linux-kernel - [PATCH v4 3/4] mm/swap: remove confusing checking for non_swap_entry() in swap_ra

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Date:   Sun, 25 Apr 2021 10:38:05 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <ying.huang@...el.com>, <dennis@...nel.org>,
        <tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
        <hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <willy@...radead.org>, <minchan@...nel.org>,
        <richard.weiyang@...il.com>, <shy828301@...il.com>,
        <david@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <linmiaohe@...wei.com>
Subject: [PATCH v4 3/4] mm/swap: remove confusing checking for non_swap_entry() in swap_ra_info()

The non_swap_entry() was used for working with VMA based swap readahead
via commit ec560175c0b6 ("mm, swap: VMA based swap readahead"). At that
time, the non_swap_entry() checking is necessary because the function is
called before checking that in do_swap_page(). Then it's moved to
swap_ra_info() since commit eaf649ebc3ac ("mm: swap: clean up swap
readahead"). After that, the non_swap_entry() checking is unnecessary,
because swap_ra_info() is called after non_swap_entry() has been checked
already. The resulting code is confusing as the non_swap_entry() check
looks racy now because while we released the pte lock, somebody else might
have faulted in this pte. So we should check whether it's swap pte first
to guard against such race or swap_type will be unexpected. But the race
isn't important because it will not cause problem. We would have enough
checking when we really operate the PTE entries later. So we remove the
non_swap_entry() check here to avoid confusion.

Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/swap_state.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 272ea2108c9d..df5405384520 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -721,7 +721,6 @@ static void swap_ra_info(struct vm_fault *vmf,
 {
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long ra_val;
-	swp_entry_t entry;
 	unsigned long faddr, pfn, fpfn;
 	unsigned long start, end;
 	pte_t *pte, *orig_pte;
@@ -739,11 +738,6 @@ static void swap_ra_info(struct vm_fault *vmf,

 	faddr = vmf->address;
 	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
-	entry = pte_to_swp_entry(*pte);
-	if ((unlikely(non_swap_entry(entry)))) {
-		pte_unmap(orig_pte);
-		return;
-	}

 	fpfn = PFN_DOWN(faddr);
 	ra_val = GET_SWAP_RA_VAL(vma);
-- 
2.19.1