hugetlb: move vm_fault declaration to the top of hugetlb_fault()

hugetlb_fault() currently defines a vm_fault to pass to the generic
handle_userfault() function.  We can move this definition to the top of
hugetlb_fault() so that it can be used throughout the rest of the hugetlb
fault path.

This will help cleanup a number of excess variables and function arguments
throughout the stack.  Also, since vm_fault already has space to store the
page offset, use that instead and get rid of idx.

Link: https://lkml.kernel.org/r/20240221234732.187629-3-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -6378,13 +6378,25 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	spinlock_t *ptl;
 	vm_fault_t ret;
 	u32 hash;
-	pgoff_t idx;
 	struct folio *folio = NULL;
 	struct folio *pagecache_folio = NULL;
 	struct hstate *h = hstate_vma(vma);
 	struct address_space *mapping;
 	int need_wait_lock = 0;
 	unsigned long haddr = address & huge_page_mask(h);
+	struct vm_fault vmf = {
+		.vma = vma,
+		.address = haddr,
+		.real_address = address,
+		.flags = flags,
+		.pgoff = vma_hugecache_offset(h, vma, haddr),
+		/* TODO: Track hugetlb faults using vm_fault */
+
+		/*
+		 * Some fields may not be initialized, be careful as it may
+		 * be hard to debug if called functions make assumptions
+		 */
+	};
 
 	/* TODO: Handle faults under the VMA lock */
 	if (flags & FAULT_FLAG_VMA_LOCK) {
@@ -6398,8 +6410,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * the same page in the page cache.
 	 */
 	mapping = vma->vm_file->f_mapping;
-	idx = vma_hugecache_offset(h, vma, haddr);
-	hash = hugetlb_fault_mutex_hash(mapping, idx);
+	hash = hugetlb_fault_mutex_hash(mapping, vmf.pgoff);
 	mutex_lock(&hugetlb_fault_mutex_table[hash]);
 
 	/*
@@ -6433,8 +6444,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		 * hugetlb_no_page will drop vma lock and hugetlb fault
 		 * mutex internally, which make us return immediately.
 		 */
-		return hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
-				      entry, flags);
+
+		return hugetlb_no_page(mm, vma, mapping, vmf.pgoff, address,
+					ptep, entry, flags);
 	}
 
 	ret = 0;
@@ -6480,7 +6492,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		/* Just decrements count, does not deallocate */
 		vma_end_reservation(h, vma, haddr);
 
-		pagecache_folio = filemap_lock_hugetlb_folio(h, mapping, idx);
+		pagecache_folio = filemap_lock_hugetlb_folio(h, mapping,
+							     vmf.pgoff);
 		if (IS_ERR(pagecache_folio))
 			pagecache_folio = NULL;
 	}
@@ -6495,13 +6508,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (userfaultfd_wp(vma) && huge_pte_uffd_wp(huge_ptep_get(ptep)) &&
 	    (flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
 		if (!userfaultfd_wp_async(vma)) {
-			struct vm_fault vmf = {
-				.vma = vma,
-				.address = haddr,
-				.real_address = address,
-				.flags = flags,
-			};
-
 			spin_unlock(ptl);
 			if (pagecache_folio) {
 				folio_unlock(pagecache_folio);