[PATCH 4/4] elf: Don't cache symbol nor relocation tables with mmap

["H.J. Lu" <hjl.tools@gmail.com>]

During a "-j 8" LLVM 17 debug build on a machine with 32GB RAM and 16GB
swap, ld was killed by kernel because of out of memory:

[79437.949336] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/user.slice/user-1000.slice/session-9.scope,task=ld,pid=797431,uid=1000
[79437.949349] Out of memory: Killed process 797431 (ld) total-vm:9219600kB, anon-rss:6558156kB, file-rss:1792kB, shmem-rss:0kB, UID:1000 pgtables:17552kB oom_score_adj:0

Don't cache symbol nor relocation tables if they are mapped in.  Data to
link the 3.5GB clang executable in LLVM 17 debug build on Linux/x86-64
with 32GB RAM is:

		stdio		mmap		improvement
user		86.73		87.02		-0.3%
system		9.55		9.21		3.6%
total		100.40		97.66		0.7%
maximum set(GB)	17.34		13.14		24%
page faults	4047667		3042877		25%

and data to link the 275M cc1plus executable in GCC 14 stage 1 build is:

user		5.41		5.44		-0.5%
system		0.80		0.76		5%
total		6.25		6.26		-0.2%
maximum set(MB)	1323		968		27%
page faults	323451		236371		27%

These improve the overall system performance for parallel build by
reducing memory usage and page faults.

	* linker.c (_bfd_link_keep_memory): Return false if HAVE_MMAP
	is defined.
---
 bfd/linker.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/bfd/linker.c b/bfd/linker.c
index 36cca9624c2..d6145b3272e 100644
--- a/bfd/linker.c
+++ b/bfd/linker.c
@@ -3561,8 +3561,12 @@ _bfd_nolink_bfd_define_start_stop (struct bfd_link_info *info ATTRIBUTE_UNUSED,
    and symbol tables of input files in memory.  */
 
 bool
-_bfd_link_keep_memory (struct bfd_link_info * info)
+_bfd_link_keep_memory (struct bfd_link_info * info ATTRIBUTE_UNUSED)
 {
+#ifdef HAVE_MMAP
+  /* Don't cache symbol nor relocation tables if they are mapped in.  */
+  return false;
+#else
   bfd *abfd;
   bfd_size_type size;
 
@@ -3590,4 +3594,5 @@ _bfd_link_keep_memory (struct bfd_link_info * info)
   while (1);
 
   return true;
+#endif
 }
-- 
2.44.0