On 17.08.24 08:24, Barry Song wrote:
From: Barry Song <[email protected]>When users allocate memory with the __GFP_NOFAIL flag, they might incorrectly use it alongside GFP_ATOMIC, GFP_NOWAIT, etc. This kind of non-blockable __GFP_NOFAIL is not supported and is pointless. If we attempt and still fail to allocate memory for these users, we have two choices: 1. We could busy-loop and hope that some other direct reclamation or kswapd rescues the current process. However, this is unreliable and could ultimately lead to hard or soft lockups, which might not be well supported by some architectures. 2. We could use BUG_ON to trigger a reliable system crash, avoiding exposing NULL dereference. Neither option is ideal, but both are improvements over the existing code. This patch selects the second option because, with the introduction of scoped API and GFP_NOFAIL—capable of enforcing direct reclamation for nofail users(which is in my plan), non-blockable nofail allocations will no longer be possible. Signed-off-by: Barry Song <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Uladzislau Rezki (Sony) <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Lorenzo Stoakes <[email protected]> Cc: Christoph Lameter <[email protected]> Cc: Pekka Enberg <[email protected]> Cc: David Rientjes <[email protected]> Cc: Joonsoo Kim <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: Roman Gushchin <[email protected]> Cc: Hyeonggon Yoo <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Kees Cook <[email protected]> Cc: "Eugenio Pérez" <[email protected]> Cc: Hailong.Liu <[email protected]> Cc: Jason Wang <[email protected]> Cc: Maxime Coquelin <[email protected]> Cc: "Michael S. Tsirkin" <[email protected]> Cc: Xuan Zhuo <[email protected]> --- mm/page_alloc.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index d2c37f8f8d09..fb5850ecd3ae 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4399,11 +4399,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, */ if (gfp_mask & __GFP_NOFAIL) { /* - * All existing users of the __GFP_NOFAIL are blockable, so warn - * of any new users that actually require GFP_NOWAIT + * All existing users of the __GFP_NOFAIL are blockable + * otherwise we introduce a busy loop with inside the page + * allocator from non-sleepable contexts */ - if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask)) - goto fail; + BUG_ON(!can_direct_reclaim);
No new BUG_ON(), WARN_ON_ONCE() is good enough for something that should be found during ordinary testing.
-- Cheers, David / dhildenb
