[PATCH 0/3] mm/memory: align alloc_swap_folio() logic with alloc_anon_folio()

[Wei Yang <richard.weiyang@gmail.com>]

This series cleans up the logic used when swapping in large folios to bring it
in line with the established patterns in alloc_anon_folio().

Context:

When swapping in a large folio, the kernel performs a process similar to
alloc_anon_folio(): it scans the PTE range to determine the highest suitable
order for a large folio and then attempts allocation. However, two key
discrepancies currently exist in alloc_swap_folio():

  1. Improper Fallback: It currently falls back to an order-0 folio if
     pte_offset_map_lock() returns NULL.
  2. Unnecessary Locking: It locks the page table during the initial scanning
     phase.

Improvements:

Based on an analysis of the alloc_anon_folio() implementation, this series
applies the following corrections to the swap-in path:

  1. Bail out on NULL PTE: If pte_offset_map_lock() returns NULL, it indicates
     the page table has changed under us. In this scenario, the operation is
     likely to fail later anyway; continuing the process is inefficient and
     unnecessary.

  2. Lockless Initial Scan: The initial scan only reads PTEs to determine the
     appropriate folio order. Since no modifications are made at this stage,
     and the table is re-verified and locked before the actual write, the
     initial lock is redundant. Removing it reduces lock contention and system
     load.

Wei Yang (2):
  mm: bail out do_swap_page() when no PTE table exist
  mm: avoid unnecessary PTE table lock during initial swap folio scan

 mm/memory.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

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2.34.1