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
Anton
On Tue, Jun 5, 2018 at 8:27 AM, Christopher Lameter <cl@xxxxxxxxx> 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@xxxxxxxxx>
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;
}
