[PATCH 1/4] rmap: add page_wrprotect() function,

[Izik Eidus]

From: Izik Eidus <izike@xxxxxxxxxxxx>

this function is useful for cases you want to compare page and know
that its value wont change during you compare it.

this function is working by walking over the whole rmap of a page
and mark every pte related to the page as write_protect.

the odirect_sync paramter is used to notify the caller of
page_wrprotect() if one pte or more was not marked readonly
in order to avoid race with odirect.

Signed-off-by: Izik Eidus <izike@xxxxxxxxxxxx>
---
 include/linux/rmap.h |    7 ++++
 mm/rmap.c            |   97 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+), 0 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 89f0564..2a37fb7 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -121,6 +121,8 @@ static inline int try_to_munlock(struct page *page)
 }
 #endif
 
+int page_wrprotect(struct page *page, int *odirect_sync);
+
 #else	/* !CONFIG_MMU */
 
 #define anon_vma_init()		do {} while (0)
@@ -135,6 +137,11 @@ static inline int page_mkclean(struct page *page)
 	return 0;
 }
 
+static inline int page_wrprotect(struct page *page, int *odirect_sync)
+{
+	return 0;
+}
+
 
 #endif	/* CONFIG_MMU */
 
diff --git a/mm/rmap.c b/mm/rmap.c
index 1099394..3684edd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -576,6 +576,103 @@ int page_mkclean(struct page *page)
 }
 EXPORT_SYMBOL_GPL(page_mkclean);
 
+static int page_wrprotect_one(struct page *page, struct vm_area_struct *vma,
+			      int *odirect_sync)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long address;
+	pte_t *pte;
+	spinlock_t *ptl;
+	int ret = 0;
+
+	address = vma_address(page, vma);
+	if (address == -EFAULT)
+		goto out;
+
+	pte = page_check_address(page, mm, address, &ptl, 0);
+	if (!pte)
+		goto out;
+
+	if (pte_write(*pte)) {
+		pte_t entry;
+
+		if (page_mapcount(page) != page_count(page)) {
+			*odirect_sync = 0;
+			goto out_unlock;
+		}
+		flush_cache_page(vma, address, pte_pfn(*pte));
+		entry = ptep_clear_flush_notify(vma, address, pte);
+		entry = pte_wrprotect(entry);
+		set_pte_at(mm, address, pte, entry);
+	}
+	ret = 1;
+
+out_unlock:
+	pte_unmap_unlock(pte, ptl);
+out:
+	return ret;
+}
+
+static int page_wrprotect_file(struct page *page, int *odirect_sync)
+{
+	struct address_space *mapping;
+	struct prio_tree_iter iter;
+	struct vm_area_struct *vma;
+	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+	int ret = 0;
+
+	mapping = page_mapping(page);
+	if (!mapping)
+		return ret;
+
+	spin_lock(&mapping->i_mmap_lock);
+
+	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff)
+		ret += page_wrprotect_one(page, vma, odirect_sync);
+
+	spin_unlock(&mapping->i_mmap_lock);
+
+	return ret;
+}
+
+static int page_wrprotect_anon(struct page *page, int *odirect_sync)
+{
+	struct vm_area_struct *vma;
+	struct anon_vma *anon_vma;
+	int ret = 0;
+
+	anon_vma = page_lock_anon_vma(page);
+	if (!anon_vma)
+		return ret;
+
+	list_for_each_entry(vma, &anon_vma->head, anon_vma_node)
+		ret += page_wrprotect_one(page, vma, odirect_sync);
+
+	page_unlock_anon_vma(anon_vma);
+
+	return ret;
+}
+
+/**
+ * set all the ptes pointed to a page as read only,
+ * odirect_sync is set to 0 in case we cannot protect against race with odirect
+ * return the number of ptes that were set as read only
+ * (ptes that were read only before this function was called are couned as well)
+ */
+int page_wrprotect(struct page *page, int *odirect_sync)
+{
+	int ret =0;
+
+	*odirect_sync = 1;
+	if (PageAnon(page))
+		ret = page_wrprotect_anon(page, odirect_sync);
+	else
+		ret = page_wrprotect_file(page, odirect_sync);
+
+	return ret;
+}
+EXPORT_SYMBOL(page_wrprotect);
+
 /**
  * __page_set_anon_rmap - setup new anonymous rmap
  * @page:	the page to add the mapping to
-- 
1.6.0.3

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