mm: provide mapping_wrprotect_range() function

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_range() - which allows for
this operation to be performed for a specified address_space, offset, PFN
and size, without requiring a folio nor, of course, an inappropriate use
of page->index, mapping.

With this provided, we can subsequently 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 and mapping fields.

Link: https://lkml.kernel.org/r/e5bf969d64e7f2f2ae944d42341fc8994b736a81.1739029358.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Jaya Kumar <jayakumar.lkml@gmail.com>
Cc: Kajtar Zsolt <soci@c64.rulez.org>
Cc: Maíra Canal <mcanal@igalia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Thomas Zimemrmann <tzimmermann@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 86425d42c1a900a05a6e6647b00ba18867e10b5d..69e9a431a40e7a25617f8f9ffed3a8cf845a9a3d 100644 (file)
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -738,6 +738,9 @@ unsigned long page_address_in_vma(const struct folio *folio,
  */
 int folio_mkclean(struct folio *);
 
+int mapping_wrprotect_range(struct address_space *mapping, pgoff_t pgoff,
+               unsigned long pfn, unsigned long nr_pages);
+
 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 3da1ca49881c811c1c6e45d332c9f9338e0ae45d..24bacce9971f1a3188d06a3d6b188397d9882117 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1135,6 +1135,80 @@ 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_range_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_range() - Write-protect all mappings in a specified range.
+ *
+ * @mapping:   The mapping whose reverse mapping should be traversed.
+ * @pgoff:     The page offset at which @pfn is mapped within @mapping.
+ * @pfn:       The PFN of the page mapped in @mapping at @pgoff.
+ * @nr_pages:  The number of physically contiguous base pages spanned.
+ *
+ * Traverses the reverse mapping, finding all VMAs which contain a shared
+ * mapping of the pages in the specified range in @mapping, and write-protects
+ * them (that is, updates the page tables to mark the mappings read-only such
+ * that a write protection fault arises when the mappings are written to).
+ *
+ * The @pfn value need not refer to a folio, but rather can reference a kernel
+ * allocation which is mapped into userland. We therefore do not require that
+ * the page maps to a folio with a valid mapping or index field, rather the
+ * caller specifies these in @mapping and @pgoff.
+ *
+ * Return: the number of write-protected PTEs, or an error.
+ */
+int mapping_wrprotect_range(struct address_space *mapping, pgoff_t pgoff,
+               unsigned long pfn, unsigned long nr_pages)
+{
+       struct wrprotect_file_state state = {
+               .cleaned = 0,
+               .pgoff = pgoff,
+               .pfn = pfn,
+               .nr_pages = nr_pages,
+       };
+       struct rmap_walk_control rwc = {
+               .arg = (void *)&state,
+               .rmap_one = mapping_wrprotect_range_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_range);
+
 /**
  * pfn_mkclean_range - Cleans the PTEs (including PMDs) mapped with range of
  *                     [@pfn, @pfn + @nr_pages) at the specific offset (@pgoff)