[RFC PATCH 1/3] mm/memory.c: convert __copy_remote_vm_str() to folios

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

Use kmap_local_folio() instead of kmap_local_page().
Replaces 2 calls to compound_head() from unmap_and_put_page() with one.

This prepares us for the removal of unmap_and_put_page(), and helps
prepare for the eventual gup folio conversions since this function
now supports individual subpages from large folios.

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
---
 mm/memory.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0f9b32a20e5b..747866060658 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6820,9 +6820,10 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
 	}
 
 	while (len) {
-		int bytes, offset, retval;
+		int bytes, folio_offset, page_offset retval;
 		void *maddr;
 		struct page *page;
+		struct folio *folio;
 		struct vm_area_struct *vma = NULL;
 
 		page = get_user_page_vma_remote(mm, addr, gup_flags, &vma);
@@ -6837,17 +6838,20 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
 			goto out;
 		}
 
+		folio = page_folio(page);
 		bytes = len;
-		offset = addr & (PAGE_SIZE - 1);
-		if (bytes > PAGE_SIZE - offset)
-			bytes = PAGE_SIZE - offset;
+		folio_offset = offset_in_folio(folio, addr);
+		page_offset = offset_in_page(folio_offset);
+
+		if (bytes > PAGE_SIZE - page_offset)
+			bytes = PAGE_SIZE - page_offset;
 
-		maddr = kmap_local_page(page);
-		retval = strscpy(buf, maddr + offset, bytes);
+		maddr = kmap_local_folio(folio, folio_offset);
+		retval = strscpy(buf, maddr, bytes);
 		if (retval >= 0) {
 			/* Found the end of the string */
 			buf += retval;
-			unmap_and_put_page(page, maddr);
+			folio_release_kmap(folio, maddr);
 			break;
 		}
 
@@ -6859,13 +6863,16 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
 		 */
 		if (bytes != len) {
 			addr += bytes - 1;
-			copy_from_user_page(vma, page, addr, buf, maddr + (PAGE_SIZE - 1), 1);
+			copy_from_user_page(vma,
+				folio_page(folio, folio_offset / PAGE_SIZE),
+				addr, buf,
+				maddr + (PAGE_SIZE - page_offset - 1), 1);
 			buf += 1;
 			addr += 1;
 		}
 		len -= bytes;
 
-		unmap_and_put_page(page, maddr);
+		folio_release_kmap(folio, maddr);
 	}
 
 out:
-- 
2.49.0