CVS commit: src/sys/arch

["Chuck Silvers" <chs%netbsd.org@localhost>]

Module Name:    src
Committed By:   chs
Date:           Sat Jan 27 23:07:36 UTC 2018

Modified Files:
        src/sys/arch/alpha/alpha: pmap.c
        src/sys/arch/m68k/m68k: pmap_motorola.c
        src/sys/arch/powerpc/oea: pmap.c
        src/sys/arch/sparc64/sparc64: pmap.c

Log Message:
apply the change from arch/x86/x86/pmap.c rev. 1.266 commitid vZRjvmxG7YTHLOfA:

In pmap_enter_ma(), only try to allocate pves if we might need them,
and even if that fails, only fail the operation if we later discover
that we really do need them.  If we are replacing an existing mapping,
reuse the pv structure where possible.

This implements the requirement that pmap_enter(PMAP_CANFAIL) must not fail
when replacing an existing mapping with the first mapping of a new page,
which is an unintended consequence of the changes from the rmind-uvmplock
branch in 2011.

The problem arises when pmap_enter(PMAP_CANFAIL) is used to replace an existing
pmap mapping with a mapping of a different page (eg. to resolve a copy-on-write).
If that fails and leaves the old pmap entry in place, then UVM won't hold
the right locks when it eventually retries.  This entanglement of the UVM and
pmap locking was done in rmind-uvmplock in order to improve performance,
but it also means that the UVM state and pmap state need to be kept in sync
more than they did before.  It would be possible to handle this in the UVM code
instead of in the pmap code, but these pmap changes improve the handling of
low memory situations in general, and handling this in UVM would be clunky,
so this seemed like the better way to go.

This somewhat indirectly fixes PR 52706 on the remaining platforms where
this problem existed.


To generate a diff of this commit:
cvs rdiff -u -r1.261 -r1.262 src/sys/arch/alpha/alpha/pmap.c
cvs rdiff -u -r1.69 -r1.70 src/sys/arch/m68k/m68k/pmap_motorola.c
cvs rdiff -u -r1.94 -r1.95 src/sys/arch/powerpc/oea/pmap.c
cvs rdiff -u -r1.307 -r1.308 src/sys/arch/sparc64/sparc64/pmap.c

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