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linux-cxl@vger.kernel.org, driver-core@lists.linux.dev, linux-kselftest@vger.kernel.org, kernel-team@meta.com, david@kernel.org, osalvador@suse.de, gregkh@linuxfoundation.org, rafael@kernel.org, dakr@kernel.org, djbw@kernel.org, vishal.l.verma@intel.com, dave.jiang@intel.com, alison.schofield@intel.com, akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org, iweiny@kernel.org, Smita.KoralahalliChannabasappa@amd.com, apopple@nvidia.com Subject: Re: [PATCH v6 06/10] mm/memory_hotplug: add offline_and_remove_memory_ranges() Message-ID: References: <20260630211842.2252800-1-gourry@gourry.net> <20260630211842.2252800-7-gourry@gourry.net> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260630211842.2252800-7-gourry@gourry.net> X-ClientProxiedBy: SI2P153CA0018.APCP153.PROD.OUTLOOK.COM (2603:1096:4:140::9) To BL0PR12MB2370.namprd12.prod.outlook.com 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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) > Signed-off-by: Gregory Price > --- > 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 > >