Re: PF_MEMALLOC in 2.6

[Nick Piggin]

Oliver Neukum wrote:
> Am Freitag, 20. August 2004 10:06 schrieb Nick Piggin:
>
> > > > So I'd say try to find a way to only use PF_MEMALLOC on behalf of
> > > > a PF_MEMALLOC thread or use a mempool or something.
> > >
> > >
> > > Then the SCSI layer should pass down the flag.
> >
> > It would be ideal from the memory allocator's point of view to do it
> > on a per-request basis like that.
> >
> > When the rubber hits the road, I think it is probably going to be very
> > troublesome to do it right that way. For example, what happens when
> > your usb-thingy-thread blocks on a memory allocation while handling a
> > read request, then the system gets low on memory and someone tries to
> > free some by submitting a write request to the USB device?
>
>
> The write request will have to wait. Storage cannot do concurrent IO.
> But all memory allocated in the read request will be GFP_NOIO or
> GFP_ATOMIC so the conclusion of the memory allocation should not
> wait for IO. Either it fails and we report that to the SCSI layer or it
> is completed and the write serviced in turn.
> At least that's the intent.

In that case, having the SCSI layer pass down the flag may be a viable
option.

Just FYI, non atomic allocations need to be __GFP_NORETRY otherwise they
won't fail (unless order >= 3). I suspect this detail is fairly important.
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