linux-ext4 - Re: [PATCH] mm: split vm_normal_pages for LRU and non-LRU handling

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Date:   Tue, 1 Mar 2022 09:03:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alex Sierra <alex.sierra@....com>, jgg@...dia.com
Cc:     Felix.Kuehling@....com, linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] mm: split vm_normal_pages for LRU and non-LRU handling

On 28.02.22 21:34, Alex Sierra wrote:
> DEVICE_COHERENT pages introduce a subtle distinction in the way
> "normal" pages can be used by various callers throughout the kernel.
> They behave like normal pages for purposes of mapping in CPU page
> tables, and for COW. But they do not support LRU lists, NUMA
> migration or THP. Therefore we split vm_normal_page into two
> functions vm_normal_any_page and vm_normal_lru_page. The latter will
> only return pages that can be put on an LRU list and that support
> NUMA migration and THP.

Why not s/vm_normal_any_page/vm_normal_page/ and avoid code churn?

> 
> We also introduced a FOLL_LRU flag that adds the same behaviour to
> follow_page and related APIs, to allow callers to specify that they
> expect to put pages on an LRU list.

[...]
> -#define FOLL_WRITE	0x01	/* check pte is writable */
> -#define FOLL_TOUCH	0x02	/* mark page accessed */
> -#define FOLL_GET	0x04	/* do get_page on page */
> -#define FOLL_DUMP	0x08	/* give error on hole if it would be zero */
> -#define FOLL_FORCE	0x10	/* get_user_pages read/write w/o permission */
> -#define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
> -				 * and return without waiting upon it */
> -#define FOLL_POPULATE	0x40	/* fault in pages (with FOLL_MLOCK) */
> -#define FOLL_NOFAULT	0x80	/* do not fault in pages */
> -#define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
> -#define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
> -#define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
> -#define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
> -#define FOLL_MLOCK	0x1000	/* lock present pages */
> -#define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
> -#define FOLL_COW	0x4000	/* internal GUP flag */
> -#define FOLL_ANON	0x8000	/* don't do file mappings */
> -#define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
> -#define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
> -#define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
> -#define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
> +#define FOLL_WRITE	0x01	 /* check pte is writable */
> +#define FOLL_TOUCH	0x02	 /* mark page accessed */
> +#define FOLL_GET	0x04	 /* do get_page on page */
> +#define FOLL_DUMP	0x08	 /* give error on hole if it would be zero */
> +#define FOLL_FORCE	0x10	 /* get_user_pages read/write w/o permission */
> +#define FOLL_NOWAIT	0x20	 /* if a disk transfer is needed, start the IO
> +				  * and return without waiting upon it */
> +#define FOLL_POPULATE	0x40	 /* fault in pages (with FOLL_MLOCK) */
> +#define FOLL_NOFAULT	0x80	 /* do not fault in pages */
> +#define FOLL_HWPOISON	0x100	 /* check page is hwpoisoned */
> +#define FOLL_NUMA	0x200	 /* force NUMA hinting page fault */
> +#define FOLL_MIGRATION	0x400	 /* wait for page to replace migration entry */
> +#define FOLL_TRIED	0x800	 /* a retry, previous pass started an IO */
> +#define FOLL_MLOCK	0x1000	 /* lock present pages */
> +#define FOLL_REMOTE	0x2000	 /* we are working on non-current tsk/mm */
> +#define FOLL_COW	0x4000	 /* internal GUP flag */
> +#define FOLL_ANON	0x8000	 /* don't do file mappings */
> +#define FOLL_LONGTERM	0x10000	 /* mapping lifetime is indefinite: see below */
> +#define FOLL_SPLIT_PMD	0x20000	 /* split huge pmd before returning */
> +#define FOLL_PIN	0x40000	 /* pages must be released via unpin_user_page */
> +#define FOLL_FAST_ONLY	0x80000	 /* gup_fast: prevent fall-back to slow gup */
> +#define FOLL_LRU	0x100000 /* return only LRU (anon or page cache) */
>  

Can we minimize code churn, please?


>  		if (PageReserved(page))
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c31d04b46a5e..17d049311b78 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1614,7 +1614,7 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>  		goto out;
>  
>  	/* FOLL_DUMP to ignore special (like zero) pages */
> -	follflags = FOLL_GET | FOLL_DUMP;
> +	follflags = FOLL_GET | FOLL_DUMP | FOLL_LRU;
>  	page = follow_page(vma, addr, follflags);

Why wouldn't we want to dump DEVICE_COHERENT pages? This looks wrong.


-- 
Thanks,

David / dhildenb