Date: Sat, 15 Apr 2000 20:13:20 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Alan Cox <alc@cs.rice.edu> Cc: Brian Fundakowski Feldman <green@FreeBSD.ORG>, current@FreeBSD.ORG Subject: Re: panic: vm_object_shadow: source object has OBJ_ONEMAPPING set. Message-ID: <200004160313.UAA82143@apollo.backplane.com> References: <20000415140945.B3462@cs.rice.edu> <Pine.BSF.4.21.0004151801140.17351-100000@green.dyndns.org> <20000415185718.E3462@cs.rice.edu>
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:Here's what I worry about: We only clear OBJ_ONEMAPPING on the top-level :object, and none of its backing objects. Nothing guarantees that these :backing objects have OBJ_ONEMAPPING cleared. The page that we're "double" :mapping may, however, reside in one of its backing objects. This is :bad. I don't think this is an issue. The only double-mapped pages we care about are those at the top-level (vm_object's connected directly to a vm_map_entry). This is because these are the only pages effected by write-faults and copy-on-write issues. For example, if you dirty a page that is mapped privately the system must copy-on-write the page. More importantly, if you fork() and either parent or child dirties what is now a shared page, they need to copy on write and the other process cannot see that change. OBJ_ONEMAPPING is an optimization that allows a process to dirty an anonymous (not vnode-backed) page without doing a copy-on-write in the case where that process is the ONLY process mapping the page. Copy-on-write is an issue that only effects the top level object. So we shouldn't care whether OBJ_ONEMAPPING is set or cleared in deeper objects. Copying a page that was previously shared is an expensive operation not only in having to do the copy, but also in having to split the vm_map_entry and create a new holding vm_object to hold the copy. This new object must have OBJ_ONEMAPPING set so that when pages are dirtied around it, it can be pre-pended or post-pended with the new pages rather then have to create a new vm_object (and thus not be able to coalesce the associated vm_map_entry structures) every time we take a copy-on-write fault. In this respect, the OBJ_ONEMAPPING optimization is *CRITICAL* to the scaleability of our VM system. -Matt To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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