[PATCH v5 3/4] elf: Use mmap to map in symbol and relocation tables

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

Add _bfd_mmap_read_temporary to mmap in symbol tables and relocations
whose sizes >= 4 * page size.  Don't cache external relocations when
mmap is used.

When mmap is used to map in all ELF sections, 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		84.28		85.04		-0.9%
system		12.46		10.16		14%
total		96		95.35		0.7%
page faults	4837944		4047667		16%

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

user		5.22		5.27		-1%
system		0.94		0.84		11%
total		6.20		6.13		0.7%
page faults	361272		323377		10%

	* elf.c (bfd_elf_get_elf_syms): Replace bfd_read with
	_bfd_mmap_read_temporary.
	* elflink.c (elf_link_read_relocs_from_section): Add 2 arguments
	to return mmap memory address and size.
	(_bfd_elf_link_info_read_relocs); Replace bfd_read with
	_bfd_mmap_read_temporary.
	(bfd_elf_final_link): Don't cache external relocations when mmap
	is used.
	* libbfd.c (_bfd_mmap_read_temporary ): New.
	* libbfd-in.h (_bfd_mmap_read_temporary): Likewise.
	* libbfd.h: Regenerated.
---
 bfd/elf.c       | 47 +++++++++++++++++++++++++++++++++++------------
 bfd/elflink.c   | 44 ++++++++++++++++++++++++++++++++------------
 bfd/libbfd-in.h |  3 +++
 bfd/libbfd.c    | 33 +++++++++++++++++++++++++++++++++
 bfd/libbfd.h    |  3 +++
 5 files changed, 106 insertions(+), 24 deletions(-)

diff --git a/bfd/elf.c b/bfd/elf.c
index c5e7a7ca2fb..9cf76d910eb 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -464,19 +464,30 @@ bfd_elf_get_elf_syms (bfd *ibfd,
       goto out;
     }
   pos = symtab_hdr->sh_offset + symoffset * extsym_size;
+  size_t alloc_ext_size = amt;
   if (extsym_buf == NULL)
     {
-      alloc_ext = bfd_malloc (amt);
-      extsym_buf = alloc_ext;
+#ifdef USE_MMAP
+      if ((ibfd->flags & BFD_PLUGIN) != 0
+	  || amt < _bfd_minimum_mmap_size)
+	{
+#endif
+	  alloc_ext = bfd_malloc (amt);
+	  extsym_buf = alloc_ext;
+#ifdef USE_MMAP
+	}
+#endif
     }
-  if (extsym_buf == NULL
-      || bfd_seek (ibfd, pos, SEEK_SET) != 0
-      || bfd_read (extsym_buf, amt, ibfd) != amt)
+
+  if (bfd_seek (ibfd, pos, SEEK_SET) != 0
+      || !_bfd_mmap_read_temporary (&extsym_buf, &alloc_ext_size,
+				    &alloc_ext, ibfd))
     {
       intsym_buf = NULL;
       goto out;
     }
 
+  size_t alloc_extshndx_size = 0;
   if (shndx_hdr == NULL || shndx_hdr->sh_size == 0)
     extshndx_buf = NULL;
   else
@@ -487,15 +498,27 @@ bfd_elf_get_elf_syms (bfd *ibfd,
 	  intsym_buf = NULL;
 	  goto out;
 	}
+      alloc_extshndx_size = amt;
       pos = shndx_hdr->sh_offset + symoffset * sizeof (Elf_External_Sym_Shndx);
       if (extshndx_buf == NULL)
 	{
-	  alloc_extshndx = (Elf_External_Sym_Shndx *) bfd_malloc (amt);
-	  extshndx_buf = alloc_extshndx;
+#ifdef USE_MMAP
+	  if ((ibfd->flags & BFD_PLUGIN) != 0
+	      || amt < _bfd_minimum_mmap_size)
+	    {
+#endif
+	      alloc_extshndx
+		= (Elf_External_Sym_Shndx *) bfd_malloc (amt);
+	      extshndx_buf = alloc_extshndx;
+#ifdef USE_MMAP
+	    }
+#endif
 	}
-      if (extshndx_buf == NULL
-	  || bfd_seek (ibfd, pos, SEEK_SET) != 0
-	  || bfd_read (extshndx_buf, amt, ibfd) != amt)
+      if (bfd_seek (ibfd, pos, SEEK_SET) != 0
+	  || !_bfd_mmap_read_temporary ((void **) &extshndx_buf,
+					&alloc_extshndx_size,
+					(void **) &alloc_extshndx,
+					ibfd))
 	{
 	  intsym_buf = NULL;
 	  goto out;
@@ -534,8 +557,8 @@ bfd_elf_get_elf_syms (bfd *ibfd,
       }
 
  out:
-  free (alloc_ext);
-  free (alloc_extshndx);
+  _bfd_munmap_readonly_temporary (alloc_ext, alloc_ext_size);
+  _bfd_munmap_readonly_temporary (alloc_extshndx, alloc_extshndx_size);
 
   return intsym_buf;
 }
diff --git a/bfd/elflink.c b/bfd/elflink.c
index 216c124b207..2a0586722e8 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -2644,8 +2644,11 @@ _bfd_elf_link_assign_sym_version (struct elf_link_hash_entry *h, void *data)
    may be either a REL or a RELA section.  The relocations are
    translated into RELA relocations and stored in INTERNAL_RELOCS,
    which should have already been allocated to contain enough space.
-   The EXTERNAL_RELOCS are a buffer where the external form of the
-   relocations should be stored.
+   The *EXTERNAL_RELOCS_P are a buffer where the external form of the
+   relocations should be stored.  If *EXTERNAL_RELOCS_ADDR is NULL,
+   *EXTERNAL_RELOCS_ADDR and *EXTERNAL_RELOCS_SIZE returns the mmap
+   memory address and size.  Otherwise, *EXTERNAL_RELOCS_ADDR is
+   unchanged and *EXTERNAL_RELOCS_SIZE returns 0.
 
    Returns FALSE if something goes wrong.  */
 
@@ -2653,7 +2656,8 @@ static bool
 elf_link_read_relocs_from_section (bfd *abfd,
 				   asection *sec,
 				   Elf_Internal_Shdr *shdr,
-				   void *external_relocs,
+				   void **external_relocs_addr,
+				   size_t *external_relocs_size,
 				   Elf_Internal_Rela *internal_relocs)
 {
   const struct elf_backend_data *bed;
@@ -2663,13 +2667,17 @@ elf_link_read_relocs_from_section (bfd *abfd,
   Elf_Internal_Rela *irela;
   Elf_Internal_Shdr *symtab_hdr;
   size_t nsyms;
+  void *external_relocs = *external_relocs_addr;
 
   /* Position ourselves at the start of the section.  */
   if (bfd_seek (abfd, shdr->sh_offset, SEEK_SET) != 0)
     return false;
 
   /* Read the relocations.  */
-  if (bfd_read (external_relocs, shdr->sh_size, abfd) != shdr->sh_size)
+  *external_relocs_size = shdr->sh_size;
+  if (!_bfd_mmap_read_temporary (&external_relocs,
+				 external_relocs_size,
+				 external_relocs_addr, abfd))
     return false;
 
   symtab_hdr = &elf_tdata (abfd)->symtab_hdr;
@@ -2754,6 +2762,7 @@ _bfd_elf_link_info_read_relocs (bfd *abfd,
 				bool keep_memory)
 {
   void *alloc1 = NULL;
+  size_t alloc1_size;
   Elf_Internal_Rela *alloc2 = NULL;
   const struct elf_backend_data *bed = get_elf_backend_data (abfd);
   struct bfd_elf_section_data *esdo = elf_section_data (o);
@@ -2791,17 +2800,26 @@ _bfd_elf_link_info_read_relocs (bfd *abfd,
       if (esdo->rela.hdr)
 	size += esdo->rela.hdr->sh_size;
 
-      alloc1 = bfd_malloc (size);
-      if (alloc1 == NULL)
-	goto error_return;
-      external_relocs = alloc1;
+#ifdef USE_MMAP
+      if (size < _bfd_minimum_mmap_size)
+	{
+#endif
+	  alloc1 = bfd_malloc (size);
+	  if (alloc1 == NULL)
+	    goto error_return;
+	  external_relocs = alloc1;
+#ifdef USE_MMAP
+	}
+#endif
     }
+  else
+    alloc1 = external_relocs;
 
   internal_rela_relocs = internal_relocs;
   if (esdo->rel.hdr)
     {
       if (!elf_link_read_relocs_from_section (abfd, o, esdo->rel.hdr,
-					      external_relocs,
+					      &alloc1, &alloc1_size,
 					      internal_relocs))
 	goto error_return;
       external_relocs = (((bfd_byte *) external_relocs)
@@ -2812,7 +2830,7 @@ _bfd_elf_link_info_read_relocs (bfd *abfd,
 
   if (esdo->rela.hdr
       && (!elf_link_read_relocs_from_section (abfd, o, esdo->rela.hdr,
-					      external_relocs,
+					      &alloc1, &alloc1_size,
 					      internal_rela_relocs)))
     goto error_return;
 
@@ -2820,7 +2838,7 @@ _bfd_elf_link_info_read_relocs (bfd *abfd,
   if (keep_memory)
     esdo->relocs = internal_relocs;
 
-  free (alloc1);
+  _bfd_munmap_readonly_temporary (alloc1, alloc1_size);
 
   /* Don't free alloc2, since if it was allocated we are passing it
      back (under the name of internal_relocs).  */
@@ -2828,7 +2846,7 @@ _bfd_elf_link_info_read_relocs (bfd *abfd,
   return internal_relocs;
 
  error_return:
-  free (alloc1);
+  _bfd_munmap_readonly_temporary (alloc1, alloc1_size);
   if (alloc2 != NULL)
     {
       if (keep_memory)
@@ -12741,12 +12759,14 @@ bfd_elf_final_link (bfd *abfd, struct bfd_link_info *info)
 	goto error_return;
     }
 
+#ifndef USE_MMAP
   if (max_external_reloc_size != 0)
     {
       flinfo.external_relocs = bfd_malloc (max_external_reloc_size);
       if (flinfo.external_relocs == NULL)
 	goto error_return;
     }
+#endif
 
   if (max_internal_reloc_count != 0)
     {
diff --git a/bfd/libbfd-in.h b/bfd/libbfd-in.h
index c5a79cf932c..9d5396a0354 100644
--- a/bfd/libbfd-in.h
+++ b/bfd/libbfd-in.h
@@ -905,6 +905,9 @@ extern void _bfd_munmap_readonly_temporary
 #define _bfd_munmap_readonly_temporary(ptr, rsize) free (ptr)
 #endif
 
+extern bool _bfd_mmap_read_temporary
+  (void **, size_t *, void **, bfd *) ATTRIBUTE_HIDDEN;
+
 static inline void *
 _bfd_malloc_and_read (bfd *abfd, bfd_size_type asize, bfd_size_type rsize)
 {
diff --git a/bfd/libbfd.c b/bfd/libbfd.c
index 98b8d113d35..cff44a1f7bd 100644
--- a/bfd/libbfd.c
+++ b/bfd/libbfd.c
@@ -1180,6 +1180,39 @@ _bfd_mmap_readonly_persistent (bfd *abfd, size_t rsize)
 }
 #endif
 
+/* Attempt to read *SIZE_P bytes from ABFD's iostream to *DATA_P.
+   Return true if the full the amount has been read.  If *DATA_P is
+   NULL, mmap should be used, return the memory address at the
+   current offset in *DATA_P as well as return mmap address and size
+   in *MMAP_BASE and *SIZE_P.  Otherwise, return NULL in *MMAP_BASE
+   and 0 in *SIZE_P.  */
+
+bool
+_bfd_mmap_read_temporary (void **data_p, size_t *size_p,
+			  void **mmap_base, bfd *abfd)
+{
+  void *data = *data_p;
+  size_t size = *size_p;
+
+#ifdef USE_MMAP
+  if (data == NULL)
+    {
+      data = _bfd_mmap_readonly_temporary (abfd, size, mmap_base,
+					   size_p);
+      if (data == NULL || data == (void *) -1)
+	abort ();
+      *data_p = data;
+      return true;
+    }
+  else
+#endif
+    {
+      *mmap_base = NULL;
+      *size_p = 0;
+      return bfd_read (data, size, abfd) == size;
+    }
+}
+
 /* Default implementation */
 
 bool
diff --git a/bfd/libbfd.h b/bfd/libbfd.h
index 47f40889a95..4b605c4ce6c 100644
--- a/bfd/libbfd.h
+++ b/bfd/libbfd.h
@@ -911,6 +911,9 @@ extern void _bfd_munmap_readonly_temporary
 #define _bfd_munmap_readonly_temporary(ptr, rsize) free (ptr)
 #endif
 
+extern bool _bfd_mmap_read_temporary
+  (void **, size_t *, void **, bfd *) ATTRIBUTE_HIDDEN;
+
 static inline void *
 _bfd_malloc_and_read (bfd *abfd, bfd_size_type asize, bfd_size_type rsize)
 {
-- 
2.44.0