oopses in test10-pre4 (was Re: [RFC] atomic pte updates and pae changes, take 3)

[Linus Torvalds <torvalds@transmeta.com>]

Ben,
 you added these two BUG() conditions in your atomic pte patch:

> diff -ur v2.4.0-test10-pre2/mm/vmscan.c work-10-2/mm/vmscan.c
> --- v2.4.0-test10-pre2/mm/vmscan.c	Fri Oct 13 17:18:37 2000
> +++ work-10-2/mm/vmscan.c	Fri Oct 13 17:19:47 2000
> @@ -99,6 +98,10 @@
>  	if (PageSwapCache(page)) {
>  		entry.val = page->index;
>  		swap_duplicate(entry);
> +		if (pte_dirty(pte))
> +			BUG();
> +		if (pte_write(pte))
> +			BUG();
>  		set_pte(page_table, swp_entry_to_pte(entry));
>  drop_pte:
>  		UnlockPage(page);
> @@ -109,6 +112,13 @@

and people are getting them left and right when they start swapping.

As far as I can tell, the thing you test for is not actually a bug at all.
The pte may be dirty, but that's ok - the swap cache page is obviously
up-to-date, as it's actually physically the very same page.

I think you overlooked the fact that SHM mappings use the page cache, and
it's ok if such pages are dirty and writable - they will get written out
by the shm_swap() logic once there are no mappings active any more.

I like the test per se, because I think it's correct for the "normal"
case of a private page, but I really think those two BUG()'s are not bugs
at all in general, and we should just remove the two tests.

Comments? Anything I've overlooked?

			Linus

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