Re: [RFC PATCH] fs: dcache: Delete the associated dentry when deleting a file

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, May 11, 2024 at 04:36:19AM +0100, Al Viro wrote:

> Said that, I seriously suspect that there are loads where it would become
> painful.  unlink() + creat() is _not_ a rare sequence, and this would
> shove an extra negative lookup into each of those.
> 
> I would like to see the details on original posters' setup.  Note that
> successful rmdir() evicts all children, so that it would seem that their
> arseloads of negative dentries come from a bunch of unlinks in surviving
> directories.
> 
> It would be interesting to see if using something like
> 	mkdir graveyard
> 	rename victim over there, then unlink in new place
> 	rename next victim over there, then unlink in new place
> 	....
> 	rmdir graveyard
> would change the situation with memory pressure - it would trigger
> eviction of all those negatives at controlled point(s) (rmdir).
> I'm not saying that it's a good mitigation, but it would get more
> details on that memory pressure.

BTW, how about adding to do_vfs_ioctl() something like
	case FS_IOC_FORGET:
		shrink_dcache_parent(file->f_path.dentry);
		return 0;
possibly with option for dropping only negatives?

Even in the minimal form it would allow userland to force eviction
in given directory tree, without disrupting things anywhere else.
That's a trivial (completely untested) patch, really -

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 1d5abfdf0f22..342bb71cf76c 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -878,6 +878,12 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
 	case FS_IOC_GETFSSYSFSPATH:
 		return ioctl_get_fs_sysfs_path(filp, argp);
 
+	case FS_IOC_FORGET:
+		if (arg != 0)	// only 0 for now
+			break;
+		shrink_dcache_parent(filp->f_path.dentry);
+		return 0;
+
 	default:
 		if (S_ISREG(inode->i_mode))
 			return file_ioctl(filp, cmd, argp);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 45e4e64fd664..143129510289 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -222,6 +222,7 @@ struct fsxattr {
 
 #define	FS_IOC_GETFLAGS			_IOR('f', 1, long)
 #define	FS_IOC_SETFLAGS			_IOW('f', 2, long)
+#define	FS_IOC_FORGET			_IOW('f', 255, int)
 #define	FS_IOC_GETVERSION		_IOR('v', 1, long)
 #define	FS_IOC_SETVERSION		_IOW('v', 2, long)
 #define FS_IOC_FIEMAP			_IOWR('f', 11, struct fiemap)