Re: [PATCH] fs/dax: get the folio of an entry only after the entry has one

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  1 Sep 2026 21:56:58 +0200 Kiara Grouwstra <cinereal@riseup.net> wrote:

> dax_associate_entry(), dax_disassociate_entry() and dax_busy_page() each
> compute dax_to_folio(entry) in the declaration, one statement before the
> test that returns early when the entry is a zero entry or an empty entry.
> An empty entry holds no pfn, so dax_to_folio() reads vmemmap[0].
> 
> Where the memory map starts at pfn 0 this reads a struct page that exists,
> the value is discarded, and nothing shows. Where the lowest present section
> is above pfn 0 there is no struct page for pfn 0 and the read faults.
> grab_mapping_entry() gives the first fault on a file an empty entry, so on
> such a machine every first DAX fault on a file ends in:
> 
>   BUG: unable to handle page fault for address: ffffea0000000008
>   RIP: 0010:dax_to_folio+0x14/0x60
>    dax_insert_entry+0xb2/0x3c0
>    dax_fault_iter+0x200/0x600
>    dax_iomap_pte_fault+0x193/0x3d0
> 
> Found on a kernel that boots on one high region of system RAM, which has
> no struct page for the memory below it.
> 
> Move each call after the early return. The other callers of dax_to_folio()
> in this file already only run for an entry that holds a pfn.
> 

Thanks.  This is near-identical to
https://lore.kernel.org/all/m2y0enxtzk.fsf@arm.com/, which I have
queued for 7.3-rcX, with cc:stable.

Seunguk's fix was set a month ago, but balls were dropped :(