[PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another

[Izik Eidus <ieidus@redhat.com>]

this function is needed in cases you want to change the userspace
virtual mapping into diffrent physical page,
KSM need this for merging the identical pages.

this function is working by removing the oldpage from the rmap and
calling put_page on it, and by setting the virtual address pte
to point into the new page.
(note that the new page (the page that we change the pte to map to)
cannot be anonymous page)

Signed-off-by: Izik Eidus <izike@qumranet.com>
---
 include/linux/mm.h |    3 ++
 mm/memory.c        |   68 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 0 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ffee2f7..4da7fa8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1207,6 +1207,9 @@ int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
 int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
 			unsigned long pfn);
 
+int replace_page(struct vm_area_struct *vma, struct page *oldpage,
+		 struct page *newpage, pte_t orig_pte, pgprot_t prot);
+
 struct page *follow_page(struct vm_area_struct *, unsigned long address,
 			unsigned int foll_flags);
 #define FOLL_WRITE	0x01	/* check pte is writable */
diff --git a/mm/memory.c b/mm/memory.c
index 164951c..b2c542c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1472,6 +1472,74 @@ int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
 }
 EXPORT_SYMBOL(vm_insert_mixed);
 
+/**
+ * replace _page - replace the pte mapping related to vm area between two pages
+ * (from oldpage to newpage)
+ * NOTE: you should take into consideration the impact on the VM when replacing
+ * anonymous pages with kernel non swappable pages.
+ */
+int replace_page(struct vm_area_struct *vma, struct page *oldpage,
+		 struct page *newpage, pte_t orig_pte, pgprot_t prot)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *ptep;
+	spinlock_t *ptl;
+	unsigned long addr;
+	int ret;
+
+	BUG_ON(PageAnon(newpage));
+
+	ret = -EFAULT;
+	addr = page_address_in_vma(oldpage, vma);
+	if (addr == -EFAULT)
+		goto out;
+
+	pgd = pgd_offset(mm, addr);
+	if (!pgd_present(*pgd))
+		goto out;
+
+	pud = pud_offset(pgd, addr);
+	if (!pud_present(*pud))
+		goto out;
+
+	pmd = pmd_offset(pud, addr);
+	if (!pmd_present(*pmd))
+		goto out;
+
+	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
+	if (!ptep)
+		goto out;
+
+	if (!pte_same(*ptep, orig_pte)) {
+		pte_unmap_unlock(ptep, ptl);
+		goto out;
+	}
+
+	ret = 0;
+	get_page(newpage);
+	page_add_file_rmap(newpage);
+
+	flush_cache_page(vma, addr, pte_pfn(*ptep));
+	ptep_clear_flush(vma, addr, ptep);
+	set_pte_at(mm, addr, ptep, mk_pte(newpage, prot));
+
+	page_remove_rmap(oldpage, vma);
+	if (PageAnon(oldpage)) {
+		dec_mm_counter(mm, anon_rss);
+		inc_mm_counter(mm, file_rss);
+	}
+	put_page(oldpage);
+
+	pte_unmap_unlock(ptep, ptl);
+
+out:
+	return ret;
+}
+EXPORT_SYMBOL(replace_page);
+
 /*
  * maps a range of physical memory into the requested pages. the old
  * mappings are removed. any references to nonexistent pages results
-- 
1.6.0.3

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