On Tue, Jun 30, 2026 at 05:18:38PM +0800, Gregory Price wrote:
> offline_and_remove_memory() handles a single contiguous range.
>
> Callers that manage a device composed of several ranges (dax/kmem)
> currently have to call it in a loop, which gives up atomicity.
>
> In addition to pushing rollback logic into the driver, the lack
> of atomicity creates a race condition between system daemons trying
> to manage the same resource:
>
> - Manager 1: Offlines memory blocks. Removes device.
> ^^^^
> - Manager 2: Detects offline memory blocks, re-onlines them.
>
> Add offline_and_remove_memory_ranges(), which takes an array of ranges
> and processes them as one operation under a single lock_device_hotplug():
>
> - Phase 1 offlines every block of every range.
> - Phase 2 removes the ranges only if all ranges are offline.
> - If any offline fails, the whole operation is reverted.
>
> This gives callers all-or-nothing semantics for the offline step, so a
> failed or interrupted unplug leaves the device in a consistent state.
>
> This also resolves the battling managers race - the second manager's
> operation simply fails when the block is destroyed / cannot be onlined.
>
> offline_and_remove_memory() becomes a thin wrapper that passes its single
> range to the new helper, so the offline/rollback logic lives in one place.
>
> Suggested-by: David Hildenbrand (Arm) <[email protected]>
> Signed-off-by: Gregory Price <[email protected]>
> ---
> include/linux/memory_hotplug.h | 8 +++
> mm/memory_hotplug.c | 93 ++++++++++++++++++++++++----------
> 2 files changed, 73 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index ff3b865ea7e7..db10d50f30ae 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -268,6 +268,8 @@ extern int offline_pages(unsigned long start_pfn,
> unsigned long nr_pages,
> extern int remove_memory(u64 start, u64 size);
> extern void __remove_memory(u64 start, u64 size);
> extern int offline_and_remove_memory(u64 start, u64 size);
> +int offline_and_remove_memory_ranges(const struct range *ranges,
> + unsigned int nr_ranges);
>
> #else
> static inline void try_offline_node(int nid) {}
> @@ -284,6 +286,12 @@ static inline int remove_memory(u64 start, u64 size)
> }
>
> static inline void __remove_memory(u64 start, u64 size) {}
> +
> +static inline int offline_and_remove_memory_ranges(const struct range
> *ranges,
> + unsigned int nr_ranges)
> +{
> + return -EBUSY;
> +}
> #endif /* CONFIG_MEMORY_HOTREMOVE */
>
> #ifdef CONFIG_MEMORY_HOTPLUG
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index a66346def504..3225364bec2f 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -2429,58 +2429,95 @@ static int try_reonline_memory_block(struct
> memory_block *mem, void *arg)
> */
> int offline_and_remove_memory(u64 start, u64 size)
> {
> - const unsigned long mb_count = size / memory_block_size_bytes();
> + struct range range = {
> + .start = start,
> + .end = start + size - 1,
> + };
> +
> + return offline_and_remove_memory_ranges(&range, 1);
> +}
> +EXPORT_SYMBOL_GPL(offline_and_remove_memory);
> +
> +/**
> + * offline_and_remove_memory_ranges - offline and remove multiple memory
> ranges
> + * @ranges: array of physical address ranges to offline and remove
> + * @nr_ranges: number of entries in @ranges
> + *
> + * Offline and remove several memory ranges as one operation, serialized
> + * against other hotplug operations by a single lock_device_hotplug().
> + *
> + * This offlines all ranges before removing any of them. If offlining any
> + * range fails, the entire process is reverted and nothing is removed.
> + * This provides a fully atomic semantic for unplugging an entire device.
> + *
> + * Each range must be memory-block aligned in start and size.
> + *
> + * Return: 0 on success, negative errno otherwise. On failure no range has
> + * been removed.
> + */
I think this can return 1, and it shouldn't.
device_offline() returns 1 when a block is already offline, and phase 1 passes
that value through as-is.
Easy to hit with patch 0, offline one memory block via memoryN/state, then write
"unplugged" to daxX.Y/state. The store returns 1, userspace treats it as a
partial write of 1 byte,
and retries the write with the rest of the string.
Maybe
"""
if (rc > 0)
rc = -EBUSY;
"""
Best regards,
Richard Cheng.
> +int offline_and_remove_memory_ranges(const struct range *ranges,
> + unsigned int nr_ranges)
> +{
> + unsigned long mb_count = 0;
> uint8_t *online_types, *tmp;
> - int rc;
> + unsigned int i;
> + int rc = 0;
>
> - if (!IS_ALIGNED(start, memory_block_size_bytes()) ||
> - !IS_ALIGNED(size, memory_block_size_bytes()) || !size)
> + if (!ranges || !nr_ranges)
> return -EINVAL;
>
> + for (i = 0; i < nr_ranges; i++) {
> + const u64 start = ranges[i].start;
> + const u64 size = range_len(&ranges[i]);
> +
> + if (!IS_ALIGNED(start, memory_block_size_bytes()) ||
> + !IS_ALIGNED(size, memory_block_size_bytes()) || !size)
> + return -EINVAL;
> + mb_count += size / memory_block_size_bytes();
> + }
> +
> /*
> - * We'll remember the old online type of each memory block, so we can
> - * try to revert whatever we did when offlining one memory block fails
> - * after offlining some others succeeded.
> + * Remember the old online type of every memory block across all ranges,
> + * so we can revert if offlining a later block fails. All entries start
> + * as MMOP_OFFLINE so blocks we never touched are skipped on rollback.
> */
> online_types = kmalloc_array(mb_count, sizeof(*online_types),
> GFP_KERNEL);
> if (!online_types)
> return -ENOMEM;
> - /*
> - * Initialize all states to MMOP_OFFLINE, so when we abort processing in
> - * try_offline_memory_block(), we'll skip all unprocessed blocks in
> - * try_reonline_memory_block().
> - */
> memset(online_types, MMOP_OFFLINE, mb_count);
>
> lock_device_hotplug();
>
> + /* Phase 1: offline every block in every range. */
> tmp = online_types;
> - rc = walk_memory_blocks(start, size, &tmp, try_offline_memory_block);
> -
> - /*
> - * In case we succeeded to offline all memory, remove it.
> - * This cannot fail as it cannot get onlined in the meantime.
> - */
> - if (!rc) {
> - rc = try_remove_memory(start, size);
> + for (i = 0; i < nr_ranges; i++) {
> + rc = walk_memory_blocks(ranges[i].start, range_len(&ranges[i]),
> + &tmp, try_offline_memory_block);
> if (rc)
> - pr_err("%s: Failed to remove memory: %d", __func__, rc);
> + break;
> }
>
> - /*
> - * Rollback what we did. While memory onlining might theoretically fail
> - * (nacked by a notifier), it barely ever happens.
> - */
> + /* If any failure occurred at all, rollback any changes and bail */
> if (rc) {
> tmp = online_types;
> - walk_memory_blocks(start, size, &tmp,
> - try_reonline_memory_block);
> + for (i = 0; i < nr_ranges; i++)
> + walk_memory_blocks(ranges[i].start,
> + range_len(&ranges[i]), &tmp,
> + try_reonline_memory_block);
> + goto out_unlock;
> }
> +
> + /* Phase 2: Remove. This should never fail holding the hotplug lock */
> + for (i = 0; i < nr_ranges; i++)
> + WARN_ON_ONCE(try_remove_memory(ranges[i].start,
> + range_len(&ranges[i])));
> +
> +out_unlock:
> unlock_device_hotplug();
>
> kfree(online_types);
> return rc;
> }
> -EXPORT_SYMBOL_GPL(offline_and_remove_memory);
> +EXPORT_SYMBOL_GPL(offline_and_remove_memory_ranges);
> #endif /* CONFIG_MEMORY_HOTREMOVE */
> --
> 2.53.0-Meta
>
>
