[PATCH 2/3] mm: provide mapping_wrprotect_page() function

Lorenzo Stoakes
2025-01-31T18:30:26Z

in the fb_defio video driver, page dirty state is used to determine when
frame buffer pages have been changed, allowing for batched, deferred I/O to
be performed for efficiency.

This implementation had only one means of doing so effectively - the use of
the folio_mkclean() function.

However, this use of the function is inappropriate, as the fb_defio
implementation allocates kernel memory to back the framebuffer, and then is
forced to specified page->index, mapping fields in order to permit the
folio_mkclean() rmap traversal to proceed correctly.

It is not correct to specify these fields on kernel-allocated memory, and
moreover since these are not folios, page->index, mapping are deprecated
fields, soon to be removed.

We therefore need to provide a means by which we can correctly traverse the
reverse mapping and write-protect mappings for a page backing an
address_space page cache object at a given offset.

This patch provides this - mapping_wrprotect_page() allows for this
operation to be performed for a specified address_space, offset and page,
without requiring a folio nor, of course, an inappropriate use of
page->index, mapping.

With this provided, we can subequently adjust the fb_defio implementation
to make use of this function and avoid incorrect invocation of
folio_mkclean() and more importantly, incorrect manipulation of
page->index, mapping fields.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
include/linux/rmap.h | 3 ++
mm/rmap.c | 73 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 76 insertions(+)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 683a04088f3f..0bf5f64884df 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -739,6 +739,9 @@ unsigned long page_address_in_vma(const struct folio *folio,
*/
int folio_mkclean(struct folio *);

+int mapping_wrprotect_page(struct address_space *mapping, pgoff_t pgoff,
+ unsigned long nr_pages, struct page *page);
+
int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
struct vm_area_struct *vma);

diff --git a/mm/rmap.c b/mm/rmap.c
index a2ff20c2eccd..bb5a42d95c48 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1127,6 +1127,79 @@ int folio_mkclean(struct folio *folio)
}
EXPORT_SYMBOL_GPL(folio_mkclean);

+struct wrprotect_file_state {
+ int cleaned;
+ pgoff_t pgoff;
+ unsigned long pfn;
+ unsigned long nr_pages;
+};
+
+static bool mapping_wrprotect_page_one(struct folio *folio,
+ struct vm_area_struct *vma, unsigned long address, void *arg)
+{
+ struct wrprotect_file_state *state = (struct wrprotect_file_state *)arg;
+ struct page_vma_mapped_walk pvmw = {
+ .pfn = state->pfn,
+ .nr_pages = state->nr_pages,
+ .pgoff = state->pgoff,
+ .vma = vma,
+ .address = address,
+ .flags = PVMW_SYNC,
+ };
+
+ state->cleaned += page_vma_mkclean_one(&pvmw);
+
+ return true;
+}
+
+static void __rmap_walk_file(struct folio *folio, struct address_space *mapping,
+ pgoff_t pgoff_start, unsigned long nr_pages,
+ struct rmap_walk_control *rwc, bool locked);
+
+/**
+ * mapping_wrprotect_page() - Write protect all mappings of this page.
+ *
+ * @mapping: The mapping whose reverse mapping should be traversed.
+ * @pgoff: The page offset at which @page is mapped within @mapping.
+ * @nr_pages: The number of physically contiguous base pages spanned.
+ * @page: The page mapped in @mapping at @pgoff.
+ *
+ * Traverses the reverse mapping, finding all VMAs which contain a shared
+ * mapping of the single @page in @mapping at offset @pgoff and write-protecting
+ * the mappings.
+ *
+ * The page does not have to be a folio, but rather can be a kernel allocation
+ * that is mapped into userland. We therefore do not require that the page maps
+ * to a folio with a valid mapping or index field, rather these are specified in
+ * @mapping and @pgoff.
+ *
+ * Return: the number of write-protected PTEs, or an error.
+ */
+int mapping_wrprotect_page(struct address_space *mapping, pgoff_t pgoff,
+ unsigned long nr_pages, struct page *page)
+{
+ struct wrprotect_file_state state = {
+ .cleaned = 0,
+ .pgoff = pgoff,
+ .pfn = page_to_pfn(page),
+ .nr_pages = nr_pages,
+ };
+ struct rmap_walk_control rwc = {
+ .arg = (void *)&state,
+ .rmap_one = mapping_wrprotect_page_one,
+ .invalid_vma = invalid_mkclean_vma,
+ };
+
+ if (!mapping)
+ return 0;
+
+ __rmap_walk_file(/* folio = */NULL, mapping, pgoff, nr_pages, &rwc,
+ /* locked = */false);
+
+ return state.cleaned;
+}
+EXPORT_SYMBOL_GPL(mapping_wrprotect_page);
+
/**
* pfn_mkclean_range - Cleans the PTEs (including PMDs) mapped with range of
* [@pfn, @pfn + @nr_pages) at the specific offset (@pgoff)
--
2.48.1