Date: Wed, 4 Sep 2019 20:28:16 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r351830 - head/sys/vm Message-ID: <201909042028.x84KSGHS060580@repo.freebsd.org>
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Author: kib Date: Wed Sep 4 20:28:16 2019 New Revision: 351830 URL: https://svnweb.freebsd.org/changeset/base/351830 Log: madvise(MADV_FREE): Quick fix to time rewind. Don't free pages in a shadowing object. While this degrades MADV_FREE to a no-op (and we could, instead, choose to fall back to MADV_DONTNEED, at the cost of changing pmap_madvise), this is presently considered a temporary fix. We may prefer to risk a little fragmentation of the map by creating a zero/OBJT_DEFAULT entry over top of the existing object and, simultaneously, revert to the existing marking any pages in the former shadowing object in the advised region as reclaimable. At least one consumer of MADV_FREE (snmalloc) may use mmap() to construct zeroed pages "eventually" here anyway, so the fragmentation may be coming anyway. Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk> PR: 240061 Reviewed by: markj MFC after: 1 week Differential revision: https://reviews.freebsd.org/D21517 Modified: head/sys/vm/vm_map.c Modified: head/sys/vm/vm_map.c ============================================================================== --- head/sys/vm/vm_map.c Wed Sep 4 20:14:21 2019 (r351829) +++ head/sys/vm/vm_map.c Wed Sep 4 20:28:16 2019 (r351830) @@ -2736,6 +2736,18 @@ vm_map_madvise( if (current->eflags & MAP_ENTRY_IS_SUB_MAP) continue; + /* + * MADV_FREE would otherwise rewind time to + * the creation of the shadow object. Because + * we hold the VM map read-locked, neither the + * entry's object nor the presence of a + * backing object can change. + */ + if (behav == MADV_FREE && + current->object.vm_object != NULL && + current->object.vm_object->backing_object != NULL) + continue; + pstart = OFF_TO_IDX(current->offset); pend = pstart + atop(current->end - current->start); useStart = current->start;
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