From: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
To: Muchun Song <songmuchun@bytedance.com>,
Dan Williams <djbw@kernel.org>,
Vishal Verma <vishal.l.verma@intel.com>,
Dave Jiang <dave.jiang@intel.com>,
Alison Schofield <alison.schofield@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Joao Martins <joao.m.martins@oracle.com>,
Pavithra Prakash <pavrampu@linux.ibm.com>,
Muchun Song <muchun.song@linux.dev>,
stable@vger.kernel.org, nvdimm@lists.linux.dev,
linux-cxl@vger.kernel.org, linux-kernel@vger.kernel.org,
Muchun Song <songmuchun@bytedance.com>,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2] dax/pmem: fix unaligned compound devmap initialization
Date: Mon, 09 Nov 2026 07:27:12 +0530 [thread overview]
Message-ID: <cxseojgn.ritesh.list@gmail.com> (raw)
In-Reply-To: <20261107030126.90894-1-songmuchun@bytedance.com>
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>
next prev parent reply other threads:[~2026-11-09 2:03 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-07 3:01 Muchun Song
2026-11-09 1:57 ` Ritesh Harjani [this message]
2026-11-09 2:34 ` Muchun Song
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