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Re: [Xen-devel] [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots

On 30/05/14 09:06, Stefan Bader wrote:
On 16.05.2014 18:29, Zoltan Kiss wrote:
On 16/05/14 16:34, Wei Liu wrote:
On Fri, May 16, 2014 at 08:22:19AM -0700, Eric Dumazet wrote:
On Fri, 2014-05-16 at 15:36 +0100, Wei Liu wrote:
On Fri, May 16, 2014 at 07:21:08AM -0700, Eric Dumazet wrote:
On Fri, 2014-05-16 at 14:11 +0100, Wei Liu wrote:

It's not that common to trigger this, I only saw a few reports. In fact
Stefan's report is the first one that comes with a method to reproduce

I tested with redis-benchmark on a guest with 256MB RAM and only saw a
few "failed to linearize", never saw a single one with 1GB guest.
Well, I am just saying. This is asking order-5 allocations, and yes,
this is going to fail after few days of uptime, no matter what you try.

Hmm... I see what you mean -- memory fragmentation leads to allocation
failure. Thanks.
In the mean time, have you tried to lower gso_max_size ?

Setting it witk netif_set_gso_max_size() to something like 56000 might
avoid the problem.

(Not sure if it is applicable in your case)

It works, at least in this Redis testcase. Could you explain a bit where
this 56000 magic number comes from? :-)

Presumably I can derive it from some constant in core network code?
I guess it just makes more unlikely to have packets with problematic layout. 
But the following packet would still fail:
linear buffer : 80 bytes, on 2 pages
17 frags, 80 bytes each, each spanning over page boundary.

I just had an idea: a modified version of xenvif_handle_frag_list function from 
netback would be useful for us here. It recreates the frags array on fully 
utilized 4k pages. Plus we can use pskb_expand_head to reduce the page number 
on the linear buffer (although it might not work, see my comment in the patch)
The worst case linear buffer then spans N+1 pages, and has N*PAGE_SIZE+1 bytes. 
Then the frags after this coalescing should have 16*PAGE_SIZE - (N*PAGE_SIZE+2) 
bytes at most, which is 16-N pages. Altogether that's 16+1 page, which should 
definitely fit!
This is what I mean:

I had been idly wondering about this onwards. And trying to understand the whole
skb handling environment, I tried to come up with some idea as well. It may be
totally stupid and using the wrong assumptions. It seems to work in the sense
that things did not blow up into my face immediately and somehow I did not see
dropped packages due to the number of slots either.
But again, I am not sure I am doing the right thing. The idea was to just try to
get rid of so many compound pages (which I believe are the only ones that can
have an offset big enough to allow some alignment savings)...

This probably helps in a lot of scenarios, but not for everything. E.g. if the skb has 15 frags, each PAGE_SIZE+1 bytes, it'll still consume 30 slots for the frags, and this function won't be able to help that. It's hard to come up with an algorithm which handles all the scenarios we can have here, and does that with the least possible amount of copy. I'll keep looking into this in the next weeks, but don't hesitate, if you have an idea!




 From 8571b106643b32296e58526e2fbe97c330877ac8 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Date: Thu, 29 May 2014 12:18:01 +0200
Subject: [PATCH] xen-netfront: Align frags to fit max slots

In cases where the frags in a skb require more than MAX_SKB_FRAGS + 1
(= 18) 4K pages of grant pages, try to reduce the footprint by moving
the data to new pages and have it aligned to the beginning.
Then replace the page in the frag and release the old one. This sure is
more expensive in compute but should happen not too often and sounds
better than to just drop the packet in that case.

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
  drivers/net/xen-netfront.c | 65 +++++++++++++++++++++++++++++++++++++++++++---
  1 file changed, 62 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 158b5e6..ad71e5c 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -544,6 +544,61 @@ static int xennet_count_skb_frag_slots(struct sk_buff *skb)
        return pages;

+ * Align data to new pages in order to save slots required to
+ * transmit this buffer.
+ * @skb - socket buffer
+ * @target - number of pages to save
+ * returns the number of pages the fragments have been reduced of
+ */
+static int xennet_align_frags(struct sk_buff *skb, int target)
+       int i, frags = skb_shinfo(skb)->nr_frags;
+       int reduced = 0;
+       for (i = 0; i < frags; i++) {
+               skb_frag_t *frag = skb_shinfo(skb)->frags + i;
+               struct page *fpage = skb_frag_page(frag);
+               struct page *npage;
+               unsigned long size;
+               unsigned long offset;
+               gfp_t gfp;
+               int order;
+               if (!PageCompound(fpage))
+                       continue;
+               size = skb_frag_size(frag);
+               offset = frag->page_offset & ~PAGE_MASK;
+               /*
+                * If the length of data in the last subpage of a compound
+                * page is smaller than the offset into the first data sub-
+                * page, we can save a subpage by copying data around.
+                */
+               if ( ((offset + size) & ~PAGE_MASK) > offset )
+                       continue;
+               gfp = GFP_ATOMIC | __GFP_COLD;
+               order = PFN_UP(size);
+               if (order)
+                       gfp |= __GFP_COMP | __GFP_NOWARN;
+               npage = alloc_pages(gfp, order);
+               if (!npage)
+                       break;
+               memcpy(page_address(npage), skb_frag_address(frag), size);
+               frag->page.p = npage;
+               frag->page_offset = 0;
+               put_page(fpage);
+               if (++reduced >= target)
+                       break;
+       }
+       return reduced;
  static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
        unsigned short id;
@@ -573,9 +628,13 @@ static int xennet_start_xmit(struct sk_buff *skb, struct
net_device *dev)
        slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
        if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
-               net_alert_ratelimited(
-                       "xennet: skb rides the rocket: %d slots\n", slots);
-               goto drop;
+               slots -= xennet_align_frags(skb, slots - (MAX_SKB_FRAGS + 1));
+               if (slots > MAX_SKB_FRAGS + 1) {
+                       net_alert_ratelimited(
+                               "xennet: skb rides the rocket: %d slots\n",
+                               slots);
+                       goto drop;
+               }

        spin_lock_irqsave(&np->tx_lock, flags);

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