数据库内核月报 - 2015 / 08

PgSQL · 捉虫动态 · 执行大SQL语句提示无效的内存申请大小

背景

我们执行一个大SQL时(长度大于512M),会返回如下错误:

ERROR: invalid memory alloc request size 1073741824

复现

我们首先复现出来这个问题

  1. 创建表

     create table byteatable(id int, obj bytea);
    
  2. 插入512M大对象

     #!/bin/bash
     data='a'
     for ((i=1;i<=29;i++));
     do
     data=$data$data
     done
     echo 'build ok'
     psql -U postgres -d postgres << EOF
     insert into byteatable(id,obj) values (1,"$data");
     EOF
     echo 'OK'
    

执行脚本后就能复现出来了

ERROR: invalid memory alloc request size 1073741824

BUG分析

我们先找到出现这个错误的位置。

源码位置:

void *
MemoryContextAlloc(MemoryContext context, Size size)
{
    void *ret;
    AssertArg(MemoryContextIsValid(context));
    if (!AllocSizeIsValid(size))
      elog(ERROR, "invalid memory alloc request size %zu", size);
    context->isReset = false;
    ret = (*context->methods->alloc) (context, size);
    VALGRIND_MEMPOOL_ALLOC(context, ret, size);
    return ret;
}

#define AllocSizeIsValid(size) ((Size) (size) <= MaxAllocSize)

#define MaxAllocSize ((Size) 0x3fffffff) /* 1 gigabyte - 1 */

这里限制的内存是1G - 1,而我们插入的大SQL需要的内存没有1G,那么为什么还提示非法的申请内存大小呢?通过调试跟踪,我们发现是在词法分析的时刻出错的。主要问题是在词法分析的内存申请机制上,申请的内存肯定不能少于sql的长度,如果当前申请的内存不够用,那么将重新申请当前内存乘以2的内存大小。

int literallen; /* actual current string length */

int literalalloc; /* current allocated buffer size */
//当前申请内存初始化
yyext->literalalloc = 1024;
static void
addlit(char *ytext, int yleng, core_yyscan_t yyscanner)
{
    /* enlarge buffer if needed */
    if ((yyextra->literallen + yleng) >= yyextra->literalalloc)
    {
       do {
           yyextra->literalalloc *= 2;
       } while ((yyextra->literallen + yleng) >= yyextra->literalalloc);
       yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf,
                              yyextra->literalalloc);
     }
     /* append new data */
     memcpy(yyextra->literalbuf + yyextra->literallen, ytext, yleng);
     yyextra->literallen += yleng;
}
static void
addlitchar(unsigned char ychar, core_yyscan_t yyscanner)
{
     /* enlarge buffer if needed */
     if ((yyextra->literallen + 1) >= yyextra->literalalloc)
     {
          yyextra->literalalloc *= 2;
          yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf,
         yyextra->literalalloc);
     }
}

从源码中可以看出,每次申请原申请内存的2倍,即yyextra->literalalloc *= 2; 而最大申请内存限制是

#define MaxAllocSize ((Size) 0x3fffffff) /* 1 gigabyte - 1 */

所以我们在词法分析能申请的最大内存是2^29 = 536870912,如果词法分析SQL语句需要的内存大于536870912,那么申请的内存需要再乘2,就会得到2^30 = 1073741824,超过MaxAllocSize=0x3fffffff= 1073741823。所以会提示错误:

ERROR: invalid memory alloc request size 1073741824

当然不仅仅是插入一个大对象才会引起这个问题,只要是SQL语句长度大于512M都是出现这个错误,我们可以使用select复现:

do language plpgsql $$
declare
  v_text text := 'a';
begin
  for i in 1..29 loop
    v_text:=v_text||v_text;
  end loop;
  execute $_$select '$_$||v_text||$_$'$_$;
  raise notice 'execute a sql large than 512MB success.';
exception when others then
  raise notice 'execute a sql large than 512MB failed.';
end;
$$;

BUG修复

其实申请MaxAllocSize是可行的,通过修改源码实现,每当申请的内存大于MaxAllocSize并且SQL需要的长度小于MaxAllocSize时,我们就申请MaxAllocSize大小的内存。

static void
addlit(char *ytext, int yleng, core_yyscan_t yyscanner)
{
    /* enlarge buffer if needed */
    if ((yyextra->literallen + yleng) >= yyextra->literalalloc)
    {
        do {
                yyextra->literalalloc *= 2;
        } while ((yyextra->literallen + yleng) >= yyextra->literalalloc);
        /* we can not alloc more than MaxAllocSize */
        if (yyextra->literalalloc > MaxAllocSize && (yyextra->literallen + yleng) < MaxAllocSize)
                yyextra->literalalloc = MaxAllocSize;
        yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf,
                                                                                        yyextra->literalalloc);
    }
    /* append new data */
    memcpy(yyextra->literalbuf + yyextra->literallen, ytext, yleng);
    yyextra->literallen += yleng;
}
static void
addlitchar(unsigned char ychar, core_yyscan_t yyscanner)
{
    /* enlarge buffer if needed */
    if ((yyextra->literallen + 1) >= yyextra->literalalloc)
    {
        yyextra->literalalloc *= 2;
        /* we can not alloc more than MaxAllocSize */
        if (yyextra->literalalloc > MaxAllocSize && (yyextra->literallen + 1) < MaxAllocSize)
                yyextra->literalalloc = MaxAllocSize;
        yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf,
                                                                                        yyextra->literalalloc);
    }
    /* append new data */
    yyextra->literalbuf[yyextra->literallen] = ychar;
    yyextra->literallen += 1;
}

修复之后可以正常插入了

$ sh pgtest.sh
build ok
INSERT 0 1
OK