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

[Joao Martins]

On 16/10/2026 06:49, Muchun Song wrote:
> 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
> 
> 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 advancing the pgmap start to the aligned Device DAX data
> range for non-altmap compound mappings. Leave base-page namespaces
> unchanged, and leave the altmap path unchanged because
> memmap_init_zone_device() already skips the altmap reservation itself.
> 
> Fixes: 14606001efb4 ("device-dax: compound devmap support")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Reported-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> Closes: https://lore.kernel.org/r/wlrkqqvg.ritesh.list@xxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> --->  drivers/dax/pmem.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
> index bee93066a849..d3a9b85107b0 100644
> --- a/drivers/dax/pmem.c
> +++ b/drivers/dax/pmem.c
> @@ -51,6 +51,13 @@ static struct dev_dax *__dax_pmem_probe(struct device *dev)
>  	/* adjust the dax_region range to the start of data */
>  	range = pgmap.range;
>  	range.start += offset;
> +	/*
> +	 * When the memmap is stored in RAM, exclude metadata and alignment
> +	 * padding from compound page initialization.  Otherwise, the pgmap
> +	 * may start before the naturally aligned Device DAX data range.
> +	 */
> +	if (!pgmap_altmap(&pgmap) && le32_to_cpu(pfn_sb->align) > PAGE_SIZE)
> +		pgmap.range.start = range.start;

Should we need to check here for pgmap.type == MEMORY_DEVICE_GENERIC ?

Considering the dedup is not quite applicable for FSDAX.