Re: [PATCH v2] dax/pmem: fix unaligned compound devmap initialization

[Muchun Song <muchun.song@linux.dev>]

> On Nov 9, 2026, at 09:57, Ritesh Harjani (IBM) <ritesh.list@gmail.com> wrote:
> 
> Muchun Song <songmuchun@bytedance.com> writes:
> 
>> Static pmem Device DAX namespaces reserve space for namespace metadata
>> before the user-visible data range, and may also include padding needed
>> to satisfy the Device DAX alignment. __dax_pmem_probe() adjusts the
>> dax_region range to start at the data range, but the dev_pagemap range
>> still starts at the beginning of the namespace.
>> 
>> That is not valid for compound devmap mappings whose memmap is stored in
>> system RAM. Compound page initialization requires the start PFN to be
>> naturally aligned to the compound page size, because the head and tail
>> struct pages are initialized according to that geometry. If the pgmap
>> starts before the aligned data range, memmap_init_zone_device() can build
>> compound page metadata from an unaligned PFN.
>> 
>> The resulting struct page layout is inconsistent with the Device DAX
>> mapping. A later mmap fault can then resolve the user-visible PFN through
>> invalid compound page metadata and hit a kernel panic while inserting the
>> DAX folio.
>> 
>> Fix this by rounding the pgmap start up to the Device DAX alignment for
>> non-altmap mappings. This gives compound mappings the required naturally
>> aligned start while retaining full-section vmemmap savings when the
>> original start is already aligned. The altmap path remains unchanged
>> because memmap_init_zone_device() skips the altmap reservation itself.
>> 
>> Fixes: 14606001efb4 ("device-dax: compound devmap support")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Reported-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> Closes: https://lore.kernel.org/r/wlrkqqvg.ritesh.list@gmail.com
> 
> Tested v2 also on my Power dev test setup. With this patch the reported
> problem is fixed. I am no more hitting the following warning and crash:
> 
> [    3.071811][    T1] WARNING: mm/mm_init.c:1175 at memmap_init_zone_device+0x144/0x470, CPU#0: swapper/0/1
> [    3.082276][    T1] NIP [c000000000596604] memmap_init_zone_device+0x144/0x470
> [    3.082763][    T1] LR [c0000000006d72b8] memremap_pages+0x3b0/0x974
> [    3.083345][    T1] Call Trace:
> [    3.083629][    T1] [c000000006d86e00] [c000000006d86e40] 0xc000000006d86e40 (unreliable)
> [    3.084975][    T1] [c000000006d86f10] [c0000000006d72b8] memremap_pages+0x3b0/0x974
> [    3.085449][    T1] [c000000006d86fe0] [c0000000006d78b8] devm_memremap_pages+0x3c/0xa4
> [    3.085891][    T1] [c000000006d87020] [c00000000125c084] dev_dax_probe+0x264/0x3d0
> [    3.086339][    T1] [c000000006d870c0] [c000000001259a28] dax_bus_probe+0x140/0x1c8
> [    3.086778][    T1] [c000000006d87110] [c0000000011ce640] really_probe+0x108/0x580
> [    3.087218][    T1] [c000000006d871a0] [c0000000011ceb84] __driver_probe_device+0xcc/0x284
> [    3.087668][    T1] [c000000006d87230] [c0000000011ceea0] driver_probe_device+0x5c/0x10c
> [    3.088123][    T1] [c000000006d87270] [c0000000011cf05c] __device_attach_driver+0x10c/0x1e8
> [    3.088582][    T1] [c000000006d872f0] [c0000000011cab70] bus_for_each_drv+0xc0/0x13c
> [    3.089035][    T1] [c000000006d87350] [c0000000011cf65c] __device_attach+0xdc/0x2b0
> [    3.089467][    T1] [c000000006d873f0] [c0000000011cfaac] device_initial_probe+0x80/0x98
> [    3.089910][    T1] [c000000006d87420] [c0000000011cac44] __bus_probe_device+0x58/0xf4
> [    3.090358][    T1] [c000000006d87470] [c0000000011c8030] device_add+0x850/0xa78
> [    3.090778][    T1] [c000000006d87530] [c00000000125b040] __devm_create_dev_dax+0x1c0/0x638
> [    3.091236][    T1] [c000000006d875d0] [c00000000125b6c8] devm_create_dev_dax+0x44/0x7c
> [    3.091676][    T1] [c000000006d87600] [c00000000125dbe8] __dax_pmem_probe+0x238/0x27c
> [    3.092120][    T1] [c000000006d87770] [c00000000125dc4c] dax_pmem_probe+0x20/0x44
> [    3.092542][    T1] [c000000006d87790] [c000000001237a14] nvdimm_bus_probe+0xcc/0x370
> [    3.093097][    T1] [c000000006d87830] [c0000000011ce640] really_probe+0x108/0x580
> [    3.093610][    T1] [c000000006d878c0] [c0000000011ceb84] __driver_probe_device+0xcc/0x284
> <...>
> [    3.099405][    T1] ---- interrupt: 0 at 0x0
> [    3.099837][    T1] Code: 7c0802a6 fa8100b0 faa100b8 fb8100f0 7d574030 7d4a4030 f8010120 fbe10108 3917ffff 7d294038 3109ffff 7d284910 <0b090000> 3d2201c5 7bb53664 7ed607b4
> [    3.101209][    T1] ---[ end trace 0000000000000000 ]---
> [    3.104702][    T1] BUG: Unable to handle kernel data access on write at 0xc00c000000210000
> [    3.105209][    T1] Faulting instruction address: 0xc000000000596394
> cpu 0x0: Vector: 300 (Data Access) at [c000000006d86aa0]
>    lr: c0000000005962c4: memmap_init_compound+0xac/0x2a8
>    sp: c000000006d86d40
>   msr: 8000000002009033
>   dar: c00c000000210000
> dsisr: a000000
>  current = 0xc000000006d33e00
>  paca    = 0xc000000005010000   irqmask: 0x03   irq_happened: 0x01
>    pid   = 1, comm = swapper/0
> Linux version 7.3.0-17056-gc0944ee4a2c4 (powerpc64le-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #111 SMP PREEMPT Mon Nov  9 07:25:29 IST 2026
> enter ? for help
> [c000000006d86e00] c0000000005968f8 memmap_init_zone_device+0x438/0x470
> [c000000006d86f10] c0000000006d72b8 memremap_pages+0x3b0/0x974
> [c000000006d86fe0] c0000000006d78b8 devm_memremap_pages+0x3c/0xa4
> [c000000006d87020] c00000000125c084 dev_dax_probe+0x264/0x3d0
> [c000000006d870c0] c000000001259a28 dax_bus_probe+0x140/0x1c8
> [c000000006d87110] c0000000011ce640 really_probe+0x108/0x580
> [c000000006d871a0] c0000000011ceb84 __driver_probe_device+0xcc/0x284
> [c000000006d87230] c0000000011ceea0 driver_probe_device+0x5c/0x10c
> [c000000006d87270] c0000000011cf05c __device_attach_driver+0x10c/0x1e8
> [c000000006d872f0] c0000000011cab70 bus_for_each_drv+0xc0/0x13c
> [c000000006d87350] c0000000011cf65c __device_attach+0xdc/0x2b0
> [c000000006d873f0] c0000000011cfaac device_initial_probe+0x80/0x98
> [c000000006d87420] c0000000011cac44 __bus_probe_device+0x58/0xf4
> [c000000006d87470] c0000000011c8030 device_add+0x850/0xa78
> [c000000006d87530] c00000000125b040 __devm_create_dev_dax+0x1c0/0x638
> [c000000006d875d0] c00000000125b6c8 devm_create_dev_dax+0x44/0x7c
> [c000000006d87600] c00000000125dbe8 __dax_pmem_probe+0x238/0x27c
> [c000000006d87770] c00000000125dc4c dax_pmem_probe+0x20/0x44
> [c000000006d87790] c000000001237a14 nvdimm_bus_probe+0xcc/0x370
> [c000000006d87830] c0000000011ce640 really_probe+0x108/0x580
> <...>
> 
> Feel free to add:
> 
> Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Thanks for testing again.

Muchun