Re: uvm page freelist problem

[Matthias Drochner <M.Drochner%fz-juelich.de@localhost>]

hpeyerl%beer.org@localhost said:
> What happens is a freelist TAILQ pointer becomes corrupted

Could this be explained by a page getting onto a freelist twice?

What I checked in the meantime leaves that as interpretation:
I've moved the mutex_enter before the PG_FREE check as
said in my other mail, and I've never seen that pglistalloc
panic again.
As I understand everyone who changes PG_FREE gets the page
off the freelist before, and the freelist removal/insertion
is always protected by the mutex afaics.
The my patch to move the mutex_enter does likely not
fix the real problem. So what is left as a possibility is
that a page is still on a freelist while uvm_pagefree
deals with it; that way eg pageidlezero could set the
PG_ZERO flag in that time window my change closed.

I've instrumented uvm_pagefree and pglistfree to check
whether the page is already on one of the two freelists
(with and without PG_ZERO), doing a linear search,
but didn't get a hit. The linear search is too expensive
to have it built in for longer use, and it changes
runtime behaviour. So I might have just been unlucky.

best regards
Matthias




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