Re: 2.6.24-git: kmap_atomic() WARN_ON()

Andrew Morton
2008-02-26T18:23:27Z

On Tue, 26 Feb 2008 11:32:24 -0500 Jeff Garzik <jgarzik@pobox.com> wrote:

> Ingo Molnar wrote:
> > * Jeff Garzik <jgarzik@pobox.com> wrote:
> >
> >> + unsigned long flags;
> >> +
> >> + local_irq_save(flags);
> >
> > hm, couldnt we attach the irq disabling to some spinlock, in a natural
> > way? Explicit flags fiddling is a PITA once we do things like threaded
> > irq handlers, -rt, etc.
>
> Attaching the irq disabling to some spinlock is what would be
> artificial... See the ahci.c patch earlier in this thread. It is taken
> without spin_lock_irqsave() in the interrupt handler, and there is no
> reason to disable interrupts for the entirety of the interrupt handler
> run -- only the part where we call kmap.
>
> This is only being done to satisfy kmap_atomic's requirements, not libata's.
>
> I could add a "kmap lock" but that just seems silly.
>

It's a bit sad to disable interupts across a memset (how big is it?) just
for the small proportion of cases which are accessing a highmem page.

What you could do is to add an `unsigned long *flags' arg to
ata_scsi_rbuf_get() and ata_scsi_rbuf_put(), and then, in
ata_scsi_rbuf_get() do

if (PageHighmem(page))
local_irq_disable(*flags);