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

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Date: Thu,  7 Mar 2024 00:23:38 +0100
From: Richard Weinberger <richard@....at>
To: 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,
	david@...hat.com,
	avagin@...gle.com,
	lstoakes@...il.com,
	vbabka@...e.cz,
	akpm@...ux-foundation.org,
	usama.anjum@...labora.com,
	corbet@....net,
	Richard Weinberger <richard@....at>
Subject: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable

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)
 #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) +
-- 
2.35.3