[RFC,0/6] Discard __GFP_ATOMIC

Message

Mel Gorman Nov. 29, 2022, 3:16 p.m. UTC

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.