mm: allow ->huge_fault() to be called without the mmap_lock held

Remove the checks for the VMA lock being held, allowing the page fault
path to call into the filesystem instead of retrying with the mmap_lock
held.  This will improve scalability for DAX page faults.  Also update the
documentation to match (and fix some other changes that have happened
recently).

Link: https://lkml.kernel.org/r/20230818202335.2739663-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/locking.rst

@@ -628,26 +628,29 @@ vm_operations_struct
 
 prototypes::
 
-	void (*open)(struct vm_area_struct*);
-	void (*close)(struct vm_area_struct*);
-	vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
+	void (*open)(struct vm_area_struct *);
+	void (*close)(struct vm_area_struct *);
+	vm_fault_t (*fault)(struct vm_fault *);
+	vm_fault_t (*huge_fault)(struct vm_fault *, unsigned int order);
+	vm_fault_t (*map_pages)(struct vm_fault *, pgoff_t start, pgoff_t end);
 	vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
 	vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
 	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
 
 locking rules:
 
-=============	=========	===========================
+=============	==========	===========================
 ops		mmap_lock	PageLocked(page)
-=============	=========	===========================
-open:		yes
-close:		yes
-fault:		yes		can return with page locked
-map_pages:	read
-page_mkwrite:	yes		can return with page locked
-pfn_mkwrite:	yes
-access:		yes
-=============	=========	===========================
+=============	==========	===========================
+open:		write
+close:		read/write
+fault:		read		can return with page locked
+huge_fault:	maybe-read
+map_pages:	maybe-read
+page_mkwrite:	read		can return with page locked
+pfn_mkwrite:	read
+access:		read
+=============	==========	===========================
 
 ->fault() is called when a previously not present pte is about to be faulted
 in. The filesystem must find and return the page associated with the passed in
@@ -657,6 +660,13 @@ then ensure the page is not already truncated (invalidate_lock will block
 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
 locked. The VM will unlock the page.
 
+->huge_fault() is called when there is no PUD or PMD entry present.  This
+gives the filesystem the opportunity to install a PUD or PMD sized page.
+Filesystems can also use the ->fault method to return a PMD sized page,
+so implementing this function may not be necessary.  In particular,
+filesystems should not call filemap_fault() from ->huge_fault().
+The mmap_lock may not be held when this method is called.
+
 ->map_pages() is called when VM asks to map easy accessible pages.
 Filesystem should find and map pages associated with offsets from "start_pgoff"
 till "end_pgoff". ->map_pages() is called with the RCU lock held and must

Documentation/filesystems/porting.rst

@@ -943,3 +943,14 @@ file pointer instead of struct dentry pointer.  d_tmpfile() is similarly
 changed to simplify callers.  The passed file is in a non-open state and on
 success must be opened before returning (e.g. by calling
 finish_open_simple()).
+
+---
+
+**mandatory**
+
+Calling convention for ->huge_fault has changed.  It now takes a page
+order instead of an enum page_entry_size, and it may be called without the
+mmap_lock held.  All in-tree users have been audited and do not seem to
+depend on the mmap_lock being held, but out of tree users should verify
+for themselves.  If they do need it, they can return VM_FAULT_RETRY to
+be called with the mmap_lock held.

mm/memory.c

@@ -4854,13 +4854,8 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
 	struct vm_area_struct *vma = vmf->vma;
 	if (vma_is_anonymous(vma))
 		return do_huge_pmd_anonymous_page(vmf);
-	if (vma->vm_ops->huge_fault) {
-		if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-			vma_end_read(vma);
-			return VM_FAULT_RETRY;
-		}
+	if (vma->vm_ops->huge_fault)
 		return vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
-	}
 	return VM_FAULT_FALLBACK;
 }
 
@@ -4880,10 +4875,6 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
 
 	if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
 		if (vma->vm_ops->huge_fault) {
-			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-				vma_end_read(vma);
-				return VM_FAULT_RETRY;
-			}
 			ret = vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
 			if (!(ret & VM_FAULT_FALLBACK))
 				return ret;
@@ -4904,13 +4895,8 @@ static vm_fault_t create_huge_pud(struct vm_fault *vmf)
 	/* No support for anonymous transparent PUD pages yet */
 	if (vma_is_anonymous(vma))
 		return VM_FAULT_FALLBACK;
-	if (vma->vm_ops->huge_fault) {
-		if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-			vma_end_read(vma);
-			return VM_FAULT_RETRY;
-		}
+	if (vma->vm_ops->huge_fault)
 		return vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
-	}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 	return VM_FAULT_FALLBACK;
 }
@@ -4927,10 +4913,6 @@ static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
 		goto split;
 	if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
 		if (vma->vm_ops->huge_fault) {
-			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-				vma_end_read(vma);
-				return VM_FAULT_RETRY;
-			}
 			ret = vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
 			if (!(ret & VM_FAULT_FALLBACK))
 				return ret;