Split memmap.MMapOpts.Precommit into an enum.

`Precommit=false` allows MM to delay creating PMAs (roughly speaking, host
mmaps) until they are needed (e.g. page faults). `Precommit=true` requires MM
to create PMAs, but also requires them to be precommitted (roughly speaking,
mmapped with MAP_POPULATE). Introduce an intermediate setting,
memmap.PlatformEffectPopulate, that requires MM to create PMAs but does not
require precommit.

Nothing in this CL uses the new setting, so this should result in no functional
change.

Updates #9593

PiperOrigin-RevId: 577939908

pkg/sentry/memmap/memmap.go

@@ -356,9 +356,7 @@ type MMapOpts struct {
 	// downward on guard page faults.
 	GrowsDown bool
 
-	// Precommit is true if the platform should eagerly commit resources to the
-	// mapping (see platform.AddressSpace.MapFile).
-	Precommit bool
+	PlatformEffect MMapPlatformEffect
 
 	// MLockMode specifies the memory locking behavior of the mapping.
 	MLockMode MLockMode
@@ -382,6 +380,25 @@ type MMapOpts struct {
 	SentryOwnedContent bool
 }
 
+// MMapPlatformEffect is the type of MMapOpts.PlatformEffect.
+type MMapPlatformEffect uint8
+
+// Possible values for MMapOpts.PlatformEffect:
+const (
+	// PlatformEffectDefault indicates that no specific behavior is requested
+	// from the platform.
+	PlatformEffectDefault MMapPlatformEffect = iota
+
+	// PlatformEffectPopulate indicates that platform mappings should be
+	// established for all pages in the mapping.
+	PlatformEffectPopulate
+
+	// PlatformEffectCommit is like PlatformEffectPopulate, but also requests
+	// that the platform eagerly commit resources to the mapping, as in
+	// platform.AddressSpace.MapFile(precommit=true).
+	PlatformEffectCommit
+)
+
 // File represents a host file that may be mapped into an platform.AddressSpace.
 type File interface {
 	// All pages in a File are reference-counted.

pkg/sentry/mm/syscalls.go

@@ -131,9 +131,9 @@ func (mm *MemoryManager) MMap(ctx context.Context, opts memmap.MMapOpts) (hostar
 	// mm/util.c:vm_mmap_pgoff() => mm/gup.c:__mm_populate() =>
 	// populate_vma_page_range(). Confirm this behavior.
 	switch {
-	case opts.Precommit || opts.MLockMode == memmap.MLockEager:
-		// Get pmas and map with precommit as requested.
-		mm.populateVMAAndUnlock(ctx, vseg, ar, true)
+	case opts.PlatformEffect >= memmap.PlatformEffectPopulate || opts.MLockMode == memmap.MLockEager:
+		// Get pmas and map as requested.
+		mm.populateVMAAndUnlock(ctx, vseg, ar, opts.PlatformEffect == memmap.PlatformEffectCommit)
 
 	case opts.Mappable == nil && length <= privateAllocUnit:
 		// NOTE(b/63077076, b/63360184): Get pmas and map eagerly in the hope

pkg/sentry/syscalls/linux/sys_mmap.go

@@ -67,7 +67,9 @@ func Mmap(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *
 		},
 		MaxPerms:  hostarch.AnyAccess,
 		GrowsDown: linux.MAP_GROWSDOWN&flags != 0,
-		Precommit: linux.MAP_POPULATE&flags != 0,
+	}
+	if linux.MAP_POPULATE&flags != 0 {
+		opts.PlatformEffect = memmap.PlatformEffectCommit
 	}
 	if linux.MAP_LOCKED&flags != 0 {
 		opts.MLockMode = memmap.MLockEager