On Wed, Feb 05, 2014 at 17:53, Bob Beck wrote:
> On Wed, Feb 5, 2014 at 3:17 PM, Ted Unangst <[email protected]> wrote:
>> We are missing back pressure channels from uvm to the buf cache. The
>> buf cache will happily sit on 9000 free pages while uvm churns around
>> trying to scavenge up one more page.
> Or are you in a situation here where the cache has *not* backed off?
Talked to Bob and hashed out better ideas of the problem. The page
daemon does tell the buffer cache to make some room, but...
If you have a huge mmap file, the pdaemon will try to flush it out via
VOP_WRITE, which circles back via ffs into buf_get, which eats those
previously freed pages, and then some, as the pagedaemon continues
pushing more and more of the mmap file out.
We discussed some other changes and fixes that this situation has
clearly highlighted, but here's a slightly revised diff. It now uses
the correct bufbackoff() function to communicate uvm's needs. Any
other fix is rather precarious for this release, but as stated before,
this keeps the change to the deadlock paths. You were already dead,
but now you have a second chance.
(We don't currently use the pmemrange argument; we'll have to adjust
accordingly when the bufcache becomes range aware.)
Index: uvm_pdaemon.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pdaemon.c,v
retrieving revision 1.64
diff -u -p -r1.64 uvm_pdaemon.c
--- uvm_pdaemon.c 30 May 2013 16:29:46 -0000 1.64
+++ uvm_pdaemon.c 6 Feb 2014 03:09:53 -0000
@@ -117,6 +117,8 @@ uvm_wait(const char *wmsg)
*/
if (curproc == uvm.pagedaemon_proc) {
+ if (bufbackoff(NULL, 4) == 0)
+ return;
/*
* now we have a problem: the pagedaemon wants to go to
* sleep until it frees more memory. but how can it
Index: uvm_pmemrange.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pmemrange.c,v
retrieving revision 1.36
diff -u -p -r1.36 uvm_pmemrange.c
--- uvm_pmemrange.c 29 Jan 2013 19:55:48 -0000 1.36
+++ uvm_pmemrange.c 6 Feb 2014 03:10:32 -0000
@@ -22,6 +22,7 @@
#include <sys/malloc.h>
#include <sys/proc.h> /* XXX for atomic */
#include <sys/kernel.h>
+#include <sys/mount.h>
/*
* 2 trees: addr tree and size tree.
@@ -1883,6 +1884,13 @@ uvm_wait_pla(paddr_t low, paddr_t high,
const char *wmsg = "pmrwait";
if (curproc == uvm.pagedaemon_proc) {
+ uvm_unlock_fpageq();
+ if (bufbackoff(NULL, atop(size)) == 0) {
+ uvm_lock_fpageq();
+ return 0;
+ }
+ uvm_lock_fpageq();
+
/*
* XXX detect pagedaemon deadlock - see comment in
* uvm_wait(), as this is exactly the same issue.
