filemap: convert filemap_map_pmd() to take a folio

Patch series "Some more filemap folio conversions".

Three more places which could easily be converted to folios.  The third
one fixes a minor bug in readahead_expand(), but it's only a performance
bug and there are few users of readahead_expand(), so I don't think it's
worth backporting.


This patch (of 3):

Save a few calls to compound_head().  We specify exactly which page from
the folio to use by passing in start_pgoff, which means this will work for
a folio which is larger than PMD size.  The rest of the VM isn't prepared
for that yet, but now this function is.

Link: https://lkml.kernel.org/r/20230116193941.2148487-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20230116193941.2148487-2-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/filemap.c

@@ -3259,22 +3259,24 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 }
 EXPORT_SYMBOL(filemap_fault);
 
-static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page)
+static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
+		pgoff_t start)
 {
 	struct mm_struct *mm = vmf->vma->vm_mm;
 
 	/* Huge page is mapped? No need to proceed. */
 	if (pmd_trans_huge(*vmf->pmd)) {
-		unlock_page(page);
-		put_page(page);
+		folio_unlock(folio);
+		folio_put(folio);
 		return true;
 	}
 
-	if (pmd_none(*vmf->pmd) && PageTransHuge(page)) {
+	if (pmd_none(*vmf->pmd) && folio_test_pmd_mappable(folio)) {
+		struct page *page = folio_file_page(folio, start);
 		vm_fault_t ret = do_set_pmd(vmf, page);
 		if (!ret) {
 			/* The page is mapped successfully, reference consumed. */
-			unlock_page(page);
+			folio_unlock(folio);
 			return true;
 		}
 	}
@@ -3284,8 +3286,8 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page)
 
 	/* See comment in handle_pte_fault() */
 	if (pmd_devmap_trans_unstable(vmf->pmd)) {
-		unlock_page(page);
-		put_page(page);
+		folio_unlock(folio);
+		folio_put(folio);
 		return true;
 	}
 
@@ -3368,7 +3370,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	if (!folio)
 		goto out;
 
-	if (filemap_map_pmd(vmf, &folio->page)) {
+	if (filemap_map_pmd(vmf, folio, start_pgoff)) {
 		ret = VM_FAULT_NOPAGE;
 		goto out;
 	}