Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

Marek Szyprowski
2022-06-15T22:49:14Z

Hi Mel,

On 13.06.2022 14:56, Mel Gorman wrote:
> struct per_cpu_pages is no longer strictly local as PCP lists can be
> drained remotely using a lock for protection. While the use of local_lock
> works, it goes against the intent of local_lock which is for "pure
> CPU local concurrency control mechanisms and not suited for inter-CPU
> concurrency control" (Documentation/locking/locktypes.rst)
>
> local_lock protects against migration between when the percpu pointer is
> accessed and the pcp->lock acquired. The lock acquisition is a preemption
> point so in the worst case, a task could migrate to another NUMA node
> and accidentally allocate remote memory. The main requirement is to pin
> the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.
>
> Replace local_lock with helpers that pin a task to a CPU, lookup the
> per-cpu structure and acquire the embedded lock. It's similar to local_lock
> without breaking the intent behind the API. It is not a complete API
> as only the parts needed for PCP-alloc are implemented but in theory,
> the generic helpers could be promoted to a general API if there was
> demand for an embedded lock within a per-cpu struct with a guarantee
> that the per-cpu structure locked matches the running CPU and cannot use
> get_cpu_var due to RT concerns. PCP requires these semantics to avoid
> accidentally allocating remote memory.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

This patch landed in linux next-20220614 as commit 54bcdc6744e3
("mm/page_alloc: replace local_lock with normal spinlock").
Unfortunately it causes some serious issues when some kernel debugging
options (CONFIG_PROVE_LOCKING and CONFIG_DEBUG_ATOMIC_SLEEP) are
enabled. I've observed this on various ARM 64bit and 32bit boards.

In the logs I see lots of errors like:

BUG: sleeping function called from invalid context at
./include/linux/sched/mm.h:274

BUG: scheduling while atomic: systemd-udevd/288/0x00000002

BUG: sleeping function called from invalid context at mm/filemap.c:2647

however there are also a fatal ones like:

Unable to handle kernel paging request at virtual address 00000000017a87b4

The issues seems to be a bit random. Looks like memory trashing.
Reverting $subject on top of current linux-next fixes all those issues.

Let me know if how I can help debugging this.

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland