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cmd/internal/bio: resolve todo to query vm.max_map_count on linux for mmapLimit #65107

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cmd/internal/bio: resolve todo to query vm.max_map_count on linux for mmapLimit #65107

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21 changes: 19 additions & 2 deletions src/cmd/internal/bio/buf_mmap.go
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,10 @@
package bio

import (
"os"
"runtime"
"strconv"
"strings"
"sync/atomic"
"syscall"
)
Expand All @@ -20,18 +23,32 @@ import (
// Darwin unlimited
// DragonFly 1000000 (vm.max_proc_mmap)
// FreeBSD unlimited
// Linux 65530 (vm.max_map_count) // TODO: query /proc/sys/vm/max_map_count?
// Linux reads from /proc/sys/vm/max_map_count
// NetBSD unlimited
// OpenBSD unlimited
var mmapLimit int32 = 1<<31 - 1

func init() {
// Linux is the only practically concerning OS.
if runtime.GOOS == "linux" {
mmapLimit = 30000
mmapLimit = getLinuxMaxMapCount()
}
}

func getLinuxMaxMapCount() int32 {
data, err := os.ReadFile("/proc/sys/vm/max_map_count")
if err != nil {
return 1<<31 - 1
}

maxMapCount, err := strconv.Atoi(strings.TrimSpace(string(data)))
if err != nil {
return 1<<31 - 1
}

return int32(maxMapCount)
}

func (r *Reader) sliceOS(length uint64) ([]byte, bool) {
// For small slices, don't bother with the overhead of a
// mapping, especially since we have no way to unmap it.
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