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    SubjectRe: [PATCH 02/16] DRBD: lru_cache
    On Sat, May 02, 2009 at 10:58:23AM -0700, Andrew Morton wrote:
    > On Sat, 2 May 2009 17:26:20 +0200 Lars Ellenberg <lars.ellenberg@linbit.com> wrote:
    >
    > > in memory structure is
    > >
    > > struct lru_cache {
    > > struct list_head active;
    > > struct list_head quiet;
    > > struct list_head free;
    > > size_t element_size; <-- parameter to "lc_alloc"
    > > unsigned int nr_elements; <-- parameter to "lc_alloc"
    > > unsigned int new_number;
    > >
    > > unsigned int used;
    > > unsigned long flags;
    > > unsigned long hits, misses, starving, dirty, changed;
    > >
    > > struct lc_element *changing_element; /* just for paranoia */
    > >
    > > const char *name;
    > >
    > > struct hlist_head slot[0];
    > > /* hash colision chains here, then element storage. */
    > > };
    > >
    > > so we have fixed size list heads,
    > > size of a single such "element", to allow the user
    > > to add small payload;
    > > number of hash slots and "elements" following this header;
    > > some counters;
    > > hlist_slot[0];
    > > }
    > > following:
    > > struct hlist_head[nr_elements];
    > > array of element_size blobs[nr_elements];
    > >
    > > these "blobs" start with the struct lru_element,
    > > possibly followed by some user payload.
    > >
    > > the "index" you are asking about later is
    > > index into that "blob" array,
    > > and is used primarily to initialize the state of this thing
    > > from an on-disk representation (the "activity log", "AL"),
    > > for crash recovery purposes.
    > >
    > > the typecasting is necessary to get from the slot[0] to the "elements"
    > > skipping the hash slots.
    > > using "container of" or something like that would obscure the fact that,
    > > as currently implemented, the "lru_element" _must_ be the first member
    > > of any payload structure.
    >
    > I still don't see why the lru_element must be the first member of the
    > user's outer, containing structure.


    ok, arguably one could also record the offset_of beneath the element_size,
    and add that in when doing the lc_element *e = blob[index] + offset.
    would not make it much more appealing, though.

    Lars


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