[PATCH v2] swap: warn when a swap area overflows the maximum size

[Raymond Jennings]

Turned the comparison around for clarity of "bigger than"

No semantic changes, if it still compiles it should do the same thing so
I've omitted the testing this time.  Will be happy to retest if required
but I'm on an atom 330 and kernel rebuilds are a nightmare.

----

swap: warn when a swap area overflows the maximum size

It is possible to swapon a swap area that is too big for the pte width
to handle.

Presently this failure happens silently.

Instead, emit a diagnostic to warn the user.

Signed-off-by: Raymond Jennings <shentino@xxxxxxxxx>
Acked-by: Valdis Kletnieks <valdis.kletnieks@xxxxxx>

----

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 36af6ee..5a4ce53 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1953,6 +1953,12 @@ static unsigned long read_swap_header(struct
swap_info_struct *p,
         */
        maxpages = swp_offset(pte_to_swp_entry(
                        swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
+       if (swap_header->info.last_page > maxpages) {
+               printk(KERN_WARNING
+                      "Truncating oversized swap area, only using %luk
out of %luk
\n",
+                      maxpages << (PAGE_SHIFT - 10),
+                      swap_header->info.last_page << (PAGE_SHIFT -
10));
+       }
        if (maxpages > swap_header->info.last_page) {
                maxpages = swap_header->info.last_page + 1;
                /* p->max is an unsigned int: don't overflow it */

----

Testing results, root prompt commands and kernel log messages:

# lvresize /dev/system/swap --size 16G
# mkswap /dev/system/swap
# swapon /dev/system/swap

Jul  7 04:27:22 warfang kernel: Adding 16777212k swap
on /dev/mapper/system-swap.  Priority:-1 extents:1 across:16777212k 

# lvresize /dev/system/swap --size 16G
# mkswap /dev/system/swap
# swapon /dev/system/swap

Jul  7 04:27:22 warfang kernel: Truncating oversized swap area, only
using 33554432k out of 67108860k
Jul  7 04:27:22 warfang kernel: Adding 33554428k swap
on /dev/mapper/system-swap.  Priority:-1 extents:1 across:33554428k 


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