nfsdv4 leaks file descriptors

[Gerhard Heift <ml-nfs-linux-20110412-ef47@gheift.de>]

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Hello,

I have a linux kernel (2.6.38.2, armv5tel) running which exports a
directory with following options:

/data ip/32(rw,async,crossmnt,insecure,root_squash,no_subtree_check,fsid=0)

On the client (Debian unstable, kernel 2.6.38-2-amd64) I mounted the
directory under /media(nfs4import with

# mount -t nfs4 ip:/ /media/nfs4import

After a lot running fdupes on this folder the server kernel starts
throwing messages like this one:

  VFS: file-max limit [number] reached

After a reboot I recognised that the number of used file descriptors
increases during the run of fdupes (see /proc/sys/fs/file-nr). Even
after unmounting the exported filesystem on the client, unexport the
file system on the server and stopping the nfs-server the file
descriptors are still allocated and unmounting the filesystem is not
possible.

After a little bit of research I could create a small program, which
increases the used file descriptors by one.

Regards,
  Gerhard

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#include <stdio.h>

#define DIR "/media/nfs4import/"

int main(void) {
	FILE *f1;
	FILE *f2;
	char b[1024];
	int i;

	// create file1
	f1 = fopen(DIR "file1.txt", "w");
	fwrite("test\n", 5, 1, f1);
	close(f1);

	// open file1 in read mode
	f1 = fopen(DIR "file1.txt", "r");

	for (i = 0; i < 1; i++) {
		// open file2 in write mode and close it
		f2 = fopen(DIR "file2.txt", "w");
		fclose(f2);
	
		// open file2 in read mode and close it
		// this causes the leak of a file descriptor on the server
		// just watch /proc/sys/fs/file-nr on the server
		f2 = fopen(DIR "file2.txt", "r");
		fclose(f2);
	}
	fclose(f1);

	return 0;
}

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