Re: unused swap offset / bad page map.

[Dave Jones <davej@redhat.com>]

On Mon, Aug 26, 2013 at 03:08:45PM -0700, Hugh Dickins wrote:
 
 > > That said, google does find "swap_free: Unused swap offset entry"
 > > reports from over the years. Most of them seem to be single-bit
 > > errors, though (ie when the entry is 00000100 or similar I'm more
 > > inclined to blame a bit error
 > 
 > Yes, historically they have usually represented either single-bit
 > errors, or corruption of page tables by other kernel data.  The
 > swap subsystem discovers it, but it's rarely an error of swap.
 
Just to rule out bad hardware, I've seen this on two systems
(admittedly the exact same spec, but still..)

 > So I don't care for Dave's suggestion much earlier in this thread,
 > that swapoff should fail with -EINVAL if there has been a bad page
 > taint: that doesn't necessarily interfere with swapoff at all.
 > 
 > And besides, swapoff is killable: yes, if counts go wrong, it
 > can cycle around endlessly, but it checks for signal_pending()
 > each time around the loop.

It might be killable, but if I've done /sbin/reboot, and the
kernel dies in sys_swapoff because of the corruption, I won't
get a chance to kill it, because at that point the shutdown process
has killed my shell, sshd, and just about everything else.
It mieans a grumpy walk to the other side of the house to prod a
reset button.  So yeah, it might not be a mergable thing, but
at least while bisecting it's pretty much a must-have.

 > I just did a quick diff of 3.11-rc7/mm against 3.10, and here's
 > a line in mremap which worries me.  That set_pte_at() is operating
 > on anything that isn't pte_none(), so the pte_mksoft_dirty() looks
 > prone to corrupt a swap entry.
 > 
 > I've not tried matching up bits with Dave's reports, and just going
 > into a meeting now, but this patch looks worth a try: probably Cyrill
 > can improve it meanwhile to what he actually wants there (I'm
 > surprised anything special is needed for just moving a pte).
 > 
 > Hugh
 > 
 > --- 3.11-rc7/mm/mremap.c	2013-07-14 17:10:16.640003652 -0700
 > +++ linux/mm/mremap.c	2013-08-26 14:46:14.460027627 -0700
 > @@ -126,7 +126,7 @@ static void move_ptes(struct vm_area_str
 >  			continue;
 >  		pte = ptep_get_and_clear(mm, old_addr, old_pte);
 >  		pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
 > -		set_pte_at(mm, new_addr, new_pte, pte_mksoft_dirty(pte));
 > +		set_pte_at(mm, new_addr, new_pte, pte);
 >  	}

I'll give this a shot once I'm done with the bisect.

	Dave

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