PR/52706 CVS commit: src/sys/arch/x86/x86

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

The following reply was made to PR kern/52706; it has been noted by GNATS.

From: "Chuck Silvers" <chs%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/52706 CVS commit: src/sys/arch/x86/x86
Date: Mon, 20 Nov 2017 20:57:59 +0000

 Module Name:	src
 Committed By:	chs
 Date:		Mon Nov 20 20:57:58 UTC 2017
 
 Modified Files:
 	src/sys/arch/x86/x86: pmap.c
 
 Log Message:
 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.  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, as well as the failing assertion
 about "uvm_page_locked_p(old_pg)".  (but only on x86, various other platforms
 will need their own changes to handle this issue.)
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.265 -r1.266 src/sys/arch/x86/x86/pmap.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.