[PATCH net-next v4 0/3] Minor cleanups to skb frag ref/unref

[Mina Almasry]

v4:
- Rebased to net-next.
- Clarified skb_shift() code change in commit message.
- Use skb->pp_recycle in a couple of places where I previously hardcoded
  'false'.

v3:
- Fixed patchwork build errors/warnings from patch-by-patch modallconfig
  build

v2:
- Removed RFC tag.
- Rebased on net-next after the merge window opening.
- Added 1 patch at the beginning, "net: make napi_frag_unref reuse
  skb_page_unref" because a recent patch introduced some code
  duplication that can also be improved.
- Addressed feedback from Dragos & Yunsheng.
- Added Dragos's Reviewed-by.

This series is largely motivated by a recent discussion where there was
some confusion on how to properly ref/unref pp pages vs non pp pages:

https://lore.kernel.org/netdev/CAHS8izOoO-EovwMwAm9tLYetwikNPxC0FKyVGu1TPJWSz4bGoA@xxxxxxxxxxxxxx/T/#t

There is some subtely there because pp uses page->pp_ref_count for
refcounting, while non-pp uses get_page()/put_page() for ref counting.
Getting the refcounting pairs wrong can lead to kernel crash.

Additionally currently it may not be obvious to skb users unaware of
page pool internals how to properly acquire a ref on a pp frag. It
requires checking of skb->pp_recycle & is_pp_page() to make the correct
calls and may require some handling at the call site aware of arguable pp
internals.

This series is a minor refactor with a couple of goals:

1. skb users should be able to ref/unref a frag using
   [__]skb_frag_[un]ref() functions without needing to understand pp
   concepts and pp_ref_count vs get/put_page() differences.

2. reference counting functions should have a mirror opposite. I.e. there
   should be a foo_unref() to every foo_ref() with a mirror opposite
   implementation (as much as possible).

This is RFC to collect feedback if this change is desirable, but also so
that I don't race with the fix for the issue Dragos is seeing for his
crash.

https://lore.kernel.org/lkml/CAHS8izN436pn3SndrzsCyhmqvJHLyxgCeDpWXA4r1ANt3RCDLQ@xxxxxxxxxxxxxx/T/

Cc: Dragos Tatulea <dtatulea@xxxxxxxxxx>


Mina Almasry (3):
  net: make napi_frag_unref reuse skb_page_unref
  net: mirror skb frag ref/unref helpers
  net: remove napi_frag_unref

 .../chelsio/inline_crypto/ch_ktls/chcr_ktls.c |  2 +-
 drivers/net/ethernet/sun/cassini.c            |  4 +-
 drivers/net/veth.c                            |  2 +-
 include/linux/skbuff.h                        | 38 +++++++------
 net/core/skbuff.c                             | 55 ++++++-------------
 net/ipv4/esp4.c                               |  2 +-
 net/ipv6/esp6.c                               |  2 +-
 net/tls/tls_device_fallback.c                 |  2 +-
 8 files changed, 44 insertions(+), 63 deletions(-)

-- 
2.44.0.478.gd926399ef9-goog