Re: 2.6.24-rc8-mm1: powerpc oopses

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 17 Jan 2008 17:39:54 -0600
Matt Mackall <mpm@selenic.com> wrote:

> 
> On Thu, 2008-01-17 at 14:51 -0800, Andrew Morton wrote:
> > > proc_loop: /proc/3731/task/3731/pagemap
> > > kernel: BUG: sleeping function called from invalid context at fs/proc/task_mmu.c:554
> > > kernel: in_atomic():1, irqs_disabled():0
> > > kernel: Call Trace:
> > > kernel: [cf1cddf0] [c000840c] show_stack+0x3c/0x194 (unreliable)
> > > kernel: [cf1cde20] [c002b2ec] __might_sleep+0xf4/0x108
> > > kernel: [cf1cde30] [c00d2d54] add_to_pagemap+0x40/0x11c
> > > kernel: [cf1cde50] [c00d2f44] pagemap_pte_range+0xa8/0x10c
> > > kernel: [cf1cde70] [c0081b30] walk_page_range+0x148/0x23c
> > > kernel: [cf1cdeb0] [c00d3104] pagemap_read+0x15c/0x244
> > > kernel: [cf1cdef0] [c0092144] vfs_read+0xc4/0x16c
> > > kernel: [cf1cdf10] [c009261c] sys_read+0x4c/0x90
> > > kernel: [cf1cdf40] [c001328c] ret_from_syscall+0x0/0x40
> > 
> > It's not really an oops - it's a warning.  add_to_pagemap() is doing a
> > put_user() inside pagemap_pte_range->pte_offset_map->kmap_atomic.
> > 
> > A known bug, I'm afraid.
> > 
> > How to fix?
> > 
> > - double-buffer the data to be copied to userspace or
> > 
> > - take a local copy of the pte page then work on that instead or
> > 
> > - play copy_to_user_inatomic() tricks.
> 
> Hmm, this fell off my radar. How about something like this as a minimal
> fix (untested as -mm is a complete doorstop for me at the moment)?
> 
> diff -r 5595adaea70f fs/proc/task_mmu.c
> --- a/fs/proc/task_mmu.c	Thu Jan 17 13:26:54 2008 -0600
> +++ b/fs/proc/task_mmu.c	Thu Jan 17 17:29:21 2008 -0600
> @@ -582,20 +583,26 @@
>  {
>  	struct pagemapread *pm = private;
>  	pte_t *pte;
> -	int err = 0;
> +	int offset = 0, err = 0;
>  
>  	pte = pte_offset_map(pmd, addr);
> -	for (; addr != end; pte++, addr += PAGE_SIZE) {
> +	for (; addr != end; offset++, addr += PAGE_SIZE) {
>  		u64 pfn = PM_NOT_PRESENT;
> -		if (is_swap_pte(*pte))
> -			pfn = swap_pte_to_pagemap_entry(*pte);
> -		else if (pte_present(*pte))
> -			pfn = pte_pfn(*pte);
> +		if (is_swap_pte(pte[offset]))
> +			pfn = swap_pte_to_pagemap_entry(pte[offset]);
> +		else if (pte_present(pte[offset]))
> +			pfn = pte_pfn(pte[offset]);
> +#ifdef CONFIG_HIGHPTE
> +		pte_unmap(pte);
>  		err = add_to_pagemap(addr, pfn, pm);
> +		pte = pte_offset_map(pmd, addr);
> +#else
> +		err = add_to_pagemap(addr, pfn, pm);
> +#endif
>  		if (err)
>  			return err;
>  	}
> -	pte_unmap(pte - 1);
> +	pte_unmap(pte);
>  
>  	cond_resched();
>  

Good point, it really can be taht simple.

Do we need the ifdef?  pte_offset_map/pte_unmap should be super-cheap on
!CONFIG_HIGHPTE builds.