Re: 2.6.22 -mm merge plans: mm-more-rmap-checking

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Hugh Dickins wrote:
On Mon, 30 Apr 2007, Andrew Morton wrote:

... mm-more-rmap-checking.patch
...

Misc MM things.  Will merge.

Would Nick mind very much if I ask you to drop this one?
You did CC me ages ago, but I've only just run across it.
It's a small matter, but I'd prefer it dropped for now.
I guess I would prefer it to go under CONFIG_DEBUG_VM. Speaking
of which, it would be nice to be able to turn that on unconditionally
in -rc1. Although I may have put a few too many things under it, so
it might slow down too much...


Re-introduce rmap verification patches that Hugh removed when he removed
PG_map_lock. PG_map_lock actually isn't needed to synchronise access to
anonymous pages, because PG_locked and PTL together already do.

These checks were important in discovering and fixing a rare rmap corruption
in SLES9.

It introduces some silly checks which were never in mainline,
nor so far as I can tell in SLES9: I'm thinking of those
+	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
Yes, but IIRC I put that in because there was another check in
SLES9 that I actually couldn't put in, but used this one instead
because it also caught the bug we saw.


There are few callsites for these rmap functions, I don't think
they need to be checking their arguments in that way.

It also changes the inline page_dup_rmap (a single atomic increment)
into a bugchecking out-of-line function: do we really want to slow
down fork in that way, for 2.6.22 to fix a rare corruption in SLES9?
This was actually a rare corruption that is also in 2.6.21, and
as few rmap callsites as we have, it was never noticed until the
SLES9 bug check was triggered.


What I really like about the patch is Nick's observation that my
	/* else checking page index and mapping is racy */
is no longer true: a change we made to the do_swap_page sequence
some while ago has indeed cured that raciness, and I'm happy to
reintroduce the check on mapping and index in page_add_anon_rmap,
and his BUG_ON(!PageLocked(page)) there (despite BUG_ONs falling
out of fashion very recently).
Hmm, I didn't notice the do_swap_page change, rather just derived
its safety by looking at the current state of the code (which I
guess must have been post-do_swap_page change)...

Do you have a pointer to the patch, for my interest?

--
SUSE Labs, Novell Inc.
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