Hi Christopher,

This eliminates the failure as expected (at the source).
I do not think such a solution is required, and it probably affect performance.

As Matthew said, slab objects should not be used in sk_buff fragments.
The source of these is my kernel TCP sockets, where kernel_sendpage() is used with slab payload.
I eliminated this, and the failure disappeared, even though with this kind of fine timing issues, no failure does not mean anything
Moreover, I tried triggering on slab in sk_buff fragments and nothing came up.

So far:
1) Use of slab payload in kernel_sendpage() is not polite, even though we do not BUG on this and documentation does not tell it was just wrong.
2) RX path cannot bring sk_buffs in slab: drivers use alloc_pagexxx or page_frag_alloc().

What I am still wondering about (and investigating), is how kernel_sendpage() with slab payload results in slab payload on another socket RX.
Do you see how page ref-counting can be broken with extra references taken on a slab page containing the fragments, and dropped when networking is done with them?

Thanks,
Anton



On Tue, Jun 5, 2018 at 8:27 AM, Christopher Lameter <cl@linux.com> wrote:
On Fri, 1 Jun 2018, Anton Eidelman wrote:

> I do not have a way of reproducing this decent enough to recommend: I'll
> keep digging.

If you can reproduce it: Could you try the following patch?



Subject: [NET] Fix false positives of skb_can_coalesce

Skb fragments may be slab objects. Two slab objects may reside
in the same slab page. In that case skb_can_coalesce() may return
true althought the skb cannot be expanded because it would
cross a slab boundary.

Enabling slab debugging will avoid the issue since red zones will
be inserted and thus the skb_can_coalesce() check will not detect
neighboring objects and return false.

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/include/linux/skbuff.h
===================================================================
--- linux.orig/include/linux/skbuff.h
+++ linux/include/linux/skbuff.h
@@ -3010,8 +3010,29 @@ static inline bool skb_can_coalesce(stru
        if (i) {
                const struct skb_frag_struct *frag = &skb_shinfo(skb)->frags[i - 1];

-               return page == skb_frag_page(frag) &&
-                      off == frag->page_offset + skb_frag_size(frag);
+               if (page != skb_frag_page(frag))
+                       return false;
+
+               if (off != frag->page_offset + skb_frag_size(frag))
+                       return false;
+
+               /*
+                * This may be a slab page and we may have pointers
+                * to different slab objects in the same page
+                */
+               if (!PageSlab(skb_frag_page(frag)))
+                       return true;
+
+               /*
+                * We could still return true if we would check here
+                * if the two fragments are within the same
+                * slab object. But that is complicated and
+                * I guess we would need a new slab function
+                * to check if two pointers are within the same
+                * object.
+                */
+               return false;
+
        }
        return false;
 }