CVS commit: src/sys/kern

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Fri Oct 20 14:48:43 UTC 2017

Modified Files:
        src/sys/kern: kern_exec.c

Log Message:
Carve out KVA for execargs on boot from an exec_map like we used to.

Candidate fix for PR kern/45718: `processes sometimes get stuck and
spin in vm_map', a problem that has been plaguing all our 32-bit
ports for years.

Since we currently use large (256k) buffers for execargs, and since
nobody has stepped up to tackle breaking them into bite-sized (or at
least page-sized) chunks, after KVA gets sufficiently fragmented we
can't allocate new execargs buffers from kernel_map.

Until 2008, we always carved out KVA for execargs on boot with a uvm
submap exec_map of kernel_map.  Then ad@ found that the uvm_km_free
call, to discard them when done, cost about 100us, which a pool
avoided:

https://mail-index.NetBSD.org/tech-kern/2008/06/25/msg001854.html
https://mail-index.NetBSD.org/tech-kern/2008/06/26/msg001859.html

ad@ _simultaneously_ introduced a pool _and_ eliminated the reserved
KVA in the exec_map submap.  This change preserves the pool, but
restores exec_map (with less code, by putting it in MI code instead
of copying it in every MD initialization routine).

Patch proposed on tech-kern:
https://mail-index.NetBSD.org/tech-kern/2017/10/19/msg022461.html

Patch tested by bouyer@:
https://mail-index.NetBSD.org/tech-kern/2017/10/20/msg022465.html

I previously discussed the issue on tech-kern before I knew of the
history around exec_map:
https://mail-index.NetBSD.org/tech-kern/2012/12/09/msg014695.html

The candidate workaround I proposed of using pool_setlowat to force
preallocation of KVA would also force preallocation of physical RAM,
which is a waste not incurred by using exec_map, and which is part of
why I never committed it.

There may remain a general problem that if thread A calls pool_get
and tries to service that request by a uvm_km_alloc call that hangs
because KVA is scarce, and thread B does pool_put, the pool_put in
thread B will not notify the pool_get in thread A that it doesn't
need to wait for KVA, and so thread A may continue to hang in
uvm_km_alloc.  However,

(a) That won't apply here, because there is exactly as much KVA
available in exec_map as exec_pool will ever try to use.

(b) It is possible that may not even matter in other cases as long as
the page daemon eventually tries to shrink the pool, which will cause
a uvm_km_free that can unhang the hung uvm_km_alloc.

XXX pullup-8
XXX pullup-7
XXX pullup-6
XXX pullup-5, perhaps...


To generate a diff of this commit:
cvs rdiff -u -r1.447 -r1.448 src/sys/kern/kern_exec.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.