linux-kernel - [RFC v3 4/5] fs: Optimize credentials reference count for backing file ops

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Message-ID: <20240216051640.197378-5-vinicius.gomes@intel.com>
Date: Thu, 15 Feb 2024 21:16:39 -0800
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: brauner@...nel.org,
	amir73il@...il.com,
	hu1.chen@...el.com
Cc: miklos@...redi.hu,
	malini.bhandaru@...el.com,
	tim.c.chen@...el.com,
	mikko.ylinen@...el.com,
	lizhen.you@...el.com,
	linux-unionfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: [RFC v3 4/5] fs: Optimize credentials reference count for backing file ops

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@...el.com>
---
 fs/backing-file.c | 27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

diff --git a/fs/backing-file.c b/fs/backing-file.c
index a681f38d84d8..2cd015f49382 100644
--- a/fs/backing-file.c
+++ b/fs/backing-file.c
@@ -140,7 +140,6 @@ 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;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -153,7 +152,7 @@ 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);
+	guard(cred)(ctx->cred);
 	if (is_sync_kiocb(iocb)) {
 		rwf_t rwf = iocb_to_rw_flags(flags);
 
@@ -174,8 +173,6 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 			backing_aio_cleanup(aio, ret);
 	}
 out:
-	revert_creds(old_cred);
-
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
 
@@ -187,7 +184,6 @@ 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;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -210,7 +206,7 @@ ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
 	 */
 	flags &= ~IOCB_DIO_CALLER_COMP;
 
-	old_cred = override_creds(ctx->cred);
+	guard(cred)(ctx->cred);
 	if (is_sync_kiocb(iocb)) {
 		rwf_t rwf = iocb_to_rw_flags(flags);
 
@@ -222,12 +218,12 @@ ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
 
 		ret = backing_aio_init_wq(iocb);
 		if (ret)
-			goto out;
+			return ret;
 
 		ret = -ENOMEM;
 		aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
 		if (!aio)
-			goto out;
+			return ret;
 
 		aio->orig_iocb = iocb;
 		aio->end_write = ctx->end_write;
@@ -240,9 +236,6 @@ ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
 		if (ret != -EIOCBQUEUED)
 			backing_aio_cleanup(aio, ret);
 	}
-out:
-	revert_creds(old_cred);
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(backing_file_write_iter);
@@ -252,15 +245,13 @@ 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;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(in->f_mode & FMODE_BACKING)))
 		return -EIO;
 
-	old_cred = override_creds(ctx->cred);
+	guard(cred)(ctx->cred);
 	ret = vfs_splice_read(in, ppos, pipe, len, flags);
-	revert_creds(old_cred);
 
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
@@ -274,7 +265,6 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 				  unsigned int flags,
 				  struct backing_file_ctx *ctx)
 {
-	const struct cred *old_cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(out->f_mode & FMODE_BACKING)))
@@ -284,11 +274,10 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 	if (ret)
 		return ret;
 
-	old_cred = override_creds(ctx->cred);
+	guard(cred)(ctx->cred);
 	file_start_write(out);
 	ret = iter_file_splice_write(pipe, out, ppos, len, flags);
 	file_end_write(out);
-	revert_creds(old_cred);
 
 	if (ctx->end_write)
 		ctx->end_write(ctx->user_file);
@@ -300,7 +289,6 @@ 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;
 	int ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)) ||
@@ -312,9 +300,8 @@ int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
 
 	vma_set_file(vma, file);
 
-	old_cred = override_creds(ctx->cred);
+	guard(cred)(ctx->cred);
 	ret = call_mmap(vma->vm_file, vma);
-	revert_creds(old_cred);
 
 	if (ctx->accessed)
 		ctx->accessed(ctx->user_file);
-- 
2.43.1