linux-kernel - Re: [PATCH 02/16] DRBD: lru

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Message-ID: <20090502181311.GG6466@racke>
Date:	Sat, 2 May 2009 20:13:12 +0200
From:	Lars Ellenberg <lars.ellenberg@...bit.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Philipp Reisner <philipp.reisner@...bit.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Greg KH <gregkh@...e.de>, Neil Brown <neilb@...e.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Sam Ravnborg <sam@...nborg.org>, Dave Jones <davej@...hat.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	Lars Marowsky-Bree <lmb@...e.de>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Bart Van Assche <bart.vanassche@...il.com>
Subject: Re: [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@...bit.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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