linux-kernel - Re: [PATCH v6 18/26] mm/gup: Don't allow FOLL

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Message-ID: <6785c8f6511d4_20fa2944a@dwillia2-xfh.jf.intel.com.notmuch>
Date: Mon, 13 Jan 2025 18:16:22 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, <akpm@...ux-foundation.org>,
	<dan.j.williams@...el.com>, <linux-mm@...ck.org>
CC: <alison.schofield@...el.com>, Alistair Popple <apopple@...dia.com>,
	<lina@...hilina.net>, <zhang.lyra@...il.com>,
	<gerald.schaefer@...ux.ibm.com>, <vishal.l.verma@...el.com>,
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Subject: Re: [PATCH v6 18/26] mm/gup: Don't allow FOLL_LONGTERM pinning of FS
 DAX pages

Alistair Popple wrote:
> Longterm pinning of FS DAX pages should already be disallowed by
> various pXX_devmap checks. However a future change will cause these
> checks to be invalid for FS DAX pages so make
> folio_is_longterm_pinnable() return false for FS DAX pages.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
> Acked-by: David Hildenbrand <david@...hat.com>
> ---
>  include/linux/mm.h | 4 ++++
>  1 file changed, 4 insertions(+)

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f267b06..01edca9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2078,6 +2078,10 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
>  	if (folio_is_device_coherent(folio))
>  		return false;
>  
> +	/* DAX must also always allow eviction. */

This 'eviction' terminology seems like it was copied from the
device-memory comment, but with fsdax it does not fit. How about:

/*
 * Filesystems can only tolerate transient delays to truncate and
 * hole-punch operations
 */

> +	if (folio_is_fsdax(folio))
> +		return false;
> +

After the comment fixup you can add:

Reviewed-by: Dan Williams <dan.j.williams@...el.com>