Re: panic: Lock vm object not exclusively locked _at_ /usr/src/sys/vm/vm_page.c:2637

On 04/05/2015 10:47, Gleb Smirnoff wrote:
> On Sun, Apr 05, 2015 at 06:37:58AM -0700, David Wolfskill wrote:
> D> It ocurred rather late in the transition to multi-user mode, but
> D> prior to starting xdm (on my laptop).
> D> 
> D> Previous (working) head/i386 for this machine was r281074.
> D> 
> D> Here's the first bit of the crashinfo (yes, I have a crash dump):
> D> 
> D> g1-254.catwhisker.org dumped core - see /var/crash/vmcore.3
> D> 
> D> Sun Apr  5 06:18:44 PDT 2015
> D> 
> D> FreeBSD g1-254.catwhisker.org 11.0-CURRENT FreeBSD 11.0-CURRENT #1561  r281106M/281106:1100067: Sun Apr  5 06:01:06 PDT 2015     root_at_g1-254.catwhisker.org:/common/S4/obj/usr/src/sys/CANARY  i386
> D> 
> D> panic: Lock vm object not exclusively locked _at_ /usr/src/sys/vm/vm_page.c:2637
>
> This is r281079.
>
> Since vm_page_advise() may call vm_page_dirty() in the MADV_DONTNEED case,
> the assertion is valid. So, looks like vm_fault_dontneed() needs W-lock on
> the first_object.
>

Actually, what I forgot was that vm_page_advise(MADV_FREE) clears the
page's dirty field, and that is why an exclusive lock is asserted.  As
explained in vm_page.h, the pmap is allowed to set the dirty field to
all ones without any locking.  Moreover, the new "fast" path in
vm_fault() sets the dirty field with only a read lock held. 
vm_page_advise(MADV_DONTNEED) isn't really any different from the fast path.

Need to think a bit ...