Increase the default memory usage limit on "low-memory" systems.

Previously the default limit was always 40 % of RAM. The
new limit is a little bit more complex:

  - If 40 % of RAM is at least 80 MiB, 40 % of RAM is used
    as the limit.

  - If 80 % of RAM is over 80 MiB, 80 MiB is used as the limit.

  - Otherwise 80 % of RAM is used as the limit.

This should make it possible to decompress files created with
"xz -9" on more systems. Swapping is generally more expected
on systems with less RAM, so higher default limit on them
shouldn't cause too bad surprises in terms of heavy swapping.
Instead, the higher default limit should reduce the number of
bad surprises when it used to prevent decompression of files
created with "xz -9". The DoS prevention system shouldn't be
a DoS itself.

Note that even with the new default limit, a system with 64 MiB
RAM cannot decompress files created with "xz -9" without user
overriding the limit. This should be OK, because if xz is going
to need more memory than the system has RAM, it will run very
very slowly and thus it's good that user has to override the limit
in that case.

src/xz/hardware.c

@@ -21,6 +21,9 @@ static uint32_t threadlimit;
 /// Memory usage limit
 static uint64_t memlimit;
 
+/// Total amount of physical RAM
+static uint64_t total_ram;
+
 
 extern void
 hardware_threadlimit_set(uint32_t new_threadlimit)
@@ -48,11 +51,27 @@ hardware_threadlimit_get(void)
 extern void
 hardware_memlimit_set(uint64_t new_memlimit)
 {
-	if (new_memlimit == 0) {
+	if (new_memlimit != 0) {
-		// The default is 40 % of total installed physical RAM.
-		hardware_memlimit_set_percentage(40);
-	} else {
 		memlimit = new_memlimit;
+	} else {
+		// The default depends on the amount of RAM but so that
+		// on "low-memory" systems the relative limit is higher
+		// to make it more likely that files created with "xz -9"
+		// will still decompress without overriding the limit
+		// manually.
+		//
+		// If 40 % of RAM is 80 MiB or more, use 40 % of RAM as
+		// the limit.
+		memlimit = 40 * total_ram / 100;
+		if (memlimit < UINT64_C(80) * 1024 * 1024) {
+			// If 80 % of RAM is less than 80 MiB,
+			// use 80 % of RAM as the limit.
+			memlimit = 80 * total_ram / 100;
+			if (memlimit > UINT64_C(80) * 1024 * 1024) {
+				// Otherwise use 80 MiB as the limit.
+				memlimit = UINT64_C(80) * 1024 * 1024;
+			}
+		}
 	}
 
 	return;
@@ -65,14 +84,7 @@ hardware_memlimit_set_percentage(uint32_t percentage)
 	assert(percentage > 0);
 	assert(percentage <= 100);
 
-	uint64_t mem = lzma_physmem();
+	memlimit = percentage * total_ram / 100;
-
-	// If we cannot determine the amount of RAM, use the assumption
-	// defined by the configure script.
-	if (mem == 0)
-		mem = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
-
-	memlimit = percentage * mem / 100;
 	return;
 }
 
@@ -87,6 +99,13 @@ hardware_memlimit_get(void)
 extern void
 hardware_init(void)
 {
+	// Get the amount of RAM. If we cannot determine it,
+	// use the assumption defined by the configure script.
+	total_ram = lzma_physmem();
+	if (total_ram == 0)
+		total_ram = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
+
+	// Set the defaults.
 	hardware_memlimit_set(0);
 	hardware_threadlimit_set(0);
 	return;

src/xz/xz.1

@@ -5,7 +5,7 @@
 .\" This file has been put into the public domain.
 .\" You can do whatever you want with this file.
 .\"
-.TH XZ 1 "2010-01-15" "Tukaani" "XZ Utils"
+.TH XZ 1 "2010-03-07" "Tukaani" "XZ Utils"
 .SH NAME
 xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
 .SH SYNOPSIS
@@ -195,9 +195,16 @@ is several gigabytes.
 .PP
 To prevent uncomfortable surprises caused by huge memory usage,
 .B xz
-has a built-in memory usage limiter. The default limit is 40 % of total
+has a built-in memory usage limiter. While some operating systems provide
-physical RAM. While operating systems provide ways to limit the memory usage
+ways to limit the memory usage of processes, relying on it wasn't deemed
-of processes, relying on it wasn't deemed to be flexible enough.
+to be flexible enough. The default limit depends on the total amount of
+physical RAM:
+.IP \(bu 3
+If 40\ % of RAM is at least 80 MiB, 40\ % of RAM is used as the limit.
+.IP \(bu 3
+If 80\ % of RAM is over 80 MiB, 80 MiB is used as the limit.
+.IP \(bu 3
+Otherwise 80\ % of RAM is used as the limit.
 .PP
 When compressing, if the selected compression settings exceed the memory
 usage limit, the settings are automatically adjusted downwards and a notice
@@ -588,9 +595,11 @@ can be specified as a percentage of physical RAM. Example:
 .IP \(bu 3
 The
 .I limit
-can be reset back to its default value (currently 40 % of physical RAM)
+can be reset back to its default value by setting it to
-by setting it to
 .BR 0 .
+See the section
+.B "Memory usage"
+for how the default limit is defined.
 .IP \(bu 3
 The memory usage limiting can be effectively disabled by setting
 .I limit

src/xzdec/xzdec.1

@@ -4,7 +4,7 @@
 .\" This file has been put into the public domain.
 .\" You can do whatever you want with this file.
 .\"
-.TH XZDEC 1 "2009-06-04" "Tukaani" "XZ Utils"
+.TH XZDEC 1 "2010-03-07" "Tukaani" "XZ Utils"
 .SH NAME
 xzdec, lzmadec \- Small .xz and .lzma decompressors
 .SH SYNOPSIS
@@ -99,8 +99,7 @@ can be specified as a percentage of physical RAM. Example:
 .IP \(bu 3
 The
 .I limit
-can be reset back to its default value (currently 40 % of physical RAM)
+can be reset back to its default value by setting it to
-by setting it to
 .BR 0 .
 .IP \(bu 3
 The memory usage limiting can be effectively disabled by setting
@@ -163,6 +162,7 @@ for executables distributed in typical non-embedded operating system
 distributions. If you need a truly small
 .B .xz
 decompressor, consider using XZ Embedded.
-.\" TODO: Provide URL to XZ Embedded.
 .SH "SEE ALSO"
 .BR xz (1)
+.PP
+XZ Embedded: <http://tukaani.org/xz/embedded.html>

src/xzdec/xzdec.c

@@ -38,6 +38,9 @@
 /// Number of bytes to use memory at maximum
 static uint64_t memlimit;
 
+/// Total amount of physical RAM
+static uint64_t total_ram;
+
 /// Error messages are suppressed if this is zero, which is the case when
 /// --quiet has been given at least twice.
 static unsigned int display_errors = 2;
@@ -103,14 +106,7 @@ version(void)
 static void
 memlimit_set_percentage(uint32_t percentage)
 {
-	uint64_t mem = lzma_physmem();
+	memlimit = percentage * total_ram / 100;
-
-	// If we cannot determine the amount of RAM, use the assumption
-	// set by the configure script.
-	if (mem == 0)
-		mem = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
-
-	memlimit = percentage * mem / 100;
 	return;
 }
 
@@ -120,15 +116,37 @@ memlimit_set_percentage(uint32_t percentage)
 static void
 memlimit_set(uint64_t new_memlimit)
 {
-	if (new_memlimit == 0)
+	if (new_memlimit != 0) {
-		memlimit_set_percentage(40);
-	else
 		memlimit = new_memlimit;
+	} else {
+		memlimit = 40 * total_ram / 100;
+		if (memlimit < UINT64_C(80) * 1024 * 1024) {
+			memlimit = 80 * total_ram / 100;
+			if (memlimit > UINT64_C(80) * 1024 * 1024)
+				memlimit = UINT64_C(80) * 1024 * 1024;
+		}
+	}
 
 	return;
 }
 
 
+/// Get the total amount of physical RAM and set the memory usage limit
+/// to the default value.
+static void
+memlimit_init(void)
+{
+	// If we cannot determine the amount of RAM, use the assumption
+	// defined by the configure script.
+	total_ram = lzma_physmem();
+	if (total_ram == 0)
+		total_ram = (uint64_t)(ASSUME_RAM) * 1024 * 1024;
+
+	memlimit_set(0);
+	return;
+}
+
+
 /// \brief      Convert a string to uint64_t
 ///
 /// This is rudely copied from src/xz/util.c and modified a little. :-(
@@ -422,7 +440,7 @@ main(int argc, char **argv)
 
 	// Set the default memory usage limit. This is needed before parsing
 	// the command line arguments.
-	memlimit_set(0);
+	memlimit_init();
 
 	// Parse the command line options.
 	parse_options(argc, argv);