Date: Wed, 2 Jun 2010 22:34:32 -0700 From: Juli Mallett <jmallett@FreeBSD.org> To: Alan Cox <alc@imimic.com> Cc: Alan Cox <alc@cs.rice.edu>, mips@freebsd.org Subject: Re: init_pte_prot() patch Message-ID: <AANLkTilICHWaFgEpjCV14wzFgtu_nSr8DGhWkXoO_Feq@mail.gmail.com> In-Reply-To: <4C0736A5.1010101@imimic.com> References: <4C0736A5.1010101@imimic.com>
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Hi Alan, On Wed, Jun 2, 2010 at 21:59, Alan Cox <alc@imimic.com> wrote: > I would appreciate it if someone would test the attached patch. =A0(A > "buildworld" would probably suffice.) =A0Essentially, it does two things: Sorry I didn't respond to this when you sent it to me. > 1. The virtual memory system only cares about the contents of a page's di= rty > field if that page is managed (i.e., it is pageable). =A0And, in fact, if= you > survey the calls to vm_page_dirty() in the MIPS or any other pmap, they a= re > generally conditioned on the mapping having been for a managed page. > > The MIPS pmap_enter() is an exception to this rule. =A0It is unconditiona= lly > calling vm_page_dirty() on any page mapped within the kernel address spac= e, > managed or otherwise. =A0In fact, it is highly unusual for pmap_enter() t= o be > calling vm_page_dirty() on the page being mapped, regardless of whether i= t > is managed. =A0This call to vm_page_dirty() shouldn't be needed if change= #2 > below is also made. =A0The attached patch eliminates the call. I believe that the reason the MIPS pmap does that is because PTE_RWPAGE includes PTE_M which is the TLB dirty bit. This means that we won't get exceptions when that page is modified, and so MIPS is pre-dirtying the VM page to allow it to make that optimization. At least, that's what the intent appears to be. Whether that's a correct model for the interaction between pmap and the VM system's management of those kernel pages, I don't know. Juli.
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