linux-kernel - Memory allocation on speculative fastpaths

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Message-ID: <20220503155913.GA1187610@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 3 May 2022 08:59:13 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     liam.howlett@...cle.com, willy@...radead.org, walken.cr@...il.com,
        mhocko@...e.com, hannes@...xchg.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Memory allocation on speculative fastpaths

Hello!

Just following up from off-list discussions yesterday.

The requirements to allocate on an RCU-protected speculative fastpath
seem to be as follows:

1.	Never sleep.
2.	Never reclaim.
3.	Leave emergency pools alone.

Any others?

If those rules suffice, and if my understanding of the GFP flags is
correct (ha!!!), then the following GFP flags should cover this:

	__GFP_NOMEMALLOC | __GFP_NOWARN

Or is this just a fancy way of always returning NULL or some such?  ;-)

							Thanx, Paul