Re: [PATCH v1] fs/proc/task_mmu: Fix uffd-wp confusion in pagemap_scan_pmd_entry()

[Muhammad Usama Anjum]

Thanks for finding and fixing Ryan!

On 4/29/24 4:41 PM, Ryan Roberts wrote:
> pagemap_scan_pmd_entry() checks if uffd-wp is set on each pte to avoid
> unnecessary if set. However it was previously checking with
> `pte_uffd_wp(ptep_get(pte))` without first confirming that the pte was
> present. It is only valid to call pte_uffd_wp() for present ptes. For
> swap ptes, pte_swp_uffd_wp() must be called because the uffd-wp bit may
> be kept in a different position, depending on the arch.
> 
> This was leading to test failures in the pagemap_ioctl mm selftest, when
> bringing up uffd-wp support on arm64 due to incorrectly interpretting
> the uffd-wp status of migration entries.
> 
> Let's fix this by using the correct check based on pte_present(). While
> we are at it, let's pass the pte to make_uffd_wp_pte() to avoid the
> pointless extra ptep_get() which can't be optimized out due to
> READ_ONCE() on many arches.
> 
> Closes: https://lore.kernel.org/linux-arm-kernel/ZiuyGXt0XWwRgFh9@x1n/
> Fixes: 12f6b01a0bcb ("fs/proc/task_mmu: add fast paths to get/clear PAGE_IS_WRITTEN flag")
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>

> ---
>  fs/proc/task_mmu.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index af4bc1da0c01..102f48668c35 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1817,10 +1817,8 @@ static unsigned long pagemap_page_category(struct pagemap_scan_private *p,
>  }
> 
>  static void make_uffd_wp_pte(struct vm_area_struct *vma,
> -			     unsigned long addr, pte_t *pte)
> +			     unsigned long addr, pte_t *pte, pte_t ptent)
>  {
> -	pte_t ptent = ptep_get(pte);
> -
>  	if (pte_present(ptent)) {
>  		pte_t old_pte;
> 
> @@ -2175,9 +2173,12 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>  	if ((p->arg.flags & PM_SCAN_WP_MATCHING) && !p->vec_out) {
>  		/* Fast path for performing exclusive WP */
>  		for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
> -			if (pte_uffd_wp(ptep_get(pte)))
> +			pte_t ptent = ptep_get(pte);
> +
> +			if ((pte_present(ptent) && pte_uffd_wp(ptent)) ||
> +			    pte_swp_uffd_wp_any(ptent))
>  				continue;
> -			make_uffd_wp_pte(vma, addr, pte);
> +			make_uffd_wp_pte(vma, addr, pte, ptent);
>  			if (!flush_end)
>  				start = addr;
>  			flush_end = addr + PAGE_SIZE;
> @@ -2190,8 +2191,10 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>  	    p->arg.return_mask == PAGE_IS_WRITTEN) {
>  		for (addr = start; addr < end; pte++, addr += PAGE_SIZE) {
>  			unsigned long next = addr + PAGE_SIZE;
> +			pte_t ptent = ptep_get(pte);
> 
> -			if (pte_uffd_wp(ptep_get(pte)))
> +			if ((pte_present(ptent) && pte_uffd_wp(ptent)) ||
> +			    pte_swp_uffd_wp_any(ptent))
>  				continue;
>  			ret = pagemap_scan_output(p->cur_vma_category | PAGE_IS_WRITTEN,
>  						  p, addr, &next);
> @@ -2199,7 +2202,7 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>  				break;
>  			if (~p->arg.flags & PM_SCAN_WP_MATCHING)
>  				continue;
> -			make_uffd_wp_pte(vma, addr, pte);
> +			make_uffd_wp_pte(vma, addr, pte, ptent);
>  			if (!flush_end)
>  				start = addr;
>  			flush_end = next;
> @@ -2208,8 +2211,9 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>  	}
> 
>  	for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
> +		pte_t ptent = ptep_get(pte);
>  		unsigned long categories = p->cur_vma_category |
> -					   pagemap_page_category(p, vma, addr, ptep_get(pte));
> +					   pagemap_page_category(p, vma, addr, ptent);
>  		unsigned long next = addr + PAGE_SIZE;
> 
>  		if (!pagemap_scan_is_interesting_page(categories, p))
> @@ -2224,7 +2228,7 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>  		if (~categories & PAGE_IS_WRITTEN)
>  			continue;
> 
> -		make_uffd_wp_pte(vma, addr, pte);
> +		make_uffd_wp_pte(vma, addr, pte, ptent);
>  		if (!flush_end)
>  			start = addr;
>  		flush_end = next;
> --
> 2.25.1
> 

-- 
BR,
Muhammad Usama Anjum