From owner-freebsd-bugs@freebsd.org Fri Aug 23 20:46:33 2019 Return-Path: Delivered-To: freebsd-bugs@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id EAA4DCDDCF for ; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from mailman.nyi.freebsd.org (unknown [127.0.1.3]) by mx1.freebsd.org (Postfix) with ESMTP id 46FYN95qKdz49wr for ; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: by mailman.nyi.freebsd.org (Postfix) id C5B5DCDDCE; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) Delivered-To: bugs@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id C5795CDDCD for ; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from mxrelay.nyi.freebsd.org (mxrelay.nyi.freebsd.org [IPv6:2610:1c1:1:606c::19:3]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) server-signature RSA-PSS (4096 bits) client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "mxrelay.nyi.freebsd.org", Issuer "Let's Encrypt Authority X3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 46FYN94vt0z49wq for ; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from kenobi.freebsd.org (kenobi.freebsd.org [IPv6:2610:1c1:1:606c::50:1d]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mxrelay.nyi.freebsd.org (Postfix) with ESMTPS id 8BB5D22377 for ; Fri, 23 Aug 2019 20:46:33 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from kenobi.freebsd.org ([127.0.1.5]) by kenobi.freebsd.org (8.15.2/8.15.2) with ESMTP id x7NKkXDw016415 for ; Fri, 23 Aug 2019 20:46:33 GMT (envelope-from bugzilla-noreply@freebsd.org) Received: (from www@localhost) by kenobi.freebsd.org (8.15.2/8.15.2/Submit) id x7NKkXpF016414 for bugs@FreeBSD.org; Fri, 23 Aug 2019 20:46:33 GMT (envelope-from bugzilla-noreply@freebsd.org) X-Authentication-Warning: kenobi.freebsd.org: www set sender to bugzilla-noreply@freebsd.org using -f From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 240061] MADV_FREE rewinds time to before fork() Date: Fri, 23 Aug 2019 20:46:33 +0000 X-Bugzilla-Reason: AssignedTo X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: Base System X-Bugzilla-Component: kern X-Bugzilla-Version: CURRENT X-Bugzilla-Keywords: X-Bugzilla-Severity: Affects Only Me X-Bugzilla-Who: nwf20@cl.cam.ac.uk X-Bugzilla-Status: New X-Bugzilla-Resolution: X-Bugzilla-Priority: --- X-Bugzilla-Assigned-To: bugs@FreeBSD.org X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version rep_platform op_sys bug_status bug_severity priority component assigned_to reporter Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: https://bugs.freebsd.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: freebsd-bugs@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Bug reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 23 Aug 2019 20:46:34 -0000 https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D240061 Bug ID: 240061 Summary: MADV_FREE rewinds time to before fork() Product: Base System Version: CURRENT Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: kern Assignee: bugs@FreeBSD.org Reporter: nwf20@cl.cam.ac.uk MADV_FREE simply cleans pages associated with the current object backing the advisee region so that the act of page reclaimation will remove it if neede= d. If there is no shadowed object, this is fine: if the page is removed, it wi= ll be demand-zeroed, and if it isn't removed, it will be as it was before MADV_FREE. In the case there is a shadowed object (i.e., these pages are CoW; say, they were mapped before fork()), however, if the page is removed, it will not be demand zeroed: it will be copied up from the shadowed object. It is theref= ore possible to selectively rewind time to before fork() and expose whatever information was in these pages. (I have a test program that demonstrates t= he effect, but as it relies on inducing memory pressure, it is rude for a test suite; it would be nice if there were a synthetic way to cause the page rec= laim subsystem to run even without true memory pressure.) #1851 makes somewhat oblique reference to an issue with MADV_FREE and I pre= sume that it is this behavior that is being commented upon, but it's hard to say= for sure. Perhaps this behavior was indeed eliminated in 1997, but it's back n= ow. In any case, I *believe*, not being very familiar with the VM subsystem, th= at vm_map_madvise() needs to... consider MADV_FREE a modify_map=3Dtrue operation and... for each map entry within the advisee region, call vm_object_split() and = then release the backing object... then call pmap_advise() and vm_object_madvise() on this new object. There may well be a better way to do this; it may, in fact, be desirable to treat MADV_FREE as if it were mmap()-ing a new entry and new object when the advisee object is OBJT_DEFAULT or OBJT_SWAP; I am less sure what MADV_FREE's semantics should be if it's OBJT_VNODE, for example. --=20 You are receiving this mail because: You are the assignee for the bug.=