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

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Date: Thu, 7 Mar 2024 11:52:17 +0100
From: David Hildenbrand <david@...hat.com>
To: Richard Weinberger <richard@....at>, linux-mm@...ck.org
Cc: 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, avagin@...gle.com, lstoakes@...il.com,
 vbabka@...e.cz, akpm@...ux-foundation.org, usama.anjum@...labora.com,
 corbet@....net
Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable

On 07.03.24 00:23, 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.
> 

But why is that required? What is the target use case? (I did not get 
the cover letter in my inbox)

We're running slowly but steadily out of bits, so we better make wise 
decisions.

Also, consider: Architectures where the dirty/access bit is not HW 
managed could indicate "writable" here although we *will* get a page 
fault to set the page dirty/accessed.

So best this can universally do is say "this PTE currently has write 
permissions".

> 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)
>   #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) +

-- 
Cheers,

David / dhildenb