[PATCH v5 14/18] fsnotify: generate pre-content permission event on page fault

[Josef Bacik <josef@toxicpanda.com>]

FS_PRE_ACCESS or FS_PRE_MODIFY will be generated on page fault depending
on the faulting method.

This pre-content event is meant to be used by hierarchical storage
managers that want to fill in the file content on first read access.

Export a simple helper that file systems that have their own ->fault()
will use, and have a more complicated helper to be do fancy things with
in filemap_fault.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 include/linux/mm.h |   1 +
 mm/filemap.c       | 116 ++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 110 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ab3d78116043..89665732b404 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3503,6 +3503,7 @@ extern vm_fault_t filemap_fault(struct vm_fault *vmf);
 extern vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		pgoff_t start_pgoff, pgoff_t end_pgoff);
 extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf);
+extern vm_fault_t filemap_fsnotify_fault(struct vm_fault *vmf);
 
 extern unsigned long stack_guard_gap;
 /* Generic expand stack which grows the stack according to GROWS{UP,DOWN} */
diff --git a/mm/filemap.c b/mm/filemap.c
index 8b1684b62177..b2d29947ce7f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -46,6 +46,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/splice.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/fsnotify.h>
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 #include "internal.h"
@@ -3112,13 +3113,13 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
  * that.  If we didn't pin a file then we return NULL.  The file that is
  * returned needs to be fput()'ed when we're done with it.
  */
-static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
+static struct file *do_sync_mmap_readahead(struct vm_fault *vmf,
+					   struct file *fpin)
 {
 	struct file *file = vmf->vma->vm_file;
 	struct file_ra_state *ra = &file->f_ra;
 	struct address_space *mapping = file->f_mapping;
 	DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
-	struct file *fpin = NULL;
 	unsigned long vm_flags = vmf->vma->vm_flags;
 	unsigned int mmap_miss;
 
@@ -3190,12 +3191,12 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
  * was pinned if we have to drop the mmap_lock in order to do IO.
  */
 static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
-					    struct folio *folio)
+					    struct folio *folio,
+					    struct file *fpin)
 {
 	struct file *file = vmf->vma->vm_file;
 	struct file_ra_state *ra = &file->f_ra;
 	DEFINE_READAHEAD(ractl, file, ra, file->f_mapping, vmf->pgoff);
-	struct file *fpin = NULL;
 	unsigned int mmap_miss;
 
 	/* See comment in do_sync_mmap_readahead. */
@@ -3260,6 +3261,93 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
 	return ret;
 }
 
+/*
+ * If we have pre-content watches on this file we will need to emit an event for
+ * this range.  We will handle dropping the lock and emitting the event.
+ *
+ * If FAULT_FLAG_RETRY_NOWAIT is set then we'll return VM_FAULT_RETRY.
+ *
+ * If no event was emitted then *fpin will be NULL and we will return 0.
+ *
+ * If any error occurred we will return VM_FAULT_SIGBUS, *fpin could still be
+ * set and will need to have fput() called on it.
+ *
+ * If we emitted the event then we will return 0 and *fpin will be set, this
+ * must have fput() called on it, and the caller must call VM_FAULT_RETRY after
+ * any other operations it does in order to re-fault the page and make sure the
+ * appropriate locking is maintained.
+ *
+ * Return: the appropriate vm_fault_t return code, 0 on success.
+ */
+static vm_fault_t __filemap_fsnotify_fault(struct vm_fault *vmf,
+					   struct file **fpin)
+{
+	struct file *file = vmf->vma->vm_file;
+	loff_t pos = vmf->pgoff << PAGE_SHIFT;
+	int mask = (vmf->flags & FAULT_FLAG_WRITE) ? MAY_WRITE : MAY_ACCESS;
+	int ret;
+
+	/*
+	 * We already did this and now we're retrying with everything locked,
+	 * don't emit the event and continue.
+	 */
+	if (vmf->flags & FAULT_FLAG_TRIED)
+		return 0;
+
+	/* No watches, return NULL. */
+	if (!fsnotify_file_has_pre_content_watches(file))
+		return 0;
+
+	/* We are NOWAIT, we can't wait, just return EAGAIN. */
+	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
+		return VM_FAULT_RETRY;
+
+	/*
+	 * If this fails then we're not allowed to drop the fault lock, return a
+	 * SIGBUS so we don't errantly populate pagecache with bogus data for
+	 * this file.
+	 */
+	*fpin = maybe_unlock_mmap_for_io(vmf, *fpin);
+	if (*fpin == NULL)
+		return VM_FAULT_SIGBUS;
+
+	/*
+	 * We can't fput(*fpin) at this point because we could have been passed
+	 * in fpin from a previous call.
+	 */
+	ret = fsnotify_file_area_perm(*fpin, mask, &pos, PAGE_SIZE);
+	if (ret)
+		return VM_FAULT_SIGBUS;
+
+	return 0;
+}
+
+/**
+ * filemap_fsnotify_fault - maybe emit a pre-content event.
+ * @vmf:	struct vm_fault containing details of the fault.
+ *
+ * If we have a pre-content watch on this file we will emit an event for this
+ * range.  If we return anything the fault caller should return immediately, we
+ * will return VM_FAULT_RETRY if we had to emit an event, which will trigger the
+ * fault again and then the fault handler will run the second time through.
+ *
+ * Return: a bitwise-OR of %VM_FAULT_ codes, 0 if nothing happened.
+ */
+vm_fault_t filemap_fsnotify_fault(struct vm_fault *vmf)
+{
+	struct file *fpin = NULL;
+	vm_fault_t ret;
+
+	ret = __filemap_fsnotify_fault(vmf, &fpin);
+	if (fpin) {
+		fput(fpin);
+		if (!ret)
+			ret = VM_FAULT_RETRY;
+	}
+	return ret;
+}
+EXPORT_SYMBOL_GPL(filemap_fsnotify_fault);
+
 /**
  * filemap_fault - read in file data for page fault handling
  * @vmf:	struct vm_fault containing details of the fault
@@ -3299,6 +3387,17 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	if (unlikely(index >= max_idx))
 		return VM_FAULT_SIGBUS;
 
+	/*
+	 * If we have pre-content watchers then we need to generate events on
+	 * page fault so that we can populate any data before the fault.
+	 */
+	ret = __filemap_fsnotify_fault(vmf, &fpin);
+	if (unlikely(ret)) {
+		if (fpin)
+			fput(fpin);
+		return ret;
+	}
+
 	/*
 	 * Do we have something in the page cache already?
 	 */
@@ -3309,21 +3408,24 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 		 * the lock.
 		 */
 		if (!(vmf->flags & FAULT_FLAG_TRIED))
-			fpin = do_async_mmap_readahead(vmf, folio);
+			fpin = do_async_mmap_readahead(vmf, folio, fpin);
 		if (unlikely(!folio_test_uptodate(folio))) {
 			filemap_invalidate_lock_shared(mapping);
 			mapping_locked = true;
 		}
 	} else {
 		ret = filemap_fault_recheck_pte_none(vmf);
-		if (unlikely(ret))
+		if (unlikely(ret)) {
+			if (fpin)
+				goto out_retry;
 			return ret;
+		}
 
 		/* No page in the page cache at all */
 		count_vm_event(PGMAJFAULT);
 		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
 		ret = VM_FAULT_MAJOR;
-		fpin = do_sync_mmap_readahead(vmf);
+		fpin = do_sync_mmap_readahead(vmf, fpin);
 retry_find:
 		/*
 		 * See comment in filemap_create_folio() why we need
-- 
2.43.0