Re: [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas

From: Nick Piggin
Date: Fri Feb 16 2007 - 02:49:22 EST

Andrew Morton wrote:
On Thu, 15 Feb 2007 23:08:02 -0800 Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

Andrew Morton wrote:

This won't work when CONFIG_PREEMPT=y. The pagefault handler will see
in_atomic() and will scram.

Is there some other way to get the pagetable populated for the address

If you really need to run atomically, that gets ugly. Even of one were to
run handle_mm_fault() by hand, it still needs to allocate memory.

Two ugly options might be:

a) touch all the pages, then go atomic, then touch them all again. If
one of them faults (ie: you raced with swapout) then go back and try
again. Obviously susceptible to livelocking.

b) Do get_user_pages() against all the pages, then go atomic, then do
put_page() against them all. Of course, they can immediately get
swapped out.

But that function's already racy against swapout and I guess it works OK. I don't have clue what it is actually trying to do, so I'm guessing madly

Well its only operating on kernel pages, and against a vmalloc region.
So it would be immune to any sort of unmapping or swapout.

Andrew's option a) should work. What's this for, and how is it not Xen
specific if nothing else in the tree needs such a weird hack?

SUSE Labs, Novell Inc.
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