linux-kernel - Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable

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Date: Thu, 7 Mar 2024 15:44:20 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: Richard Weinberger <richard@....at>, linux-mm@...ck.org
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, upstream+pagemap@...ma-star.at,
 adobriyan@...il.com, wangkefeng.wang@...wei.com, ryan.roberts@....com,
 hughd@...gle.com, peterx@...hat.com, david@...hat.com, avagin@...gle.com,
 lstoakes@...il.com, vbabka@...e.cz, akpm@...ux-foundation.org, corbet@....net
Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable

On 3/7/24 4:23 AM, Richard Weinberger wrote:
> Is a PTE present and writable, bit 58 will be set.
> This allows detecting CoW memory mappings and other mappings
> where a write access will cause a page fault.
> 
> Signed-off-by: Richard Weinberger <richard@....at>
> ---
>  fs/proc/task_mmu.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 3f78ebbb795f..7c7e0e954c02 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1341,6 +1341,7 @@ struct pagemapread {
>  #define PM_SOFT_DIRTY		BIT_ULL(55)
>  #define PM_MMAP_EXCLUSIVE	BIT_ULL(56)
>  #define PM_UFFD_WP		BIT_ULL(57)
> +#define PM_WRITE		BIT_ULL(58)
The name doesn't mention present from its "present and writable"
definition. Maybe some other name like PM_PRESENT_WRITE?

>  #define PM_FILE			BIT_ULL(61)
>  #define PM_SWAP			BIT_ULL(62)
>  #define PM_PRESENT		BIT_ULL(63)
> @@ -1417,6 +1418,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>  			flags |= PM_SOFT_DIRTY;
>  		if (pte_uffd_wp(pte))
>  			flags |= PM_UFFD_WP;
> +		if (pte_write(pte))
> +			flags |= PM_WRITE;
>  	} else if (is_swap_pte(pte)) {
>  		swp_entry_t entry;
>  		if (pte_swp_soft_dirty(pte))
> @@ -1483,6 +1486,8 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  				flags |= PM_SOFT_DIRTY;
>  			if (pmd_uffd_wp(pmd))
>  				flags |= PM_UFFD_WP;
> +			if (pmd_write(pmd))
> +				flags |= PM_WRITE;
>  			if (pm->show_pfn)
>  				frame = pmd_pfn(pmd) +
>  					((addr & ~PMD_MASK) >> PAGE_SHIFT);
> @@ -1586,6 +1591,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
>  		if (huge_pte_uffd_wp(pte))
>  			flags |= PM_UFFD_WP;
>  
> +		if (pte_write(pte))
> +			flags |= PM_WRITE;
> +
>  		flags |= PM_PRESENT;
>  		if (pm->show_pfn)
>  			frame = pte_pfn(pte) +

-- 
BR,
Muhammad Usama Anjum