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SubjectRe: [PATCH] swapfile: avoid NULL pointer dereference in swapon when s_bdev is NULL
Hugh Dickins wrote:
> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
>> Hugh Dickins wrote:
>>> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
>>>> The call sequence is add_to_swap() -> get_swap_page() -> scan_swap_map()
>>>> -> discard_swap_cluster() -> blkdev_issue_discard().
>>>>
>>>> Wrapping the code around a NULL check fixes the Oops for me.
>>> That's odd: scan_swap_map() should only discard_swap_cluster() if
>>> SWP_DISCARDABLE got set, and your first patch made sure that it wasn't.
>> I forgot to mention, this is not on loopback NFS mount but an remote NFS
>> mount (so possibly s_bdev is not NULL) when doing swapon. The oops was
>> triggered when memhog program tries to use the swap space on the newly
>> created swapfile on NFS. I have not completely investigated the issue,
>> perhaps s_bdev is not being set when it ought to be..
>
> I'm happy to see your first patch already in 2.6.32-rc3, but still
> suspicious of this second patch you sent afterwards. A quick skim
> through your patchset suggests 23/31 is probably responsible:

Thanks for skimming through those patches. I noticed this already during
my review and thought I had already fixed this, but apparently I missed
out. I'll test after fixing this and report.

> --- mmotm.orig/include/linux/swap.h
> +++ mmotm/include/linux/swap.h
> @@ -120,6 +120,7 @@ struct swap_extent {
> enum {
> SWP_USED = (1 << 0), /* is slot in swap_info[] used? */
> SWP_WRITEOK = (1 << 1), /* ok to write to this swap? */
> + SWP_FILE = (1 << 2), /* file swap area */
> SWP_DISCARDABLE = (1 << 2), /* blkdev supports discard */
> SWP_DISCARDING = (1 << 3), /* now discarding a free cluster */
> SWP_SOLIDSTATE = (1 << 4), /* blkdev seeks are cheap */
>
> Hugh

Thanks,

--
Suresh Jayaraman


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