Messages in this thread | | | Date | Sat, 2 May 2009 20:13:12 +0200 | From | Lars Ellenberg <> | Subject | Re: [PATCH 02/16] DRBD: lru_cache |
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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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