Date: Thu, 19 Jul 2018 17:01:10 +0000 (UTC) From: Alan Cox <alc@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r336491 - head/sys/vm Message-ID: <201807191701.w6JH1AP7036953@repo.freebsd.org>
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Author: alc Date: Thu Jul 19 17:01:10 2018 New Revision: 336491 URL: https://svnweb.freebsd.org/changeset/base/336491 Log: Revert r329254. The underlying cause for the copy-on-write problem in multithreaded programs that was addressed by r329254 was in the implementation of pmap_enter() on some architectures, notably, amd64. kib@, markj@ and I have audited all of the pmap_enter() implementations, and fixed the broken ones, specifically, amd64 (r335784, r335971), i386 (r336092), mips (r336248), and riscv (r336294). To be clear, the reason to address the problem within pmap_enter() and revert r329254 is not just a matter of principle. An effect of r329254 was that a copy-on-write fault actually entailed two page faults, not one, even for single-threaded programs. Now, in the expected case for either single- or multithreaded programs, we are back to a single page fault to complete a copy-on-write operation. (In extremely rare circumstances, a multithreaded program could suffer two page faults.) Reviewed by: kib, markj Tested by: truckman MFC after: 3 weeks Differential Revision: https://reviews.freebsd.org/D16301 Modified: head/sys/vm/vm_fault.c Modified: head/sys/vm/vm_fault.c ============================================================================== --- head/sys/vm/vm_fault.c Thu Jul 19 16:58:20 2018 (r336490) +++ head/sys/vm/vm_fault.c Thu Jul 19 17:01:10 2018 (r336491) @@ -1152,10 +1152,6 @@ readrest: */ pmap_copy_page(fs.m, fs.first_m); fs.first_m->valid = VM_PAGE_BITS_ALL; - if ((fault_flags & VM_FAULT_WIRE) == 0) { - prot &= ~VM_PROT_WRITE; - fault_type &= ~VM_PROT_WRITE; - } if (wired && (fault_flags & VM_FAULT_WIRE) == 0) { vm_page_lock(fs.first_m);
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