linux-kernel - Re: [PATCH v2] mm/huge_memory: Avoid PMD-size page cache if needed

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Message-ID: <ea673fe9-beed-43fd-9a54-c683b0c800ce@arm.com>
Date: Mon, 15 Jul 2024 11:41:03 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 david@...hat.com, willy@...radead.org, baohua@...nel.org, peterx@...hat.com,
 ziy@...dia.com, william.kucharski@...cle.com, baolin.wang@...ux.alibaba.com,
 ddutile@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v2] mm/huge_memory: Avoid PMD-size page cache if needed

[...]

> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2aa986a5cd1b..c73ad77fa33d 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -72,14 +72,20 @@ extern struct kobj_attribute shmem_enabled_attr;
>  #define THP_ORDERS_ALL_ANON	((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
>  
>  /*
> - * Mask of all large folio orders supported for file THP.
> + * Mask of all large folio orders supported for file THP. Folios in a DAX
> + * file is never split and the MAX_PAGECACHE_ORDER limit does not apply to
> + * it.
>   */
> -#define THP_ORDERS_ALL_FILE	(BIT(PMD_ORDER) | BIT(PUD_ORDER))
> +#define THP_ORDERS_ALL_FILE_DAX		\
> +	(BIT(PMD_ORDER) | BIT(PUD_ORDER))

Appologies if this was already discussed, but if changing _FILE_DEFAULT to
advertise all orders 1-MAX_PAGECACHE_ORDER, shouldn't we also change _FILE_DAX
to advertise all orders 1-PUD_ORDER ? Or is DAX literally limited to PTE/PMD/PUD?

> +#define THP_ORDERS_ALL_FILE_DEFAULT	\
> +	((BIT(MAX_PAGECACHE_ORDER + 1) - 1) & ~BIT(0))
>  
>  /*
>   * Mask of all large folio orders supported for THP.
>   */
> -#define THP_ORDERS_ALL		(THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE)
> +#define THP_ORDERS_ALL	\
> +	(THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE_DAX | THP_ORDERS_ALL_FILE_DEFAULT)
>  
>  #define TVA_SMAPS		(1 << 0)	/* Will be used for procfs */
>  #define TVA_IN_PF		(1 << 1)	/* Page fault handler */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2120f7478e55..4690f33afaa6 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -88,9 +88,17 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	bool smaps = tva_flags & TVA_SMAPS;
>  	bool in_pf = tva_flags & TVA_IN_PF;
>  	bool enforce_sysfs = tva_flags & TVA_ENFORCE_SYSFS;
> +	unsigned long supported_orders;
> +
>  	/* Check the intersection of requested and supported orders. */
> -	orders &= vma_is_anonymous(vma) ?
> -			THP_ORDERS_ALL_ANON : THP_ORDERS_ALL_FILE;
> +	if (vma_is_anonymous(vma))
> +		supported_orders = THP_ORDERS_ALL_ANON;
> +	else if (vma_is_dax(vma))
> +		supported_orders = THP_ORDERS_ALL_FILE_DAX;
> +	else
> +		supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
> +
> +	orders &= supported_orders;
>  	if (!orders)
>  		return 0;
>