Merge branch 'perf-optimizations-for-tcp-recv-zerocopy'

Arjun Roy says:

====================
Perf. optimizations for TCP Recv. Zerocopy

This patchset contains several optimizations for TCP Recv. Zerocopy.

Summarized:
1. It is possible that a read payload is not exactly page aligned -
that there may exist "straggler" bytes that we cannot map into the
caller's address space cleanly. For this, we allow the caller to
provide as argument a "hybrid copy buffer", turning
getsockopt(TCP_ZEROCOPY_RECEIVE) into a "hybrid" operation that allows
the caller to avoid a subsequent recvmsg() call to read the
stragglers.

2. Similarly, for "small" read payloads that are either below the size
of a page, or small enough that remapping pages is not a performance
win - we allow the user to short-circuit the remapping operations
entirely and simply copy into the buffer provided.

Some of the patches in the middle of this set are refactors to support
this "short-circuiting" optimization.

3. We allow the user to provide a hint that performing a page zap
operation (and the accompanying TLB shootdown) may not be necessary,
for the provided region that the kernel will attempt to map pages
into. This allows us to avoid this expensive operation while holding
the socket lock, which provides a significant performance advantage.

With all of these changes combined, "medium" sized receive traffic
(multiple tens to few hundreds of KB) see significant efficiency gains
when using TCP receive zerocopy instead of regular recvmsg(). For
example, with RPC-style traffic with 32KB messages, there is a roughly
15% efficiency improvement when using zerocopy. Without these changes,
there is a roughly 60-70% efficiency reduction with such messages when
employing zerocopy.
====================

Link: https://lore.kernel.org/r/20201202225349.935284-1-arjunroy.kdev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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