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    SubjectRe: [PATCH] 1/1: Device-Mapper: Remove 1024 devices limitation
    Jim Houston <jim.houston@comcast.net> wrote:
    >
    > With out the test above an id beyond the allocated space will alias
    > to one that exists. Perhaps the highest id currently allocated is
    > 100, there will be two layers in the radix tree and the while loop
    > above will only look at the 10 least significant bits. If you call
    > idr_find with 1025 it will return the pointer associated with id 1.

    OK.

    > The patch I sent was against linux-2.6.7, so I missed the change to
    > MAX_ID_SHIFT.

    How about this?

    diff -puN lib/idr.c~idr-stale-comment lib/idr.c
    --- 25/lib/idr.c~idr-stale-comment Tue Jul 6 16:12:45 2004
    +++ 25-akpm/lib/idr.c Tue Jul 6 16:15:41 2004
    @@ -27,22 +27,6 @@
    * so you don't need to be too concerned about locking and conflicts
    * with the slab allocator.

    - * What you need to do is, since we don't keep the counter as part of
    - * id / ptr pair, to keep a copy of it in the pointed to structure
    - * (or else where) so that when you ask for a ptr you can varify that
    - * the returned ptr is correct by comparing the id it contains with the one
    - * you asked for. In other words, we only did half the reuse protection.
    - * Since the code depends on your code doing this check, we ignore high
    - * order bits in the id, not just the count, but bits that would, if used,
    - * index outside of the allocated ids. In other words, if the largest id
    - * currently allocated is 32 a look up will only look at the low 5 bits of
    - * the id. Since you will want to keep this id in the structure anyway
    - * (if for no other reason than to be able to eliminate the id when the
    - * structure is found in some other way) this seems reasonable. If you
    - * really think otherwise, the code to check these bits here, it is just
    - * disabled with a #if 0.
    -
    -
    * So here are the complete details:

    * include <linux/idr.h>
    @@ -371,15 +355,11 @@ void *idr_find(struct idr *idp, int id)
    struct idr_layer *p;

    n = idp->layers * IDR_BITS;
    + if (id >= (1 << n))
    + return NULL;
    +
    p = idp->top;
    -#if 0
    - /*
    - * This tests to see if bits outside the current tree are
    - * present. If so, tain't one of ours!
    - */
    - if ( unlikely( (id & ~(~0 << MAX_ID_SHIFT)) >> (n + IDR_BITS)))
    - return NULL;
    -#endif
    +
    /* Mask off upper bits we don't use for the search. */
    id &= MAX_ID_MASK;

    _
    -
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