On 7/3/2024 10:34 PM, Christian Brauner wrote:
On Wed, Jul 03, 2024 at 10:33:09AM GMT, Yu Ma wrote:
alloc_fd() has a sanity check inside to make sure the struct file mapping to the
allocated fd is NULL. Remove this sanity check since it can be assured by
exisitng zero initilization and NULL set when recycling fd. Meanwhile, add
likely/unlikely and expand_file() call avoidance to reduce the work under
file_lock.
Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
---
fs/file.c | 38 ++++++++++++++++----------------------
1 file changed, 16 insertions(+), 22 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index a3b72aa64f11..5178b246e54b 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -515,28 +515,29 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
if (fd < files->next_fd)
fd = files->next_fd;
- if (fd < fdt->max_fds)
+ if (likely(fd < fdt->max_fds))
fd = find_next_fd(fdt, fd);
+ error = -EMFILE;
+ if (unlikely(fd >= fdt->max_fds)) {
+ error = expand_files(files, fd);
+ if (error < 0)
+ goto out;
+ /*
+ * If we needed to expand the fs array we
+ * might have blocked - try again.
+ */
+ if (error)
+ goto repeat;
+ }
So this ends up removing the expand_files() above the fd >= end check
which means that you can end up expanding the files_struct even though
the request fd is past the provided end. That seems odd. What's the
reason for that reordering?
Yes, you are right, thanks Christian. This incorrect reordering here is
due to historical versions with fast path inside. I'll update the order
back.
+
/*
* N.B. For clone tasks sharing a files structure, this test
* will limit the total number of files that can be opened.
*/
- error = -EMFILE;
- if (fd >= end)
- goto out;
-
- error = expand_files(files, fd);
- if (error < 0)
+ if (unlikely(fd >= end))
goto out;
- /*
- * If we needed to expand the fs array we
- * might have blocked - try again.
- */
- if (error)
- goto repeat;
-
if (start <= files->next_fd)
files->next_fd = fd + 1;
@@ -546,13 +547,6 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
else
__clear_close_on_exec(fd, fdt);
error = fd;
-#if 1
- /* Sanity check */
- if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
- printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
- rcu_assign_pointer(fdt->fd[fd], NULL);
- }
-#endif
out:
spin_unlock(&files->file_lock);
@@ -618,7 +612,7 @@ void fd_install(unsigned int fd, struct file *file)
rcu_read_unlock_sched();
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
- BUG_ON(fdt->fd[fd] != NULL);
+ WARN_ON(fdt->fd[fd] != NULL);
rcu_assign_pointer(fdt->fd[fd], file);
spin_unlock(&files->file_lock);
return;
--
2.43.0
