[RFC v2 4/4] fs: Optimize credentials reference count for backing file ops

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

For backing file operations, users are expected to pass credentials
that will outlive the backing file common operations.

Use the specialized guard statements to override/revert the
credentials.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 fs/backing-file.c | 124 ++++++++++++++++++++++------------------------
 1 file changed, 60 insertions(+), 64 deletions(-)

diff --git a/fs/backing-file.c b/fs/backing-file.c
index a681f38d84d8..9874f09f860f 100644
--- a/fs/backing-file.c
+++ b/fs/backing-file.c
@@ -140,7 +140,7 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 			       struct backing_file_ctx *ctx)
 {
 	struct backing_aio *aio = NULL;
-	const struct cred *old_cred;
+	const struct cred *old_cred = ctx->cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -153,29 +153,28 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 	    !(file->f_mode & FMODE_CAN_ODIRECT))
 		return -EINVAL;
 
-	old_cred = override_creds(ctx->cred);
-	if (is_sync_kiocb(iocb)) {
-		rwf_t rwf = iocb_to_rw_flags(flags);
+	scoped_guard(cred, old_cred) {
+		if (is_sync_kiocb(iocb)) {
+			rwf_t rwf = iocb_to_rw_flags(flags);
 
-		ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
-	} else {
-		ret = -ENOMEM;
-		aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
-		if (!aio)
-			goto out;
+			ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
+		} else {
+			ret = -ENOMEM;
+			aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+			if (!aio)
+				goto out;
 
-		aio->orig_iocb = iocb;
-		kiocb_clone(&aio->iocb, iocb, get_file(file));
-		aio->iocb.ki_complete = backing_aio_rw_complete;
-		refcount_set(&aio->ref, 2);
-		ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
-		backing_aio_put(aio);
-		if (ret != -EIOCBQUEUED)
-			backing_aio_cleanup(aio, ret);
+			aio->orig_iocb = iocb;
+			kiocb_clone(&aio->iocb, iocb, get_file(file));
+			aio->iocb.ki_complete = backing_aio_rw_complete;
+			refcount_set(&aio->ref, 2);
+			ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
+			backing_aio_put(aio);
+			if (ret != -EIOCBQUEUED)
+				backing_aio_cleanup(aio, ret);
+		}
 	}
 out:
-	revert_creds(old_cred);
-
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
 
@@ -187,7 +186,7 @@ ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
 				struct kiocb *iocb, int flags,
 				struct backing_file_ctx *ctx)
 {
-	const struct cred *old_cred;
+	const struct cred *old_cred = ctx->cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -210,39 +209,37 @@ ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
 	 */
 	flags &= ~IOCB_DIO_CALLER_COMP;
 
-	old_cred = override_creds(ctx->cred);
-	if (is_sync_kiocb(iocb)) {
-		rwf_t rwf = iocb_to_rw_flags(flags);
+	scoped_guard(cred, old_cred) {
+		if (is_sync_kiocb(iocb)) {
+			rwf_t rwf = iocb_to_rw_flags(flags);
 
-		ret = vfs_iter_write(file, iter, &iocb->ki_pos, rwf);
-		if (ctx->end_write)
-			ctx->end_write(ctx->user_file);
-	} else {
-		struct backing_aio *aio;
+			ret = vfs_iter_write(file, iter, &iocb->ki_pos, rwf);
+			if (ctx->end_write)
+				ctx->end_write(ctx->user_file);
+		} else {
+			struct backing_aio *aio;
 
-		ret = backing_aio_init_wq(iocb);
-		if (ret)
-			goto out;
+			ret = backing_aio_init_wq(iocb);
+			if (ret)
+				return ret;
 
-		ret = -ENOMEM;
-		aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
-		if (!aio)
-			goto out;
+			ret = -ENOMEM;
+			aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+			if (!aio)
+				return ret;
 
-		aio->orig_iocb = iocb;
-		aio->end_write = ctx->end_write;
-		kiocb_clone(&aio->iocb, iocb, get_file(file));
-		aio->iocb.ki_flags = flags;
-		aio->iocb.ki_complete = backing_aio_queue_completion;
-		refcount_set(&aio->ref, 2);
-		ret = vfs_iocb_iter_write(file, &aio->iocb, iter);
-		backing_aio_put(aio);
-		if (ret != -EIOCBQUEUED)
-			backing_aio_cleanup(aio, ret);
+			aio->orig_iocb = iocb;
+			aio->end_write = ctx->end_write;
+			kiocb_clone(&aio->iocb, iocb, get_file(file));
+			aio->iocb.ki_flags = flags;
+			aio->iocb.ki_complete = backing_aio_queue_completion;
+			refcount_set(&aio->ref, 2);
+			ret = vfs_iocb_iter_write(file, &aio->iocb, iter);
+			backing_aio_put(aio);
+			if (ret != -EIOCBQUEUED)
+				backing_aio_cleanup(aio, ret);
+		}
 	}
-out:
-	revert_creds(old_cred);
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(backing_file_write_iter);
@@ -252,15 +249,15 @@ ssize_t backing_file_splice_read(struct file *in, loff_t *ppos,
 				 unsigned int flags,
 				 struct backing_file_ctx *ctx)
 {
-	const struct cred *old_cred;
+	const struct cred *old_cred = ctx->cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(in->f_mode & FMODE_BACKING)))
 		return -EIO;
 
-	old_cred = override_creds(ctx->cred);
-	ret = vfs_splice_read(in, ppos, pipe, len, flags);
-	revert_creds(old_cred);
+	scoped_guard(cred, old_cred) {
+		ret = vfs_splice_read(in, ppos, pipe, len, flags);
+	}
 
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
@@ -274,7 +271,7 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 				  unsigned int flags,
 				  struct backing_file_ctx *ctx)
 {
-	const struct cred *old_cred;
+	const struct cred *old_cred = ctx->cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(out->f_mode & FMODE_BACKING)))
@@ -284,12 +281,11 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 	if (ret)
 		return ret;
 
-	old_cred = override_creds(ctx->cred);
-	file_start_write(out);
-	ret = iter_file_splice_write(pipe, out, ppos, len, flags);
-	file_end_write(out);
-	revert_creds(old_cred);
-
+	scoped_guard(cred, old_cred) {
+		file_start_write(out);
+		ret = iter_file_splice_write(pipe, out, ppos, len, flags);
+		file_end_write(out);
+	}
 	if (ctx->end_write)
 		ctx->end_write(ctx->user_file);
 
@@ -300,7 +296,7 @@ EXPORT_SYMBOL_GPL(backing_file_splice_write);
 int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
 		      struct backing_file_ctx *ctx)
 {
-	const struct cred *old_cred;
+	const struct cred *old_cred = ctx->cred;
 	int ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)) ||
@@ -312,9 +308,9 @@ int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
 
 	vma_set_file(vma, file);
 
-	old_cred = override_creds(ctx->cred);
-	ret = call_mmap(vma->vm_file, vma);
-	revert_creds(old_cred);
+	scoped_guard(cred, old_cred) {
+		ret = call_mmap(vma->vm_file, vma);
+	}
 
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
-- 
2.43.0