Memory reserves or lack thereof

[Alan Cox]

On 11/13/2012 05:54, Konstantin Belousov wrote:
> On Mon, Nov 12, 2012 at 05:10:01PM -0600, Alan Cox wrote:
>> On 11/12/2012 3:48 PM, Konstantin Belousov wrote:
>>> On Mon, Nov 12, 2012 at 01:28:02PM -0800, Sushanth Rai wrote:
>>>> This patch still doesn't address the issue of M_NOWAIT calls driving
>>>> the memory the all the way down to 2 pages, right ? It would be nice to
>>>> have M_NOWAIT just do non-sleep version of M_WAITOK and M_USE_RESERVE
>>>> flag to dig deep.
>>> This is out of scope of the change. But it is required for any further
>>> adjustements.
>> I would suggest a somewhat different response:
>>
>> The patch does make M_NOWAIT into a "non-sleep version of M_WAITOK" and 
>> does reintroduce M_USE_RESERVE as a way to specify "dig deep".
>>
>> Currently, both M_NOWAIT and M_WAITOK can drive the cache/free memory 
>> down to two pages.  The effect of the patch is to stop M_NOWAIT at two 
>> pages rather than allowing it to continue to zero pages.
>>
>> When you say, "This is out of scope ...", I believe that you are 
>> referring to changing two pages into something larger.  I agree that 
>> this is out of scope for the current change.
> I referred exactly to the difference between M_USE_RESERVE set or not.
> IMO this is what was asked by the question author. So yes, my mean of
> the 'out of scope' is about tweaking the 'two pages reserve' in some
> way.

Since M_USE_RESERVE is no longer deprecated in HEAD, here is my proposed
man page update to malloc(9):

Index: share/man/man9/malloc.9
===================================================================
--- share/man/man9/malloc.9     (revision 243091)
+++ share/man/man9/malloc.9     (working copy)
@@ -29,7 +29,7 @@
 .\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
 .\" $FreeBSD$
 .\"
-.Dd January 28, 2012
+.Dd November 15, 2012
 .Dt MALLOC 9
 .Os
 .Sh NAME
@@ -153,13 +153,12 @@ if
 .Dv M_WAITOK
 is specified.
 .It Dv M_USE_RESERVE
-Indicates that the system can dig into its reserve in order to obtain the
-requested memory.
-This option used to be called
-.Dv M_KERNEL
-but has been renamed to something more obvious.
-This option has been deprecated and is slowly being removed from the
kernel,
-and so should not be used with any new programming.
+Indicates that the system can use its reserve of memory to satisfy the
+request.
+This option should only be used in combination with
+.Dv M_NOWAIT
+when an allocation failure cannot be tolerated by the caller without
+catastrophic effects on the system.
 .El
 .Pp
 Exactly one of either