On Tue, Nov 7, 2023 at 1:00 AM Yunsheng Lin linyunsheng@huawei.com wrote:
On 2023/11/6 10:44, Mina Almasry wrote:
Make skb_frag_page() fail in the case where the frag is not backed by a page, and fix its relevent callers to handle this case.
Correctly handle skb_frag refcounting in the page_pool_iovs case.
Signed-off-by: Mina Almasry almasrymina@google.com
...
/**
- skb_frag_page - retrieve the page referred to by a paged fragment
- @frag: the paged fragment
- Returns the &struct page associated with @frag.
- Returns the &struct page associated with @frag. Returns NULL if this frag
*/
- has no associated page.
static inline struct page *skb_frag_page(const skb_frag_t *frag) {
return frag->bv_page;
if (!page_is_page_pool_iov(frag->bv_page))return frag->bv_page;return NULL;It seems most of callers don't expect NULL returning for skb_frag_page(), and this patch only changes a few relevant callers to handle the NULL case.
Yes, I did not change code that I guessed was not likely to be affected or enable the devmem TCP case. Here is my breakdown:
➜ cos-kernel git:(tcpdevmem) ✗ ack -i "skb_frag_page(" --ignore-dir=drivers -t cc -l net/core/dev.c net/core/datagram.c net/core/xdp.c net/core/skbuff.c net/core/filter.c net/core/gro.c net/appletalk/ddp.c net/wireless/util.c net/tls/tls_device.c net/tls/tls_device_fallback.c net/ipv4/tcp.c net/ipv4/tcp_output.c net/bpf/test_run.c include/linux/skbuff.h
I'm ignoring ank skb_frag_page() calls in drivers because drivers need to add support for devmem TCP, and handle these calls at time of adding support, I think that's reasonable.
net/core/dev.c: I think I missed ilegal_highdma()
net/core/datagram.c: __skb_datagram_iter() protected by not_readable(skb) check.
net/core/skbuff.c: protected by not_readable(skb) check.
net/core/filter.c: bpf_xdp_frags_shrink_tail seems like xdp specific, not sure it's relevant here.
net/core/gro.c: skb_gro_reset_offset: protected by NULL check
net/ipv4/tcp.c: tcp_zerocopy_receive protected by NULL check.
net/ipv4/tcp_output.c: tcp_clone_payload: handles NULL return fine.
net/bpf/test_run.c: seems xdp specific and not sure if it can run into devmem issues.
include/linux/skbuff.h: I think the multiple calls here are being handled correctly, but let me know if not.
All the calls in these files, I think, are code paths not possible to hit devmem TCP with the current support, I think: net/core/xdp.c net/appletalk/ddp.c net/wireless/util.c net/tls/tls_device.c net/tls/tls_device_fallback.c
All in all I think maybe all in all I missed illegal_highdma(). I'll fix it in the next iteration.
It may make more sense to add a new helper to do the above checking, and add a warning in skb_frag_page() to catch any missing NULL checking for skb_frag_page() caller, something like below?
static inline struct page *skb_frag_page(const skb_frag_t *frag) {
return frag->bv_page;
struct page *page = frag->bv_page;BUG_ON(page_is_page_pool_iov(page));return page;+}
+static inline struct page *skb_frag_readable_page(const skb_frag_t *frag) +{
struct page *page = frag->bv_page;if (!page_is_page_pool_iov(page))return page;return NULL;}
My personal immediate reaction is that this may just introduce code churn without significant benefit. If an unsuspecting caller call skb_frag_page() on devmem frag and doesn't correctly handle NULL return, it will crash or error out anyway, and likely in some obvious way, so maybe the BUG_ON() isn't so useful that it's worth changing all the call sites. But if there is consensus on adding a change like you propose, I have no problem adding it.