Re: [rfc][patch] rmap: more sanity checks

[Nick Piggin <npiggin@suse.de>]

On Wed, Feb 14, 2007 at 05:50:47PM +0100, Andrea Arcangeli wrote:
> Hi Nick,
> 
> On Wed, Feb 14, 2007 at 10:04:25AM +0100, Nick Piggin wrote:
> > It would be nice to get some of these checks back into mainline, IMO. I
> 
> Obviously seconded. It's a bit ironic that my original implementation
> was effectively safer that what was further "sanitized" and pushed
> into mainline 8). (of course mainline over the dozen releases was
> significantly improved in the locking etc.etc.. I don't mean that, but
> as far as safety goes it clearly still lacks)

Yeah it is suboptimal to have mainline and possibly some other distros
missing out and getting silent corruption.

> This isn't the first time that we catch subtle VM bugs in sles9 that
> aren't reproducible in mainline (but that affects mainline too). I
> tried a few times to complain about the removal of my bugchecks
> (notably I recall the ones in do_no_page):
> 
> #ifndef CONFIG_DISCONTIGMEM
> 	/* this check is unreliable with numa enabled */
> 	BUG_ON(!pfn_valid(page_to_pfn(new_page)));
> #endif
> 	pageable = !PageReserved(new_page);
> 	as = !!new_page->mapping;
> 
> 	BUG_ON(!pageable && as);
> 
> 	pageable &= as;
> 
> 	/* ->nopage cannot return swapcache */
> 	BUG_ON(PageSwapCache(new_page));
> 	/* ->nopage cannot return anonymous pages */
> 	BUG_ON(PageAnon(new_page));
> 
> compare that with mainline...
> 
> I hope this incident will be enough to resurrect some of the
> "sanitized" bugchecks.

Well I added VM_BUG_ON so that we could keep the checks even just for
the value of commenting the code, and those who want to run without
them can choose to. However I may have put too much stuff under
CONFIG_DEBUG_VM which might discourage distros from turning it on.
I'll work on that...

> > wonder if I'm correct in thinking that checking the page index and mapping
> > is not actually racy?
> 
> Index and mapping shouldn't change if the page is locked, and you
> already added the BUG_ON(!PageLocked(page)).

I believe that's the case, yes.

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