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Date:      Sun, 20 Dec 1998 15:41:59 -0800 (PST)
From:      Julian Elischer <julian@whistle.com>
To:        zhihuizhang <bf20761@binghamton.edu>
Cc:        hackers <freebsd-hackers@FreeBSD.ORG>
Subject:   Re: questions/problems with vm_fault() in Stable
Message-ID:  <Pine.BSF.3.95.981220154047.8357P-100000@current1.whistle.com>
In-Reply-To: <Pine.SOL.L3.93.981219152554.6869B-100000@bingsun2>

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Are you commenting the source as you go?
Are yu writing a document on these things?
you seem to be going through the entire kernel :-)

If so. can we get a copy :-)

julian



On Sat, 19 Dec 1998, zhihuizhang wrote:

> 
> Hi,
> 
> I have some questions about the routine vm_fault() in the file vm_fault.c:
> 
> (1) The condition (!change_wiring || wired) is always TRUE!  Three possible
> values of change_wiring are FALSE (0), VM_FAULT_CHANGE_WIRING (1), and
> VM_FAULT_USER_WIRE (2).  When its value is VM_FAULT_CHANGE_WIRING or
> VM_FAULT_USER_WIRE, the wired count of the map entry has already been
> incremented (see vm_map_user_pageable() and vm_map_pageable()), so
> vm_map_lookup() will set wired as non-zero.  If the argment's value is
> FALSE, the condition is trivially true.
> 
> (2) Following the label readrest: in the source code, there are some codes
> trying to handle read ahead for sequential objects.  However, the following
> statement is wrong:
> 
> for (tmppindex = first_index - 1; tmppindex >=first_pindex; -- tmppindex)
> 
> We should probably use firstpindex instead of first_pindex in the comparison
> (pay attention to the underscore here).  However, the for loop will not loop
> forever, because vm_page_lookup() called within the loop will return NULL
> anyway.
> 
> (3) If the pager fails to bring in a page for the very first object in the
> shadow chain, the page will contain invalid content.  If the pager fails to
> bring in a page for other objects in the shadow chain, the page is freed
> and invalid.  Yet, we still reference to that page (m) later.  This means
> that after the page fault, we could get a page with invalid contents.
> There is a XXX near the related comment in the source code.
> 
> (4) The comment in the source code says that the pager can move a page, so
> we must relookup the page by calling vm_page_lookup().  How could this be
> the case?  Why move the page?
> 
> (5) The comment in the source code says that we do not COW read-only region
> on a user wire.   "If we do not make this restriction, the bookkeeping would
> be nearly impossible."  Can anyone explain this for me?
> 
> (6) The comment in the source code says map entries may be pageable.  I really
> doubt this because vm_map_entry_create() always allocates wired down memory
> for new map entries and enter them into pmap immediately.
> 
> (7) The comment in the source code says pmap_enter() may cause other faults.
> I can not see any reason for this to happen.
> 
> I am just wondering how this important routine in VM system can have these
> imperfect things (except (4) through (7)).  I hope I am wrong.  Please help
> me out with understanding of these points.
> 
> Any help is appreciated.
> 
> -------------------------------------------------- 
> | Zhihui Zhang, http://cs.binghamton.edu/~zzhang |
> | Dept. of Computer Science,  SUNY at Binghamton |
> --------------------------------------------------
> 
> 
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