Re: [PATCH net v2 2/2] net: add a KUnit test for the alloc_skb_with_frags() tail split

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- [Medium] The skip probe in alloc_skb_with_frags_test_tail()
  (net/core/net_test.c) cannot do what its comment says ("Don't report a…

--

net: add a KUnit test for the alloc_skb_with_frags() tail split

This adds a KUnit case that asks alloc_skb_with_frags() for
MAX_SKB_FRAGS - 2 order-3 chunks plus a 7 page tail. Before the fix it
references, splitting that tail ran out of frags. If the allocation fails
and a single order-3 page can't be allocated either, the test is meant to
skip rather than fail.

> diff --git a/net/core/net_test.c b/net/core/net_test.c
> index 2b5080c226636..de4d246f2f7ab 100644
> --- a/net/core/net_test.c
> +++ b/net/core/net_test.c
> @@ -1721,7 +1721,42 @@ static void netmem_ref_test_tcp_devmem_recv(struct kunit *test)

[ ... ]

> +	skb = alloc_skb_with_frags(0, len, PAGE_ALLOC_COSTLY_ORDER, &err,
> +				   GFP_KERNEL);
> +	if (!skb) {
> +		struct page *page;
> +
> +		/* Don't report a lack of high-order pages as a failure. */
> +		page = alloc_pages(GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY,
> +				   PAGE_ALLOC_COSTLY_ORDER);
> +		if (!page)
> +			kunit_skip(test, "no order-%d pages",
> +				   PAGE_ALLOC_COSTLY_ORDER);
> +		__free_pages(page, PAGE_ALLOC_COSTLY_ORDER);
> +	}
> +	KUNIT_ASSERT_NOT_NULL_MSG(test, skb, "err %d", err);

[Severity: Medium]
Does probing for a single order-3 page catch the case the comment
describes?

For this len, alloc_skb_with_frags() never lets a leading chunk drop
below order 3. At nr_frags = k, the check is:

net/core/skbuff.c:alloc_skb_with_frags() {
    ...
			can_shrink = DIV_ROUND_UP(data_len, smaller) <=
				     MAX_SKB_FRAGS - nr_frags;
    ...
}

This works out to 2*MAX_SKB_FRAGS - 2 - 2k <= MAX_SKB_FRAGS - k. That
holds only once k >= MAX_SKB_FRAGS - 2. So each of the first
MAX_SKB_FRAGS - 2 chunks has to get an order-3 page, or the function
does goto failure. In total the call needs roughly MAX_SKB_FRAGS - 1
order-3 pages, not one.

When the call fails partway through, the failure path does:

failure:
	kfree_skb(skb);
	return NULL;

This frees every order-3 page already placed in the frags. Since
pcp_allowed_order(3) is true, those pages land on this CPU's per-CPU
free lists right before the probe runs. The single
alloc_pages(..., PAGE_ALLOC_COSTLY_ORDER) probe will then almost always
succeed, the skip is not taken, and KUNIT_ASSERT_NOT_NULL_MSG() reports
a failure.

So the test seems to skip only when the very first order-3 allocation
fails. A host with some order-3 pages, but fewer than MAX_SKB_FRAGS - 1,
would still report a failure. That doesn't match "so that a host with
fragmented memory does not report this as a regression" in the commit
message.

Could the probe check the real requirement instead? For example, it
could try to hold MAX_SKB_FRAGS - 1 order-3 pages at once, or treat
-ENOBUFS as a skip unless a stronger check shows enough pages were
available.

> +
> +	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
> +		frags_len += skb_frag_size(&skb_shinfo(skb)->frags[i]);
> +	KUNIT_EXPECT_EQ(test, frags_len, len);
> +
> +	kfree_skb(skb);
> +}

[ ... ]

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