Re: mapped page in prep_new_page()..

[Linus Torvalds <torvalds@osdl.org>]

On Thu, 26 Feb 2004, Andrew Morton wrote:
> Linus Torvalds <torvalds@osdl.org> wrote:
> >
> > Hmm.. I've never seen this before myself, but I know there have been
> > similar reports.
> 
> There have been a few.  I don't recall seeing any against x86.

Yeah, I wouldn't be surprised if it is an architecture bug (possibly 
one that has been common but has long been fixed on x86).

> > Earlier today I got
> > 
> > 	Bad page state at prep_new_page
> > 	flags:0x00000000 mapping:0000000000000000 mapped:1 count:0
> 
> But you did not get a trace for a mapped page being freed up prior to this?

That's correct.

> Yes, I don't think we can sanely fix all these conditions.  If we really
> want to keep limping along we should just leak the page in
> __free_pages_ok(), and leak the page then pick a new one in
> __alloc_pages().  This shouldn't be worth the effort, of course.

I agree - it's only worth doing if it is simple. In this case it would 
have been simple to just refuse to add the bad page back to the free list.

> > 	Oops: Kernel access of bad area, sig: 11 [#1]
> > 	SMP NR_CPUS=2 
> > 	NIP: C00000000008D7C4 XER: 0000000020000000 LR: C000000000086F70
> > 	REGS: c00000007a43b7f0 TRAP: 0300    Not tainted
> > 	MSR: 9000000000009032 EE: 1 PR: 0 FP: 0 ME: 1 IR/DR: 11
> > 	DAR: 0000005f00000008, DSISR: 0000000040000000
> > 	TASK: c000000059819b20[8510] 'bk' THREAD: c00000007a438000 CPU: 0
> > 	GPR00: 0000000000000000 C00000007A43BA70 C0000000006AD0D0 C000000000FFFFC0 
> > 	GPR04: C00000002CBC30F0 C000000032F2F200 C000000002FD64D0 C0000000004D8050 
> > 	GPR08: 0000000002AFE480 0000000000000000 0000005F00000000 0000000000000004 
> > 	GPR12: 0000000042008488 C0000000004E0000 0000000002000000 0000000011A1E004 
> > 	GPR16: C00000005EC23400 0000000000000050 C000000054447000 4000000000000000 
> > 	GPR20: C0000000005714C8 C0000000006F6B80 0000000000001580 C000000032F2F200 
> > 	GPR24: 0000000000532000 0000000000000532 C00000000072FFB8 C000000000FFFFC0 
> > 	GPR28: CCCCCCCCCCCCCCCD 00000001A88C0397 C000000000586978 C00000002CBC30F0 
> > 	NIP [c00000000008d7c4] .page_add_rmap+0xb4/0x1b4
> > 	LR [c000000000086f70] .do_anonymous_page+0x314/0x50c
> > 	Call Trace:
> > 	[c000000000087204] .do_no_page+0x9c/0x570
> > 	[c0000000000879b0] .handle_mm_fault+0x1b0/0x26c
> > 	[c0000000000431c8] .do_page_fault+0x120/0x3f8
> > 	[c00000000000aa94] stab_bolted_user_return+0x118/0x11c
> 
> So what is the access address here?  That will tell us what value was in
> page.pte.chain.

Heh. I've had this G5 thing for a couple of weeks, I'm not very good at 
reading the oops dump either ;)

The ppc64 page fault oops thing seems to be braindead, and not even print 
out the address. Stupid. Somebody is too used to debuggers, and as a 
result users aren't helped to make good reports, hint hint..

Anyway, a little digging shows that the thing seems to be the instruction

	.. r3 is "struct page *" ..
	ld      r10,64(r3)	/* r10 is "page->pte.direct" */

	...

	ld      r0,0(r3)	/* r0 is "page->flags */
	rldicl  r0,r0,48,63
	cmpwi   r0,0		/* PageDirect(page) ? */

	... nope, direct bit not set ...

	ld      r0,8(r10)

where r10 (as per above) is 0x0000005F00000000.  So the fault address
would have been 0x0000005F00000008.

The value of r3 is interesting: C000000000FFFFC0. That's _just_ under the 
16MB mark, and the offset of the "page->pte.direct" access is 64 bytes. 
Which means that the corrupted data was at _exactly_ the 16MB mark.

Now, I have no idea why, but it's an interesting - if slightly odd -
detail.

Who would write the value quadword 0x0000005F00000000 to the physical
address 1<<24? And is that a valid "struct page *" in the first place? 
Probably. 

Bad pointer crapola? Or some subtle CPU bug with address arithmetic that
crosses the 16MB border?  Anton, BenH, any ideas?

		Linus
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