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SubjectRe: tcp_do_sendmsg() allocation still broken ?

On Sun, 29 Oct 2000, Andi Kleen wrote:

> On Sat, Oct 28, 2000 at 09:13:27PM -0200, Marcelo Tosatti wrote:
> >
> >
> > On Sun, 29 Oct 2000, Andi Kleen wrote:
> >
> > > On Sat, Oct 28, 2000 at 07:12:44PM -0200, Marcelo Tosatti wrote:
> > > >
> > > > David,
> > > >
> > > > tcp_do_sendmsg() still allocates using GFP_KERNEL when it can't, it seems:
> > >
> > > tcp_do_sendmsg() should only be called from process context, because it can
> > > sleep for other reasons anyways.
> > >
> > > If someone calls it from interrupt context it needs to be fixed.
> >
> > Andi,
> >
> > Think about nbd.
>
> Making tcp_do_sendmsg use GFP_ATOMIC would make it too unreliable for other
> situations. Penalizing the whole system just for nbd is not a good idea.

I agree that using GFP_ATOMIC is bad there.

I've fixed 2.2 with GFP_BUFFER.

>
> >
> > It allocates memory inside its request function, and since we do write
> > throttling in try_to_free_pages() we can end up with this deadlock in low
> > memory conditions:
> >
> > nbd_do_request -> nbd_send_req -> nbd_xmit -> sock_sendmsg -> ... ->
> > tcp_do_sendmsg -> tcp_send_skb -> skb_copy -> alloc_skb ->
> > kmalloc(GFP_KERNEL) -> kmem_cache_grow -> get_free_pages ->
> > try_to_free_pages -> shrink_mmap -> try_to_free_buffers ->
> > sync_page_buffers -> ll_rw_block -> make_request -> add_request ->
> > nbd_do_request -> nbd_send_req -> ...
> >
> > until there are no more free requests on the request queue and writer
> > processes stuck on __get_request_wait.
>
> Looks like a job for PF_MEMALLOC and making GFP_KERNEL fail earlier.

IMHO GFP_BUFFER is ok.




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