On Mon, Oct 6, 2025 at 1:03 PM Kairui Song ryncsn@gmail.com wrote:
From: Kairui Song kasong@tencent.com
Since commit 1b7e90020eb77 ("mm, swap: use percpu cluster as allocation fast path"), swap allocation is protected by a local lock, which means we can't do any sleeping calls during allocation.
However, the discard routine is not taken well care of. When the swap allocator failed to find any usable cluster, it would look at the pending discard cluster and try to issue some blocking discards. It may not necessarily sleep, but the cond_resched at the bio layer indicates this is wrong when combined with a local lock. And the bio GFP flag used for discard bio is also wrong (not atomic).
It's arguable whether this synchronous discard is helpful at all. In most cases, the async discard is good enough. And the swap allocator is doing very differently at organizing the clusters since the recent change, so it is very rare to see discard clusters piling up.
So far, no issues have been observed or reported with typical SSD setups under months of high pressure. This issue was found during my code review. But by hacking the kernel a bit: adding a mdelay(100) in the async discard path, this issue will be observable with WARNING triggered by the wrong GFP and cond_resched in the bio layer.
So let's fix this issue in a safe way: remove the synchronous discard in the swap allocation path. And when order 0 is failing with all cluster list drained on all swap devices, try to do a discard following the swap device priority list. If any discards released some cluster, try the allocation again. This way, we can still avoid OOM due to swap failure if the hardware is very slow and memory pressure is extremely high.
Cc: stable@vger.kernel.org Fixes: 1b7e90020eb77 ("mm, swap: use percpu cluster as allocation fast path") Signed-off-by: Kairui Song kasong@tencent.com
Seems reasonable to me.
Acked-by: Nhat Pham nphamcs@gmail.com