[PATCH 0/6 v2] Discard __GFP_ATOMIC

Mel Gorman
2023-01-09T15:19:05Z

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 | 103 ++++++++++++++++++++-------------
tools/perf/builtin-kmem.c | 1 -
8 files changed, 86 insertions(+), 60 deletions(-)

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2.35.3