pagemap_scan_private *p,
Probably missed the pagemap_page_category() for normal pages. Add
PAGE_IS_SOFT_DIRTY support for normal pages in pagemap_page_category() just
like pagemap_thp_category().

tools/testing/selftests/mm/soft-dirty.c can be updated to test if we are
getting the exactly same data from the ioctl.

>  
>  		if (is_zero_pfn(pmd_pfn(pmd)))
>  			categories |= PAGE_IS_PFNZERO;
> +		if (pmd_soft_dirty(pmd))
> +			categories |= PAGE_IS_SOFT_DIRTY;
>  	} else if (is_swap_pmd(pmd)) {
>  		swp_entry_t swp;
>  
>  		categories |= PAGE_IS_SWAPPED;
>  		if (!pmd_swp_uffd_wp(pmd))
>  			categories |= PAGE_IS_WRITTEN;
> +		if (pmd_swp_soft_dirty(pmd))
> +			categories |= PAGE_IS_SOFT_DIRTY;
>  
>  		if (p->masks_of_interest & PAGE_IS_FILE) {
>  			swp = pmd_to_swp_entry(pmd);
> @@ -1905,10 +1909,14 @@ static unsigned long pagemap_hugetlb_category(pte_t pte)
>  			categories |= PAGE_IS_FILE;
>  		if (is_zero_pfn(pte_pfn(pte)))
>  			categories |= PAGE_IS_PFNZERO;
> +		if (pte_soft_dirty(pte))
> +			categories |= PAGE_IS_SOFT_DIRTY;
>  	} else if (is_swap_pte(pte)) {
>  		categories |= PAGE_IS_SWAPPED;
>  		if (!pte_swp_uffd_wp_any(pte))
>  			categories |= PAGE_IS_WRITTEN;
> +		if (pte_swp_soft_dirty(pte))
> +			categories |= PAGE_IS_SOFT_DIRTY;
>  	}
>  
>  	return categories;
> @@ -1991,6 +1999,9 @@ static int pagemap_scan_test_walk(unsigned long start, unsigned long end,
>  	if (vma->vm_flags & VM_PFNMAP)
>  		return 1;
>  
> +	if (vma->vm_flags & VM_SOFTDIRTY)
> +		vma_category |= PAGE_IS_SOFT_DIRTY;
> +
>  	if (!pagemap_scan_is_interesting_vma(vma_category, p))
>  		return 1;
>  
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index da43810b7485..48ad69f7722e 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
Add this identical change in tools/include/uapi/linux/fs.h as well.

> @@ -316,6 +316,7 @@ typedef int __bitwise __kernel_rwf_t;
>  #define PAGE_IS_SWAPPED		(1 << 4)
>  #define PAGE_IS_PFNZERO		(1 << 5)
>  #define PAGE_IS_HUGE		(1 << 6)
> +#define PAGE_IS_SOFT_DIRTY	(1 << 7)
Maybe one or couple of lines about it in the pagemap.rst.

>  
>  /*
>   * struct page_region - Page region with flags

-- 
BR,
Muhammad Usama Anjum