[9fans] Still puzzled by Venti

[Lucio De Re <lucio@proxima.alt.za>]

... which incidentally still refuses to work as advertised :-(

The constant "IndexBase" is used exclusively in
/sys/src/cmd/venti/fmtindex.c after being defined (as 1024 * 1024) in
.../venti/dat.h.

The code that uses it seems flawed to me: it assumes that

	addr = IndexBase;

as assigned before a loop through the contents of the Venti config
file, can be used as the arenas start address, which is not entirely
unjustified if the index is meant to start, as given in a comment in
dat.h:

	IndexBase = 1024*1024,	/* initial address to use in an index */

but is wrong when used in computing the beginning of the next arena
(and the end of the present one) by adding to it the current arena
size:

	amap[n].start = addr;
	addr += ap->arenas[j]->size;
	amap[n].stop = addr;

I'm not sure what the correct adjustment should be here, but the upper
limit should certainly remain within the arena:

	amap[n].stop = addr - IndexBase + ap->arenaBase;

Whether this is not just my imagination running wild, remains to be
decided.  Unfortunately, I don't know how to test this, because I'm
not sure what the importance of the initial unused space is.  I guess
I can but try the exact change suggested above and see what its effect
is going to be.

++L