Date: Wed, 21 Apr 2004 22:17:52 -0400 From: Stephan Uphoff <ups@tree.com> To: Stephan Uphoff <ups@tree.com> Cc: Julian Elischer <julian@elischer.org> Subject: Re: how to flush out cache.? Message-ID: <200404220217.WAA28285@stups.com> In-Reply-To: Message from Stephan Uphoff <ups@tree.com> of "Wed, 21 Apr 2004 21:52:43 EDT." <200404220152.VAA26897@stups.com>
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Stephan Uphoff wrote: > Matthew Dillon wrote: > > : > > :> > > :> mmap() and msync(..MS_INVALIDATE..) should work. > > : > > :hmmm that is rather interesting.. > > :I wonder if it would work.... > > :Maybe a vm guru could confirm this.. (under 4.x) > > : > > > > Huh. If I hadn't looked at the code I would have said that > > MS_INVALIDATE doesn't work in FreeBSD, but when I look at the code > > it sure looks like it ought to work! > > > > But, alas, it does not. The invalidation request goes all the way > > through to the vnode pager but it looks like the vnode pager ignores > > it. > > > > MS_INVALIDATE -> OBJPC_INVAL -> VM_PAGER_PUT_INVAL -> IO_INVAL -> (ignored) > > > > IO_INVAL is defined to be 'invalidate after I/O completes', > > not 'throw away the dirty data', but the only place it appears to be > > implemented is in the NFS code. > > > > -Matt > > > Yes - but FreeBSD then calls vm_object_page_remove to remove the pages > from the vnode object. (vm_object_sync for 5.x or vm_map_clean for 4.x ) > > Mhhh .. 4.X removes the pages without flushing if the file is not write > mapped. > This means that anyone with read access can screw up a file. A file with dirty (unflushed) pages. > ( For example periodically throwing away pages from /var/log/wtmp ...) > 5.x looks better - but there is a race condition that can cause the same thing. > I think for msync() the vm_object_page_remove() function should be called with > clean_only set to TRUE to avoid throwing away valid changes. > ( I will send-pr tomorrow ) > > Stephan > > >
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