Re: unused swap offset / bad page map.

[Hugh Dickins <hughd@google.com>]

On Mon, 26 Aug 2013, Linus Torvalds wrote:
> On Mon, Aug 26, 2013 at 1:15 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > So I'm almost likely to think that we are more likely to have
> > something wrong in the messy magical special cases.
> 
> Of course, the good news would be if it actually ends up being the
> soft-dirty stuff, and bisection hits something recent.

I suspect so.

> 
> So maybe I'm overly pessimistic. That messy swap_map[] code really
> _is_ messy, but at the same time it should also be pretty well-tested.
> I don't think it's been touched in years.

Blame me for the byte-instead-of-short continuation stuff.
But it's never yet shown any problem (okay, perhaps that's
because it's so rare to need any continuation anyway).

> 
> 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.

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.

> - in contrast your values look like "real" swap entries).

Indeed they do.

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);
 	}
 
 	arch_leave_lazy_mmu_mode();

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