Date: Thu, 22 Feb 2001 19:26:42 -0800 (PST) From: patl@Phoenix.Volant.ORG To: Peter Wemm <peter@netplex.com.au> Cc: tlambert@primenet.com, freebsd-fs@FreeBSD.ORG Subject: Re: Status of nullfs Message-ID: <ML-3.4.982898802.5641.patl@asimov.phoenix.volant.org> In-Reply-To: <200102230242.f1N2gUf43386@mobile.wemm.org>
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On 22-Feb-01 at 18:42, Peter Wemm (peter@netplex.com.au) wrote: > > Any mmap'ed writes will fail due to cache coherency problems. If > > the system isn't quiescent long enough to flush everything out > > before an unmount, you can panic. If you have exposed the NULL > > mount , e.g. by mounting an FS on /usr, and then nullfs mounting > > it to /null_usr, then modifications to the /usr FS will not > > result in coherency notification, and files will become corrupt, > > if accessed both ways. Paging from executables can cause problems > > for multiple copies of executables, if statically initialized data > > is modified (copy on write) due to the way backing page handling > > is limited. > > > > All of these derive from the fact that the vnode in the upper > > layer (nullfs) and the vnode in the lower layer (ffs or whatever) > > have seperate vm_object_t's, even though the data contents of the > > file are supposed to be treated as identical for both. > > Terry, please check your facts. This has been recently fixed. See the > VOP_GETVOBJECT() stuff. The trick though is to get it MFC'ed into > 4.x... In Terry's support, I -was- specificly asking about -STABLE. > > There are a number of patches to manually enforce coherency; as > > far as I know, they don't deal with the pager path, and, since > > the coherency is explicit, the /usr vs. /null_usr problem is > > not resolved. Check the list archives to obtain the patches. > > nullfs is fully functional in -current. Backporting the changes should > be easy. Hmm. Easy for someone familiar with the kernel internals; or easy for someone with 30 years of systems level software engineering experience but -no- unix kernel experience? (If the later, I may see if I can free up the cycles to do it. If the former, I doubt that I'll get enough cycles to climb that specific learning curve before other priorities intervene.) -Pat To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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