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Date:   Thu,  2 Nov 2023 10:33:57 -0700
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Message-ID: <20231102173357.446157-1-avagin@google.com>
Subject: [PATCH] fs/proc/task_mmu: report SOFT_DIRTY bits through the
 PAGEMAP_SCAN ioctl
From:   Andrei Vagin <avagin@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     linux-kernel@vger.kernel.org, Andrei Vagin <avagin@google.com>,
        "=?UTF-8?q?Micha=C5=82=20Miros=C5=82aw?=" <mirq-linux@rere.qmqm.pl>
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Precedence: bulk

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: Micha=C5=82 Miros=C5=82aw <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, struc=
t 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)
=20
 struct pagemap_scan_private {
@@ -1853,12 +1853,16 @@ static unsigned long pagemap_thp_category(struct pa=
gemap_scan_private *p,
=20
 		if (is_zero_pfn(pmd_pfn(pmd)))
 			categories |=3D PAGE_IS_PFNZERO;
+		if (pmd_soft_dirty(pmd))
+			categories |=3D PAGE_IS_SOFT_DIRTY;
 	} else if (is_swap_pmd(pmd)) {
 		swp_entry_t swp;
=20
 		categories |=3D PAGE_IS_SWAPPED;
 		if (!pmd_swp_uffd_wp(pmd))
 			categories |=3D PAGE_IS_WRITTEN;
+		if (pmd_swp_soft_dirty(pmd))
+			categories |=3D PAGE_IS_SOFT_DIRTY;
=20
 		if (p->masks_of_interest & PAGE_IS_FILE) {
 			swp =3D pmd_to_swp_entry(pmd);
@@ -1905,10 +1909,14 @@ static unsigned long pagemap_hugetlb_category(pte_t=
 pte)
 			categories |=3D PAGE_IS_FILE;
 		if (is_zero_pfn(pte_pfn(pte)))
 			categories |=3D PAGE_IS_PFNZERO;
+		if (pte_soft_dirty(pte))
+			categories |=3D PAGE_IS_SOFT_DIRTY;
 	} else if (is_swap_pte(pte)) {
 		categories |=3D PAGE_IS_SWAPPED;
 		if (!pte_swp_uffd_wp_any(pte))
 			categories |=3D PAGE_IS_WRITTEN;
+		if (pte_swp_soft_dirty(pte))
+			categories |=3D PAGE_IS_SOFT_DIRTY;
 	}
=20
 	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;
=20
+	if (vma->vm_flags & VM_SOFTDIRTY)
+		vma_category |=3D PAGE_IS_SOFT_DIRTY;
+
 	if (!pagemap_scan_is_interesting_vma(vma_category, p))
 		return 1;
=20
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
@@ -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)
=20
 /*
  * struct page_region - Page region with flags
--=20
2.42.0.869.gea05f2083d-goog