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    SubjectRe: RFC: patch for suspected shm swap problem
    Date
    > 
    > Hi,
    >
    > On Thu, 22 Apr 1999 09:03:14 -0700 (PDT), kanoj@google.engr.sgi.com
    > (Kanoj Sarcar) said:
    >
    > > I suspect the problem is that shm_swap bumps up swap_id to more
    > > than max_shmid under some circumstances, leading the next call
    > > to shm_swap to trip. Note that valid values of max_shmid are 0 ..
    > > SHMMNI - 1, but shm_swap can leave swap_id set to SHMMNI.
    >
    > > MMU code maintainers, could you please review the patch and let
    > > me know whether it is good
    >
    > Agreed: the patch looks correct.
    >
    > In particular, with the patch in place we are still protected against
    > having max_shmid shrink between calls to shm_swap(): the worst that can
    > happen is that we find an unused shm_seg which will get caught by the
    > IPC_UNUSED test near the top of shm_swap. Only if the shm table is
    > full, and so the swap_id overflow forces the next shm_segs[swap_id] to
    > point to an entry not preinitialised to IPC_UNUSED or IPC_NOID, will
    > there be a danger, and the patch makes sure that we never overstep that
    > bound. (This probably explains why we haven't seen the problem before:
    > were you allocating the maximum number of shm segments during the stress
    > test?)

    Yes ... my benchmark is so resource hungry that I had to end up also
    bumping _SHM_ID_BITS to increase the number of shm segs, but that's
    irrelevant.

    Thanks.

    Kanoj

    >
    > --Stephen
    >
    > ----------------------------------------------------------------
    > From: kanoj@google.engr.sgi.com (Kanoj Sarcar)
    > Sender: owner-linux-kernel@vger.rutgers.edu
    > To: Linux-MM@kvack.org, linux-kernel@vger.rutgers.edu
    > Subject: RFC: patch for suspected shm swap problem
    > Date: Thu, 22 Apr 1999 09:03:14 -0700 (PDT)
    >
    > Hi,
    >
    > While running some heavy stress on shm code, I took a panic in
    > shm_swap coming out of do_try_to_free_pages in the context of
    > non-kswapd processes. From the register display, I suspect the
    > problem to be fixed by this patch:
    >
    > --- /usr/tmp/p_rdiff_a000PE/shm.c Tue Apr 20 16:07:02 1999
    > +++ kern/ipc/shm.c Tue Apr 20 16:05:54 1999
    > @@ -716,10 +716,10 @@
    > next_id:
    > swap_idx = 0;
    > if (++swap_id > max_shmid) {
    > + swap_id = 0;
    > if (loop)
    > goto failed;
    > loop = 1;
    > - swap_id = 0;
    > }
    > goto check_id;
    > }
    > --
    > To unsubscribe, send a message with 'unsubscribe linux-mm my@address'
    > in the body to majordomo@kvack.org. For more info on Linux MM,
    > see: http://humbolt.geo.uu.nl/Linux-MM/
    >


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