Bug#795991: /boot/vmlinuz-3.16.0-0.bpo.4-amd64: duplicate request cache is too small for NFS 4.1 servers

[Christoph Martin <martin@uni-mainz.de>]

Package: src:linux
Version: 3.16.7-ckt11-1+deb8u3~bpo70+1
Severity: normal
File: /boot/vmlinuz-3.16.0-0.bpo.4-amd64

The drc (duplicate request cache) for NFS 4.1 in the vanilla kernel
has a fixed size only depending on the RAM of the machine.

For example, when setting up a vm which should only serve as a
nfs referral server with 768 MB RAM it could only server about
20 clients. So it is roughly 32 clients per GB.

The problem is, that in nfssvc.c the size of drc is calculated with
a shift of NFSD_DRC_SIZE_SHIFT bits from the RAM size.

I attach a patch for this which I also sent to linux-nfs.
(See http://www.spinics.net/lists/linux-nfs/msg51791.html)

It implements a module variable to set the size on module load.

It hope that such a patch can make it into Debian soon.

Yours
Christoph


-- System Information:
Debian Release: 7.8
  APT prefers oldstable-updates
  APT policy: (700, 'oldstable-updates'), (700, 'oldstable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.16.7-ckt9 (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968)
Shell: /bin/sh linked to /bin/dash

Versions of packages linux-image-3.16.0-0.bpo.4-amd64 depends on:
ii  debconf [debconf-2.0]  1.5.49
ii  initramfs-tools        0.115~bpo70+1
ii  kmod                   9-3
ii  linux-base             3.5
ii  module-init-tools      9-3

Versions of packages linux-image-3.16.0-0.bpo.4-amd64 recommends:
ii  firmware-linux-free  3.2
ii  irqbalance           1.0.3-3

Versions of packages linux-image-3.16.0-0.bpo.4-amd64 suggests:
pn  debian-kernel-handbook  <none>
ii  grub-pc                 1.99-27+deb7u2
pn  linux-doc-3.16          <none>

Versions of packages linux-image-3.16.0-0.bpo.4-amd64 is related to:
pn  firmware-atheros        <none>
pn  firmware-bnx2           <none>
pn  firmware-bnx2x          <none>
pn  firmware-brcm80211      <none>
pn  firmware-intelwimax     <none>
pn  firmware-ipw2x00        <none>
pn  firmware-ivtv           <none>
pn  firmware-iwlwifi        <none>
pn  firmware-libertas       <none>
pn  firmware-linux          <none>
pn  firmware-linux-nonfree  <none>
pn  firmware-myricom        <none>
pn  firmware-netxen         <none>
pn  firmware-qlogic         <none>
pn  firmware-ralink         <none>
pn  firmware-realtek        <none>
pn  xen-hypervisor          <none>

-- debconf information:
  linux-image-3.16.0-0.bpo.4-amd64/postinst/depmod-error-initrd-3.16.0-0.bpo.4-amd64: false
  linux-image-3.16.0-0.bpo.4-amd64/postinst/mips-initrd-3.16.0-0.bpo.4-amd64:
  linux-image-3.16.0-0.bpo.4-amd64/prerm/removing-running-kernel-3.16.0-0.bpo.4-amd64: true

--- linux-source-3.16/fs/nfsd/nfssvc.c	2015-03-30 12:09:09.000000000 +0200
+++ linux-source-3.16.nfsd/fs/nfsd/nfssvc.c	2015-06-17 09:28:37.880443867 +0200
@@ -359,11 +359,19 @@ void nfsd_reset_versions(void)
  * For now this is a #defined shift which could be under admin control
  * in the future.
  */
+
+static ulong drc_size = 0;
+module_param(drc_size, ulong, 0444);
+MODULE_PARM_DESC(drc_size,
+		 "size of NFSv4.1 DRC cache memory (default and minimum: free_buffer_size >> 10)");
+
 static void set_max_drc(void)
 {
 	#define NFSD_DRC_SIZE_SHIFT	10
-	nfsd_drc_max_mem = (nr_free_buffer_pages()
-					>> NFSD_DRC_SIZE_SHIFT) * PAGE_SIZE;
+	nfsd_drc_max_mem = max(drc_size,
+			       (nr_free_buffer_pages()
+				>> NFSD_DRC_SIZE_SHIFT) * PAGE_SIZE);
+	drc_size = nfsd_drc_max_mem;
 	nfsd_drc_mem_used = 0;
 	spin_lock_init(&nfsd_drc_lock);
 	dprintk("%s nfsd_drc_max_mem %lu \n", __func__, nfsd_drc_max_mem);