Re: 2.6.17-rc5-mm1

[Hugh Dickins <hugh@veritas.com>]

On Tue, 6 Jun 2006, Martin Schwidefsky wrote:
> On Tue, 2006-06-06 at 18:59 +0100, Hugh Dickins wrote:
> > Even s390 is okay, isn't it?  MartinS's swp_type returns the highest
> > type, 31, corresponding to 32 types, as on every other architecture.
> > You and I and Martin Bligh would prefer this patch...
> 
> As long as nobody will have the smart idea to increase MAX_SWAPFILES to
> more than 32 I'm fine with it.

Thanks, Martin: we know very well that we cannot increase MAX_SWAPFILES
without going through all the architectures adjusting their swap ptes
(and maybe making MAX_SWAPFILES_SHIFT per-arch), so I don't see any
problem there.  But Christoph has observed that the comment becomes
out-of-date, so updated patch (adding your ack) below...


Remove unnecessary obfuscation from sys_swapon's range check on swap
type, which blew up causing memory corruption once swapless migration
made MAX_SWAPFILES no longer 2 ^ MAX_SWAPFILES_SHIFT.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

--- 2.6.17-rc5-mm3/mm/swapfile.c	2006-06-04 11:52:47.000000000 +0100
+++ linux/mm/swapfile.c	2006-06-06 19:08:51.000000000 +0100
@@ -1392,19 +1392,7 @@ asmlinkage long sys_swapon(const char __
 		if (!(p->flags & SWP_USED))
 			break;
 	error = -EPERM;
-	/*
-	 * Test if adding another swap device is possible. There are
-	 * two limiting factors: 1) the number of bits for the swap
-	 * type swp_entry_t definition and 2) the number of bits for
-	 * the swap type in the swap ptes as defined by the different
-	 * architectures. To honor both limitations a swap entry
-	 * with swap offset 0 and swap type ~0UL is created, encoded
-	 * to a swap pte, decoded to a swp_entry_t again and finally
-	 * the swap type part is extracted. This will mask all bits
-	 * from the initial ~0UL that can't be encoded in either the
-	 * swp_entry_t or the architecture definition of a swap pte.
-	 */
-	if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {
+	if (type >= MAX_SWAPFILES) {
 		spin_unlock(&swap_lock);
 		goto out;
 	}
-
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