From: Brendan Jackman Date: Tue, 19 May 2026 14:17:58 +0000 (+0000) Subject: mm/page_alloc: document that alloc_pages_nolock() uses RCU X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=47166f2199557e57cbab2882b033fb2949818fbb;p=thirdparty%2Flinux.git mm/page_alloc: document that alloc_pages_nolock() uses RCU The allocator interacts with cgroups which rely on RCU. RCU does not work everywhere, so the "any context" claim is slightly overstated here. This should already be enforced by objtool, since this function is not marked noinstr the x86 build should fail if you call it from a place where RCU is not watching. But, expecting readers to make that connection for themselves seems a bit cruel (I don't think there is even any documentation of what noinstr means at all, let alone the connection with RCU). Note this is not claiming that any cgroup code called from the allocator would actually break if this restriction was violated, it could very well be that there's no real way for the allocator to act on a cgroup that can disappear concurrently. But, since it's likely nobody has verified this one way or another, better to just be safe and declare that RCU is required. Allocating from an RCU-unsafe context seems a bit crazy anyway. Link: https://lore.kernel.org/20260519-nolock-rcu-comment-v1-1-4a630c8794e5@google.com Signed-off-by: Brendan Jackman Suggested-by: Junaid Shahid Acked-by: Harry Yoo (Oracle) Acked-by: Vlastimil Babka (SUSE) Cc: Alexei Starovoitov Cc: Johannes Weiner Cc: Michal Hocko Cc: Suren Baghdasaryan Cc: Zi Yan Signed-off-by: Andrew Morton --- diff --git a/mm/page_alloc.c b/mm/page_alloc.c index d7b7f9504bd8..0ebffb0bb98b 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -7937,8 +7937,8 @@ struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned * @order: allocation order size * * Allocates pages of a given order from the given node. This is safe to - * call from any context (from atomic, NMI, and also reentrant - * allocator -> tracepoint -> alloc_pages_nolock_noprof). + * call from any context where RCU is watching (from atomic, NMI, and also + * reentrant allocator -> tracepoint -> alloc_pages_nolock_noprof). * Allocation is best effort and to be expected to fail easily so nobody should * rely on the success. Failures are not reported via warn_alloc(). * See always fail conditions below.