Re: [PATCH v6 19/26] proc/task_mmu: Mark devdax and fsdax pages as always unpinned

Dan Williams
2025-01-14T02:30:01Z

Alistair Popple wrote:
> The procfs mmu files such as smaps and pagemap currently ignore devdax and
> fsdax pages because these pages are considered special. A future change
> will start treating these as normal pages, meaning they can be exposed via
> smaps and pagemap.
>
> The only difference is that devdax and fsdax pages can never be pinned for
> DMA via FOLL_LONGTERM, so add an explicit check in pte_is_pinned() to
> reflect that.

I don't understand this patch.

pin_user_pages() is also used for Direct-I/O page pinning, so the
comment about FOLL_LONGTERM is wrong, and I otherwise do not understand
what goes wrong if the only pte_is_pinned() user correctly detects the
pin state?