[Deepin-Kernel-SIG] [linux 6.6.y] [Upstream] mempolicy: remove confusing MPOL_MF_LAZY dead code by opsiff · Pull Request #1869 · deepin-community/kernel

opsiff · 2026-06-16T10:25:08Z

Summary by Sourcery

Clean up mempolicy NUMA balancing code by removing dead support for the MPOL_MF_LAZY flag and clarifying that the flag is unsupported in the UAPI header.

Enhancements:

Remove unused MPOL_MF_LAZY handling paths in mempolicy NUMA balancing code to simplify mbind behavior and queue page walking logic.
Clarify in the UAPI mempolicy header that MPOL_MF_LAZY is an unsupported flag to prevent confusion for userspace.

mainline inclusion from mainline-v6.7-rc1 category: performance v3.8 commit b24f53a ("mm: mempolicy: Add MPOL_MF_LAZY") introduced MPOL_MF_LAZY, and included it in the MPOL_MF_VALID flags; but a720094 ("mm: mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now") immediately removed it from MPOL_MF_VALID flags, pending further review. "This will need to be revisited", but it has not been reinstated. The present state is confusing: there is dead code in mm/mempolicy.c to handle MPOL_MF_LAZY cases which can never occur. Remove that: it can be resurrected later if necessary. But keep the definition of MPOL_MF_LAZY, which must remain in the UAPI, even though it always fails with EINVAL. https://lore.kernel.org/linux-mm/1553041659-46787-1-git-send-email-yang.shi@linux.alibaba.com/ links to a previous request to remove MPOL_MF_LAZY. Link: https://lkml.kernel.org/r/80c9665c-1c3f-17ba-21a3-f6115cebf7d@google.com Signed-off-by: Hugh Dickins <hughd@google.com> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Reviewed-by: Yang Shi <shy828301@gmail.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Christoph Lameter <cl@linux.com> Cc: David Hildenbrand <david@redhat.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: "Huang, Ying" <ying.huang@intel.com> Cc: Kefeng Wang <wangkefeng.wang@huawei.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Michal Hocko <mhocko@suse.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Nhat Pham <nphamcs@gmail.com> Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Tejun heo <tj@kernel.org> Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com> Cc: Yosry Ahmed <yosryahmed@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> (cherry picked from commit 2cafb58) Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

mainline inclusion from mainline-v6.9-rc1 category: bugfix Commit 2cafb58 ("mempolicy: remove confusing MPOL_MF_LAZY dead code") removes MPOL_MF_LAZY handling in queue_pages_test_walk(), and with that, there is no effective use of the local variable endvma in that function remaining. Remove the local variable endvma and its dead code. No functional change. This issue was identified with clang-analyzer's dead stores analysis. Link: https://lkml.kernel.org/r/20240122092504.18377-1-lukas.bulwahn@gmail.com Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com> Cc: Hugh Dickins <hughd@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> (cherry picked from commit 3efbe13) Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

sourcery-ai · 2026-06-16T10:25:13Z

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Removes dead MPOL_MF_LAZY handling and an unused NUMA balancing helper stub from the mempolicy implementation, while clarifying that the MPOL_MF_LAZY flag is unsupported in the UAPI header.

File-Level Changes

Change	Details	Files
Remove unused NUMA balancing helper stub when CONFIG_NUMA_BALANCING is disabled.	Delete the CONFIG_NUMA_BALANCING #else stub implementation of change_prot_numa that always returned 0 Rely solely on the CONFIG_NUMA_BALANCING-enabled implementation of change_prot_numa guarded by the existing #if	`mm/mempolicy.c`
Drop all in-kernel uses of the MPOL_MF_LAZY flag from mempolicy code paths.	Remove endvma helper variable and MPOL_MF_LAZY conditional branch from queue_pages_test_walk, eliminating the lazy NUMA protection update on VMAs Remove propagation of MPOL_MF_LAZY to internal MPOL_F_MOF flag in do_mbind Remove WARN_ON_ONCE check tied to MPOL_MF_LAZY when migrating pages after mbind	`mm/mempolicy.c`
Clarify in the UAPI that MPOL_MF_LAZY is an unsupported flag.	Update the MPOL_MF_LAZY macro comment to explicitly label it as an unsupported flag while keeping the numeric value unchanged	`include/uapi/linux/mempolicy.h`

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sourcery-ai

Hey - I've left some high level feedback:

If MPOL_MF_LAZY is now considered unsupported, consider rejecting it explicitly in do_mbind() (e.g., returning -EINVAL) rather than silently ignoring it, to avoid confusing userspace that may assume the flag has an effect.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- If MPOL_MF_LAZY is now considered unsupported, consider rejecting it explicitly in do_mbind() (e.g., returning -EINVAL) rather than silently ignoring it, to avoid confusing userspace that may assume the flag has an effect.

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Avenger-285714 · 2026-06-16T11:34:51Z

/approve

deepin-ci-robot · 2026-06-16T11:35:14Z

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Avenger-285714

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Copilot

Pull request overview

This PR removes unreachable/dead handling for the MPOL_MF_LAZY mbind flag in mempolicy code, and updates the UAPI header comment to indicate the flag is not supported.

Changes:

Drop MPOL_MF_LAZY-related dead code paths in mm/mempolicy.c (queue-page walking and mbind policy flag handling).
Update the UAPI comment for MPOL_MF_LAZY to reduce userspace confusion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
mm/mempolicy.c	Removes unreachable `MPOL_MF_LAZY` logic (including an unnecessary `change_prot_numa()` stub and lazy handling in `do_mbind()`/page-walk).
include/uapi/linux/mempolicy.h	Clarifies via comment that `MPOL_MF_LAZY` is not supported for mbind.

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 				   to policy */
 #define MPOL_MF_MOVE_ALL (1<<2)	/* Move every page to conform to policy */
-#define MPOL_MF_LAZY	 (1<<3)	/* Modifies '_MOVE:  lazy migrate on fault */
+#define MPOL_MF_LAZY	 (1<<3)	/* UNSUPPORTED FLAG: Lazy migrate on fault */


Hugh Dickins and others added 2 commits June 16, 2026 18:23

deepin-ci-robot requested review from justforlxz and winnscode June 16, 2026 10:25

sourcery-ai Bot reviewed Jun 16, 2026

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Avenger-285714 requested review from Avenger-285714 and Copilot June 16, 2026 11:32

Copilot started reviewing on behalf of Avenger-285714 June 16, 2026 11:33 View session

deepin-ci-robot added the approved label Jun 16, 2026

Copilot AI reviewed Jun 16, 2026

View reviewed changes

Comment thread include/uapi/linux/mempolicy.h

to policy */

#define MPOL_MF_MOVE_ALL (1<<2) /* Move every page to conform to policy */

#define MPOL_MF_LAZY (1<<3) /* Modifies '_MOVE: lazy migrate on fault */

#define MPOL_MF_LAZY (1<<3) /* UNSUPPORTED FLAG: Lazy migrate on fault */

opsiff merged commit 074796f into deepin-community:linux-6.6.y Jun 16, 2026
14 of 16 checks passed

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[Deepin-Kernel-SIG] [linux 6.6.y] [Upstream] mempolicy: remove confusing MPOL_MF_LAZY dead code#1869

[Deepin-Kernel-SIG] [linux 6.6.y] [Upstream] mempolicy: remove confusing MPOL_MF_LAZY dead code#1869
opsiff merged 2 commits into
deepin-community:linux-6.6.yfrom
opsiff:linux-6.6.y-2026-06-12-mempolicy

opsiff commented Jun 16, 2026 •

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