linux-kernel - [PATCH] mm: Remove stub for non_swap

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Date:   Wed, 13 Apr 2022 15:11:47 -0400
From:   Peter Xu <peterx@...hat.com>
To:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Peter Xu <peterx@...hat.com>, Alistair Popple <apopple@...dia.com>
Subject: [PATCH] mm: Remove stub for non_swap_entry()

The stub for non_swap_entry() may not help much, because MAX_SWAPFILES has
already contained all the information to decide whether a swap entry is
real swap entry of pesudo ones (migrations, ...).

There can be some performance influences on non_swap_entry() with below
conditions all met:

  !CONFIG_MIGRATION && !CONFIG_MEMORY_FAILURE && !CONFIG_DEVICE_PRIVATE

But that's definitely not the major config most machines will use, at the
meantime it's already in a slow path of swap entry (being parsed from a
swap pte), so IMHO it shouldn't be a major issue.  Also according to the
analysis from Alistair, somehow the stub didn't do the job right [1].

To make the code cleaner, let's drop the stub.

[1] https://lore.kernel.org/lkml/8735ihbw6g.fsf@nvdebian.thelocal/

Cc: Alistair Popple <apopple@...dia.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---

Note: the uffd-wp shmem & hugetlbfs series will need this patch to make
sure swap entries work as expected with below config as spotted by
Alistair:

  !CONFIG_MIGRATION &&
  !CONFIG_MEMORY_FAILURE &&
  !CONFIG_DEVICE_PRIVATE &&
  CONFIG_PTE_MARKER

(PS: this config should mostly never gonna happen, though, afaict..)

Quoting the same thread [1] as above.

Please review, thanks.
---
 include/linux/swapops.h | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index fffbba0036f6..a291f210e7f8 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -493,18 +493,10 @@ static inline void num_poisoned_pages_inc(void)
 }
 #endif
 
-#if defined(CONFIG_MEMORY_FAILURE) || defined(CONFIG_MIGRATION) || \
-    defined(CONFIG_DEVICE_PRIVATE)
 static inline int non_swap_entry(swp_entry_t entry)
 {
 	return swp_type(entry) >= MAX_SWAPFILES;
 }
-#else
-static inline int non_swap_entry(swp_entry_t entry)
-{
-	return 0;
-}
-#endif
 
 #endif /* CONFIG_MMU */
 #endif /* _LINUX_SWAPOPS_H */
-- 
2.32.0