mm/page_alloc: rename ALLOC_HIGH to ALLOC_MIN_RESERVE

Patch series "Discard __GFP_ATOMIC", v3.

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
orthogonal 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.

This patch (of 6):

__GFP_HIGH aliases to ALLOC_HIGH but the name does not really hint what it
means.  As ALLOC_HIGH is internal to the allocator, rename it to
ALLOC_MIN_RESERVE to document that the min reserves can be depleted.

Link: https://lkml.kernel.org/r/20230113111217.14134-1-mgorman@techsingularity.net
Link: https://lkml.kernel.org/r/20230113111217.14134-2-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/internal.h b/mm/internal.h
index 973b48e8b1af2af4430e2dc0b52b7da6a4d30ffd..99eb544fbded94a2eca606919b6c10490d14147e 100644 (file)
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -779,7 +779,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
 #endif
 
 #define ALLOC_HARDER            0x10 /* try to alloc harder */
-#define ALLOC_HIGH              0x20 /* __GFP_HIGH set */
+#define ALLOC_MIN_RESERVE       0x20 /* __GFP_HIGH set. Allow access to 50%
+                                      * of the min watermark.
+                                      */
 #define ALLOC_CPUSET            0x40 /* check for correct cpuset */
 #define ALLOC_CMA               0x80 /* allow allocations from CMA areas */
 #ifdef CONFIG_ZONE_DMA32

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 83be3b571fd0dab40c1f7300ce01d434051f4852..4c1f1d487c3e796c3488cfe4af0f9b39d8eacb1f 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3994,7 +3994,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
        /* free_pages may go negative - that's OK */
        free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
 
-       if (alloc_flags & ALLOC_HIGH)
+       if (alloc_flags & ALLOC_MIN_RESERVE)
                min -= min / 2;
 
        if (unlikely(alloc_harder)) {
@@ -4836,18 +4836,18 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
        unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
 
        /*
-        * __GFP_HIGH is assumed to be the same as ALLOC_HIGH
+        * __GFP_HIGH is assumed to be the same as ALLOC_MIN_RESERVE
         * and __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
         * to save two branches.
         */
-       BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
+       BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_MIN_RESERVE);
        BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);
 
        /*
         * The caller may dip into page reserves a bit more if the caller
         * cannot run direct reclaim, or if the caller has realtime scheduling
         * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
-        * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_HIGH (__GFP_HIGH).
+        * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_MIN_RESERVE(__GFP_HIGH).
         */
        alloc_flags |= (__force int)
                (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));