linux-kernel - [PATCH 0/6 v3] Discard __GFP

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Date:   Fri, 13 Jan 2023 11:12:11 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...e.com>, NeilBrown <neilb@...e.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 0/6 v3] Discard __GFP_ATOMIC

This replaces the "Discard __GFP_ATOMIC v2" series in mm-unstable. There
are changelog and patch replacements that make -fix patches impractical.

Changelog since v2
o Non-blocking (GFP_NOWAIT) allocations get no reserve access	(mhocko)
o __GFP_NOFAIL before OOM reserve access reduced		(mhocko)
o Changelog clarifications					(mhocko)
o Note that rt_task treatment to be deleted in changelog	(mhocko)
o One ack dropped as the patch changed enough to invalidate it

Changelog since v1
o Split one patch						(vbabka)
o Improve OOM reserve handling					(vbabka)
o Fix __GFP_RECLAIM vs __GFP_DIRECT_RECLAIM			(vbabka)

Neil's patch has been residing in mm-unstable as commit 2fafb4fe8f7a
("mm: discard __GFP_ATOMIC") for a long time and recently brought up
again. Most recently, I was worried that __GFP_HIGH allocations could
use high-order atomic reserves which is unintentional but there was no
response so lets revisit -- this series reworks how min reserves are used,
protects highorder reserves and then finishes with Neil's patch with very
minor modifications so it fits on top.

There was a review discussion on renaming __GFP_DIRECT_RECLAIM to
__GFP_ALLOW_BLOCKING but I didn't think it was that big an issue and is
ortogonal to the removal of __GFP_ATOMIC.

There were some concerns about how the gfp flags affect the min reserves
but it never reached a solid conclusion so I made my own attempt.

The series tries to iron out some of the details on how reserves are
used. ALLOC_HIGH becomes ALLOC_MIN_RESERVE and ALLOC_HARDER becomes
ALLOC_NON_BLOCK and documents how the reserves are affected. For example,
ALLOC_NON_BLOCK (no direct reclaim) on its own allows 25% of the min reserve.
ALLOC_MIN_RESERVE (__GFP_HIGH) allows 50% and both combined allows deeper
access again. ALLOC_OOM allows access to 75%.

High-order atomic allocations are explicitly handled with the caveat that
no __GFP_ATOMIC flag means that any high-order allocation that specifies
GFP_HIGH and cannot enter direct reclaim will be treated as if it was
GFP_ATOMIC.

 Documentation/mm/balance.rst   |   2 +-
 drivers/iommu/tegra-smmu.c     |   4 +-
 include/linux/gfp_types.h      |  12 ++--
 include/trace/events/mmflags.h |   1 -
 lib/test_printf.c              |   8 +--
 mm/internal.h                  |  15 ++++-
 mm/page_alloc.c                | 106 ++++++++++++++++++++-------------
 tools/perf/builtin-kmem.c      |   1 -
 8 files changed, 88 insertions(+), 61 deletions(-)

-- 
2.35.3