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Date: Sat, 2 May 2009 10:58:23 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Lars Ellenberg <lars.ellenberg@linbit.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>, linux-kernel@vger.kernel.org,
       Jens Axboe <jens.axboe@oracle.com>, Greg KH <gregkh@suse.de>,
       Neil Brown <neilb@suse.de>,
       James Bottomley <James.Bottomley@HansenPartnership.com>,
       Sam Ravnborg <sam@ravnborg.org>, Dave Jones <davej@redhat.com>,
       Nikanth Karthikesan <knikanth@suse.de>,
       Lars Marowsky-Bree <lmb@suse.de>,
       "Nicholas A. Bellinger" <nab@linux-iscsi.org>,
       Kyle Moffett <kyle@moffetthome.net>,
       Bart Van Assche <bart.vanassche@gmail.com>
Subject: Re: [PATCH 02/16] DRBD: lru_cache
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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.