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    SubjectRe: [PATCH 08/13] netvm: Allow skb allocation to use PFMEMALLOC reserves
    On Thu, 28 Apr 2011 11:05:06 +0100 Mel Gorman <mgorman@suse.de> wrote:

    > On Thu, Apr 28, 2011 at 04:19:33PM +1000, NeilBrown wrote:
    > > On Wed, 27 Apr 2011 17:08:06 +0100 Mel Gorman <mgorman@suse.de> wrote:
    > >
    > >
    > > > @@ -1578,7 +1589,7 @@ static inline struct sk_buff *netdev_alloc_skb_ip_align(struct net_device *dev,
    > > > */
    > > > static inline struct page *__netdev_alloc_page(struct net_device *dev, gfp_t gfp_mask)
    > > > {
    > > > - return alloc_pages_node(NUMA_NO_NODE, gfp_mask, 0);
    > > > + return alloc_pages_node(NUMA_NO_NODE, gfp_mask | __GFP_MEMALLOC, 0);
    > > > }
    > > >
    > >
    > > I'm puzzling a bit over this change.
    > > __netdev_alloc_page appears to be used to get pages to put in ring buffer
    > > for a network card to DMA received packets into. So it is OK to use
    > > __GFP_MEMALLOC for these allocations providing we mark the resulting skb as
    > > 'pfmemalloc' if a reserved page was used.
    > >
    > > However I don't see where that marking is done.
    > > I think it should be in skb_fill_page_desc, something like:
    > >
    > > if (page->pfmemalloc)
    > > skb->pfmemalloc = true;
    > >
    > > Is this covered somewhere else that I am missing?
    > >
    >
    > You're not missing anything.
    >
    > >From the context of __netdev_alloc_page, we do not know if the skb
    > is suitable for marking pfmemalloc or not (we don't have SKB_ALLOC_RX
    > flag for example that __alloc_skb has). The reserves are potentially
    > being dipped into for an unsuitable packet but it gets dropped in
    > __netif_receive_skb() and the memory is returned. If we mark the skb
    > pfmemalloc as a result of __netdev_alloc_page using a reserve page, the
    > packets would not get dropped as expected.
    >

    The only code in __netif_receive_skb that seems to drop packets is

    + if (skb_pfmemalloc(skb) && !skb_pfmemalloc_protocol(skb))
    + goto drop;
    +

    which requires that the skb have pfmemalloc set before it will be dropped.

    Actually ... I'm expecting to find code that says:
    if (skb_pfmalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
    drop_packet();

    but I cannot find it. Where is the code that discard pfmalloc packets for
    non-memalloc sockets?

    I can see similar code in sk_filter but that doesn't drop the packet, it just
    avoids filtering it.

    NeilBrown


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