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)
