Use Size instead of int64 to track allocated memory

Commit 5dd7fc1519 added block-level memory accounting, but used int64 variable to
track the amount of allocated memory. That is incorrect, because we have Size for
exactly these purposes, but it was mostly harmless until c477f3e449 which changed
how we handle with repalloc() when downsizing the chunk. Previously we've ignored
these cases and just kept using the original chunk, but now we need to update the
accounting, and the code was doing this:

    context->mem_allocated += blksize - oldblksize;

Both blksize and oldblksize are Size (so unsigned) which means the subtraction
underflows, producing a very high positive value. On 64-bit platforms (where Size
has the same size as mem_alllocated) this happens to work because the result wraps
to the right value, but on (some) 32-bit platforms this fails.

This fixes two things - it changes mem_allocated (and related variables) to Size,
and it splits the update to two separate steps, to prevent any underflows.

Discussion: https://www.postgresql.org/message-id/15151.1570163761%40sss.pgh.pa.us

diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index ea23ac2ccb90d7f04bbbba9ba69a46f242ea8659..f729d9b6decc46948494abdc4b3fbfe7e4741feb 100644 (file)
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -1154,7 +1154,9 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
                        return NULL;
                }
 
-               context->mem_allocated += blksize - oldblksize;
+               /* updated separately, not to underflow when (oldblksize > blksize) */
+               context->mem_allocated -= oldblksize;
+               context->mem_allocated += blksize;
 
                block->freeptr = block->endptr = ((char *) block) + blksize;
 
@@ -1427,7 +1429,7 @@ AllocSetCheck(MemoryContext context)
        const char *name = set->header.name;
        AllocBlock      prevblock;
        AllocBlock      block;
-       int64           total_allocated = 0;
+       Size            total_allocated = 0;
 
        for (prevblock = NULL, block = set->blocks;
                 block != NULL;

diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
index 2d24ab68bc037fc7af8c72fd0247e835ad50141c..b60a19f5a5f67be20f586680766211a3b0af5375 100644 (file)
--- a/src/backend/utils/mmgr/generation.c
+++ b/src/backend/utils/mmgr/generation.c
@@ -753,7 +753,7 @@ GenerationCheck(MemoryContext context)
        GenerationContext *gen = (GenerationContext *) context;
        const char *name = context->name;
        dlist_iter      iter;
-       int64           total_allocated = 0;
+       Size            total_allocated = 0;
 
        /* walk all blocks in this context */
        dlist_foreach(iter, &gen->blocks)

diff --git a/src/include/nodes/memnodes.h b/src/include/nodes/memnodes.h
index df0ae3625cb7696252cce3c8d8aaf11c2516232e..854bd5d22568c800ff250f17db5aa786c20c213b 100644 (file)
--- a/src/include/nodes/memnodes.h
+++ b/src/include/nodes/memnodes.h
@@ -79,7 +79,7 @@ typedef struct MemoryContextData
        /* these two fields are placed here to minimize alignment wastage: */
        bool            isReset;                /* T = no space alloced since last reset */
        bool            allowInCritSection; /* allow palloc in critical section */
-       int64           mem_allocated;  /* track memory allocated for this context */
+       Size            mem_allocated;  /* track memory allocated for this context */
        const MemoryContextMethods *methods;    /* virtual function table */
        MemoryContext parent;           /* NULL if no parent (toplevel context) */
        MemoryContext firstchild;       /* head of linked list of children */