[email protected]> <[email protected]> <20060822212150.
[email protected]> <[email protected]>
In-Reply-To: <[email protected]>
User-Agent: Mutt/1.5.11+cvs20060126
Hi. Back in August I sent out some patches for the aoe driver, and
Alan objected to the direct setting of skb->len in one of them. I
asked whether he was suggesting that we use something like this
instead of setting skb->len:
skb->data_len = 0;
skb_trim(skb, 0);
skb_put(skb, sizeof *h + sizeof *ah + DEFAULTBCNT);
... and Alan said that was acceptible because it takes care of
skb->tail, checks for overflow, and is more future proof.
So I took some time to implement the necessary changes, but then it
became apparent that there was a problem.
The skb_trim and skb_put macros are only for non-linear skbuffs, but
we are only using the area inside the skbuff itself for packet
headers, not data.
When we do something like this:
if (bio_data_dir(buf->bio) == WRITE) {
skb_fill_page_desc(skb, 0, bv->bv_page, buf->bv_off, bcnt);
ah.aflags |= AOEAFL_WRITE;
skb->len += bcnt;
skb->data_len = bcnt;
t->wpkts++;
... skb->tail isn't really relevant, because we are only using the
pre-allocated part of the skbuff for headers, and the headers aren't
expanding here. Other parts of the kernel that aren't putting data in
the skbuff itself set the skb->len directly.
e1000_main.c
ip_output.c
tcp.c
ip6_output.c
So is it correct for the callers of skb_fill_page_desc to set skb->len
or is another interface needed besides skb_put/skb_trim? Such a new
interface would be able to maintain the consistency of the skbuff
fields without assuming that the data is in the skbuff itself.
If a new interface is needed, then it seems like we should set
skb->len in this patch until the new interface is ready.
--
Ed L Cashin <[email protected]>
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