[PATCH v3 5/6] elf: Use mmap to map in symbol and relocation tables

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

Add _bfd_mmap_read_untracked 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_untracked.
	* 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_untracked.
	(bfd_elf_final_link): Don't cache external relocations when mmap
	is used.
	* libbfd.c (_bfd_mmap_read_untracked ): New.
	* libbfd-in.h (_bfd_mmap_read_untracked): 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 e2e31a93950..7427dca2ba0 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_untracked (&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_untracked ((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_untracked (alloc_ext, alloc_ext_size);
+  _bfd_munmap_readonly_untracked (alloc_extshndx, alloc_extshndx_size);
 
   return intsym_buf;
 }
diff --git a/bfd/elflink.c b/bfd/elflink.c
index 47fb890f94f..4602fb3d10f 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_P is NULL,
+   *EXTERNAL_RELOCS_P and *EXTERNAL_RELOCS_SIZE_P returns the mmap
+   memory address and size.  Otherwise, *EXTERNAL_RELOCS_SIZE_P is
+   unchanged and EXTERNAL_RELOCS_SIZE_P 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_addr,
 				   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_addr = shdr->sh_size;
+  if (!_bfd_mmap_read_untracked (&external_relocs,
+				 external_relocs_size_addr,
+				 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_untracked (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_untracked (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 7887fad9c92..effe1b86b53 100644
--- a/bfd/libbfd-in.h
+++ b/bfd/libbfd-in.h
@@ -905,6 +905,9 @@ extern void _bfd_munmap_readonly_untracked
 #define _bfd_munmap_readonly_untracked(ptr, rsize) free (ptr)
 #endif
 
+extern bool _bfd_mmap_read_untracked
+  (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 c847c4f0180..0a31f113999 100644
--- a/bfd/libbfd.c
+++ b/bfd/libbfd.c
@@ -1180,6 +1180,39 @@ _bfd_mmap_readonly_tracked (bfd *abfd, size_t rsize)
 }
 #endif
 
+/* Attempt to read *SIZE_ADDR bytes from ABFD's iostream to *PTR_P.
+   Return true if the full the amount has been read.  If *PTR_P is
+   NULL, mmap should be used, return the memory address at the
+   current offset in *PTR_P as well as return mmap address and size
+   in *PTR_ADDR and *SIZE_ADDR.  Otherwise, return NULL in *PTR_ADDR
+   and 0 in *SIZE_ADDR.  */
+
+bool
+_bfd_mmap_read_untracked (void **ptr_p, size_t *size_addr,
+			  void **ptr_addr, bfd *abfd)
+{
+  void *ptr = *ptr_p;
+  size_t size = *size_addr;
+
+#ifdef USE_MMAP
+  if (ptr == NULL)
+    {
+      ptr = _bfd_mmap_readonly_untracked (abfd, size, ptr_addr,
+					  size_addr);
+      if (ptr == NULL || ptr == (void *) -1)
+	abort ();
+      *ptr_p = ptr;
+      return true;
+    }
+  else
+#endif
+    {
+      *ptr_addr = NULL;
+      *size_addr = 0;
+      return bfd_read (ptr, size, abfd) == size;
+    }
+}
+
 /* Default implementation */
 
 bool
diff --git a/bfd/libbfd.h b/bfd/libbfd.h
index 1515a03b093..c80f5a86ed1 100644
--- a/bfd/libbfd.h
+++ b/bfd/libbfd.h
@@ -911,6 +911,9 @@ extern void _bfd_munmap_readonly_untracked
 #define _bfd_munmap_readonly_untracked(ptr, rsize) free (ptr)
 #endif
 
+extern bool _bfd_mmap_read_untracked
+  (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