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Change the main comment block to indicate why PMAP_NEW (specifically,

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pmap_kenter*()) is not required for {O,A}->K page loans.
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thorpej 1999-05-27 21:50:03 +00:00
parent ee5a52e9e7
commit 5920638afe
1 changed files with 12 additions and 7 deletions
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19
sys/uvm/uvm_loan.c
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@ -1,4 +1,4 @@
/* $NetBSD: uvm_loan.c,v 1.15 1999/04/11 04:04:11 chs Exp $ */
/* $NetBSD: uvm_loan.c,v 1.16 1999/05/27 21:50:03 thorpej Exp $ */
/*
*
@ -59,7 +59,7 @@
*
* there are 3 types of loans possible:
* O->K uvm_object page to wired kernel page (e.g. mbuf data area)
* A->K anon page to kernel wired kernel page (e.g. mbuf data area)
* A->K anon page to wired kernel page (e.g. mbuf data area)
* O->A uvm_object to anon loan (e.g. vnode page to an anon)
* note that it possible to have an O page loaned to both an A and K
* at the same time.
@ -68,10 +68,15 @@
* a uvm_object and a vm_anon, but PQ_ANON will not be set. this sort
* of page is considered "owned" by the uvm_object (not the anon).
*
* each loan of a page to a wired kernel page bumps the pg->wire_count.
* wired kernel mappings should be entered with pmap_kenter functions
* so that pmap_page_protect() will not affect the kernel mappings.
* (this requires the PMAP_NEW interface...).
* each loan of a page to the kernel bumps the pg->wire_count. the
* kernel mappings for these pages will be read-only and wired. since
* the page will also be wired, it will not be a candidate for pageout,
* and thus will never be pmap_page_protect()'d with VM_PROT_NONE. a
* write fault in the kernel to one of these pages will not cause
* copy-on-write. instead, the page fault is considered fatal. this
* is because the kernel mapping will have no way to look up the
* object/anon which the page is owned by. this is a good side-effect,
* since a kernel write to a loaned page is an error.
*
* owners that want to free their pages and discover that they are
* loaned out simply "disown" them (the page becomes an orphan). these
@ -96,7 +101,7 @@
*
* note that loaning a page causes all mappings of the page to become
* read-only (via pmap_page_protect). this could have an unexpected
* effect on normal "wired" pages if one is not careful.
* effect on normal "wired" pages if one is not careful (XXX).
*/
/*