Re: tcp_do_sendmsg() allocation still broken ?

[Andi Kleen <ak@suse.de>]

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.

> 
> 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.

-Andi
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