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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 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.

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.



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