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SubjectRe: [PATCH] swapfile: avoid NULL pointer dereference in swapon when s_bdev is NULL
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:

--- 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


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