@@ -5597,18 +5597,18 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
if (skb_cloned(to))
return false;
- /* In general, avoid mixing slab allocated and page_pool allocated
- * pages within the same SKB. However when @to is not pp_recycle and
- * @from is cloned, we can transition frag pages from page_pool to
- * reference counted.
- *
- * On the other hand, don't allow coalescing two pp_recycle SKBs if
- * @from is cloned, in case the SKB is using page_pool fragment
+ /* In general, avoid mixing page_pool and non-page_pool allocated
+ * pages within the same SKB. Additionally avoid dealing with clones
+ * containing page_pool pages, in case the SKB is using page_pool fragment
* references (PP_FLAG_PAGE_FRAG). Since we only take full page
* references for cloned SKBs at the moment that would result in
* inconsistent reference counts.
+ * In theory we could take full references if from is cloned and
+ * !@to->pp_recycle but its tricky (due to potential race with the clone
+ * disappearing) and rare, so not worth dealing with.
*/
- if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from)))
+ if (to->pp_recycle != from->pp_recycle ||
+ (from->pp_recycle && skb_cloned(from)))
return false;
if (len <= skb_tailroom(to)) {
Commit 1effe8ca4e34 ("skbuff: fix coalescing for page_pool fragment recycling") allowed coalescing to proceed with non page pool page and page pool page when @from is cloned, i.e. to->pp_recycle --> false from->pp_recycle --> true skb_cloned(from) --> true However, it actually requires skb_cloned(@from) to hold true until coalescing finishes in this situation. If the other cloned SKB is released while the merging is in process, from_shinfo->nr_frags will be set to 0 toward the end of the function, causing the increment of frag page _refcount to be unexpectedly skipped resulting in inconsistent reference counts. Later when SKB(@to) is released, it frees the page directly even though the page pool page is still in use, leading to use-after-free or double-free errors. So it should be prohibited. The double-free error message below prompted us to investigate: BUG: Bad page state in process swapper/1 pfn:0e0d1 page:00000000c6548b28 refcount:-1 mapcount:0 mapping:0000000000000000 index:0x2 pfn:0xe0d1 flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff) raw: 000fffffc0000000 0000000000000000 ffffffff00000101 0000000000000000 raw: 0000000000000002 0000000000000000 ffffffffffffffff 0000000000000000 page dumped because: nonzero _refcount CPU: 1 PID: 0 Comm: swapper/1 Tainted: G E 6.2.0+ Call Trace: <IRQ> dump_stack_lvl+0x32/0x50 bad_page+0x69/0xf0 free_pcp_prepare+0x260/0x2f0 free_unref_page+0x20/0x1c0 skb_release_data+0x10b/0x1a0 napi_consume_skb+0x56/0x150 net_rx_action+0xf0/0x350 ? __napi_schedule+0x79/0x90 __do_softirq+0xc8/0x2b1 __irq_exit_rcu+0xb9/0xf0 common_interrupt+0x82/0xa0 </IRQ> <TASK> asm_common_interrupt+0x22/0x40 RIP: 0010:default_idle+0xb/0x20 Fixes: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool") Signed-off-by: Liang Chen <liangchen.linux@gmail.com> Reviewed-by: Eric Dumazet <edumazet@google.com> --- Changes from v3: - make the comment more precise and add fixes and reviewed-by tags --- net/core/skbuff.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)