On Thu, Feb 09, 2006 at 07:01:25AM +0100, Peter Osterlund wrote:
> Adrian Bunk <[email protected]> writes:
>
> > Hi Phillip,
> >
> > your recent patch "pktcdvd: Allow larger packets" changed
> > PACKET_MAX_SIZE in the pktcdvd driver from 32 to 128.
> >
> > Unfortunately, drivers/block/pktcdvd.c contains the following:
> >
> > <-- snip -->
> >
> > ...
> > static void pkt_start_write(struct pktcdvd_device *pd, struct
> > packet_data *pkt)
> > {
> > struct bio *bio;
> > struct page *pages[PACKET_MAX_SIZE];
> > int offsets[PACKET_MAX_SIZE];
> > ...
> >
> > <-- snip -->
> >
> > With PACKET_MAX_SIZE=128, this allocates more than 1 kB on the stack
>
> Yes, I know.
>
> > which is not acceptable considering that we might have only 4 kB stack
> > altogether.
>
> Why is it not acceptable? The pkt_start_write() function is only
> called from the kcdrwd() kernel thread and the pkt_start_write()
> function doesn't call anything else in the kernel that could require
> lots of stack space.
>
> The actual I/O is started from pkt_iosched_process_queue(), which
> calls generic_make_request(). However pkt_iosched_process_queue() is
> on a different call chain than pkt_start_write(), so I don't see how
> this could be a problem.
You might be able to verify this is true today, but it is a bit fragile
and other changes might always add even more stack usage in some places.
Therefore, functions using 1 kB stack aren't a good idea.
As an exercise, pkt_open() currently uses more than 0,5 kB stack
(kernel 2.6.16-rc2-mm1, i386, gcc 4.0). Try to understand where this
stack usage comes from (hint: add up the stack usages of all only once
used static functions gcc automatically inlines).
> Peter Osterlund
cu
Adrian
--
"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed
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