Re: [PATCH 02/16] DRBD: lru_cache

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 2 May 2009 20:13:12 +0200 Lars Ellenberg <lars.ellenberg@linbit.com> wrote:

> 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.
> 

You appear to believe that I understood the relevance of all the above
text.  I didn't ;)

Let's start again.

Why can't I do

struct foo {
	int x;
	struct lc_element lc;
	..
};

and then use the lru library code to handle my foo objects?