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    SubjectRe: [patch 3/3] ipc: increase IPCMNI_MAX
    On Fri, May 21, 2010 at 01:31:36PM -0700, Andrew Morton wrote:
    > On Thu, 20 May 2010 17:07:41 +1000
    > Nick Piggin <npiggin@suse.de> wrote:
    >
    > > Just wondering whether there is a good reason to have a full 16 bits of
    > > sequence in ipc ids? 32K indexes is pretty easy to overflow, if only in
    > > stress tests for now. I was doing some big aim7 stress testing, which
    > > required this patch, but it's not exactly a realistic workload :)
    > >
    > > But the sequence seems like it just helps slightly with buggy apps, and
    > > if the app is buggy then it can by definition mess up its own ids
    > > anyway? So I don't see that such amount of seq is required.
    > >
    > > Index: linux-2.6/ipc/util.h
    > > ===================================================================
    > > --- linux-2.6.orig/ipc/util.h
    > > +++ linux-2.6/ipc/util.h
    > > @@ -14,7 +14,16 @@
    > > #include <linux/err.h>
    > >
    > > /* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */
    > > -#define IPCMNI_MAX_SHIFT 15
    > > +/*
    > > + * IPC ids consist of an index into the idr, which allocates from the bottom
    > > + * up, and a sequence number which is continually incremented. Valid indexes
    > > + * are from 0..IPCMNI_MAX (or further constrained by sysctls or other limits).
    > > + * The sequence number prevents ids from being reused quickly. The sequence
    > > + * number resides in the top part of the 'int' after IPCMNI_MAX.
    > > + *
    > > + * Increasing IPCMNI_MAX reduces the sequence wrap interval.
    > > + */
    > > +#define IPCMNI_MAX_SHIFT 20
    > > #define IPCMNI_MAX (1 << IPCMNI_MAX_SHIFT)
    > >
    > > #define SEQ_SHIFT IPCMNI_MAX_SHIFT
    >
    > Some anaylsis of the worst-case memory consumption would be mollifying.

    OK.

    > I took the absence of Signed-off-by:'s to mean "rfc" and wandered away.

    Yes. They should probably go through Manfred to you anyway.



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