linux-kernel - Re: [PATCH 11/28] mm, swap: clean up and consolidate helper for mTHP swapin check

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Message-Id: <20250519070801.2588-1-21cnbao@gmail.com>
Date: Mon, 19 May 2025 19:08:01 +1200
From: Barry Song <21cnbao@...il.com>
To: ryncsn@...il.com
Cc: akpm@...ux-foundation.org,
	baohua@...nel.org,
	baolin.wang@...ux.alibaba.com,
	bhe@...hat.com,
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	david@...hat.com,
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	hughd@...gle.com,
	kaleshsingh@...gle.com,
	kasong@...cent.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	nphamcs@...il.com,
	ryan.roberts@....com,
	shikemeng@...weicloud.com,
	tim.c.chen@...ux.intel.com,
	willy@...radead.org,
	ying.huang@...ux.alibaba.com,
	yosryahmed@...gle.com
Subject: Re: [PATCH 11/28] mm, swap: clean up and consolidate helper for mTHP swapin check

> From: Kairui Song <kasong@...cent.com>


> -static bool can_swapin_thp(struct vm_fault *vmf, pte_t *ptep, int nr_pages)
> +static bool can_swapin_thp(struct vm_fault *vmf, pte_t *ptep,
> +			   unsigned long addr, unsigned int nr_pages)

> +	if (unlikely(addr < max(addr & PMD_MASK, vmf->vma->vm_start) ||
> +		     addr_end > pmd_addr_end(addr, vmf->vma->vm_end)))


> @@ -4731,27 +4732,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	page_idx = 0;
>  	address = vmf->address;
>  	ptep = vmf->pte;
> +
>  	if (folio_test_large(folio) && folio_test_swapcache(folio)) {
> -		int nr = folio_nr_pages(folio);
> +		unsigned long nr = folio_nr_pages(folio);
>  		unsigned long idx = folio_page_idx(folio, page);
> -		unsigned long folio_start = address - idx * PAGE_SIZE;
> -		unsigned long folio_end = folio_start + nr * PAGE_SIZE;
> -		pte_t *folio_ptep;
> -		pte_t folio_pte;
> +		unsigned long folio_address = address - idx * PAGE_SIZE;
> +		pte_t *folio_ptep = vmf->pte - idx;
>  
> -		if (unlikely(folio_start < max(address & PMD_MASK, vma->vm_start)))
> -			goto check_folio;

We are handling a corner case a large folio is remapped to an unaligned address.
For example,

A 64KiB mTHP at address:  XGB + 2MB +4KB,

Its start address will be XGB + 2MB - 60KB which is another PMD.

The previous code will return false; now your can_swapin_thp() will return true
as you are using XGB + 2MB - 60KB as the argument "addr" in can_swapin_thp().

> -		if (unlikely(folio_end > pmd_addr_end(address, vma->vm_end)))
> -			goto check_folio;
> -
> -		folio_ptep = vmf->pte - idx;
> -		folio_pte = ptep_get(folio_ptep);
> -		if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) ||
> -		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> +		if (!can_swapin_thp(vmf, folio_ptep, folio_address, nr))


Thanks
Barry