Date: Tue, 06 Oct 1998 15:00:21 +0200 From: Stefan Eggers <seggers@semyam.dinoco.de> To: freebsd-current@FreeBSD.ORG Cc: seggers@semyam.dinoco.de Subject: Text: Locking in the kernel's VM subsystem Message-ID: <199810061300.PAA05872@semyam.dinoco.de>
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Surely someone knows more than I do about this but noone seems to have
written a few bits about locking in the VM subsystem, yet. ;-) So
this is a first and surely in many places incorrect description of how
it is supposed to work. Comments and corrections welcome.
Stefan.
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Locking in the VM subsystem
In the following text o denotes a VM object, p is a VM page.
splvm()/splx()
Similar to the other spl...() calls.
o->paging_in_progress
Indicator for paging going on on pages within this object.
p->busy/vm_page_io_start()/vm_page_io_finish()
vm_page_io_start() marks a page as busy for I/O use - usually this is
happening due to filesystem accesses. The reverse operation is
vm_page_io_finish() which also does the wakeup of processes waiting
for the page.
NOTE: This is different from setting the busy flag! You have to check
both if you want to make sure a page is really non-busy.
p->flags & PG_BUSY/vm_page_busy()/vm_page_wakeup()
This marks a page as busy for some operation. Using the functions
ensures the operation of setting/clearing the flags is atomar and also
does a wakeup of processes waiting for this page to become non-busy
when necessary.
NOTE: This is different from setting the busy counter! You have to
check both if you want to make sure a page is really non-busy.
p->hold_count/vm_page_hold()/vm_page_unhold()
Used (if non-zero) to indicate someone has a reference to this page.
One better doesn't make this reference invalid in any way.
p->wire_count
Wired pages are those which don't get paged anymore. The are also
fixed at their position. Useful for making something unmoveable
during I/O for example.
vm_page_flag_set()/vm_page_flag_clear()
Generic flag set/clear operation used for example by vm_page_busy()/
vm_page_wakeup(). Ensures that the operation is atomic.
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