xz: Limit --memlimit-compress to at most 4020 MiB for 32-bit xz.

See the code comment for reasoning. It's far from perfect but
hopefully good enough for certain cases while hopefully doing
nothing bad in other situations.

At presets -5 ... -9, 4020 MiB vs. 4096 MiB makes no difference
on how xz scales down the number of threads.

The limit has to be a few MiB below 4096 MiB because otherwise
things like "xz --lzma2=dict=500MiB" won't scale down the dict
size enough and xz cannot allocate enough memory. With
"ulimit -v $((4096 * 1024))" on x86-64, the limit in xz had
to be no more than 4085 MiB. Some safety margin is good though.

This is hack but it should be useful when running 32-bit xz on
a 64-bit kernel that gives full 4 GiB address space to xz.
Hopefully this is enough to solve this:

https://bugzilla.redhat.com/show_bug.cgi?id=1196786

FreeBSD has a patch that limits the result in tuklib_physmem()
to SIZE_MAX on 32-bit systems. While I think it's not the way
to do it, the results on --memlimit-compress have been good. This
commit should achieve practically identical results for compression
while leaving decompression and tuklib_physmem() and thus
lzma_physmem() unaffected.

1 files changed, 20 insertions, 1 deletions

diff --git a/src/xz/xz.1 b/src/xz/xz.1
index 6b949640..540d1364 100644
--- a/src/xz/xz.1
+++ b/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 "2019-05-11" "Tukaani" "XZ Utils"
+.TH XZ 1 "2020-02-01" "Tukaani" "XZ Utils"
 .
 .SH NAME
 xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
@@ -1005,6 +1005,25 @@ instead of
 until the details have been decided.
 .RE
 .IP ""
+For 32-bit
+.BR xz
+there is a special case: if the
+.I limit
+would be over
+.BR "4020\ MiB" ,
+the
+.I limit
+is set to
+.BR "4020\ MiB" .
+(The values
+.B 0
+and
+.B max
+aren't affected by this.
+A similar feature doesn't exist for decompression.)
+This can be helpful when a 32-bit executable has access
+to 4\ GiB address space while hopefully doing no harm in other situations.
+.IP ""
 See also the section
 .BR "Memory usage" .
 .TP