[PATCH v12 0/6] elf: Use mmap to map in section contents

[H.J. Lu]

On Thu, Apr 4, 2024 at 6:12 AM Luis Machado <luis.machado@arm.com> wrote:
>
> Hi,
>
> On 3/17/24 12:19, H.J. Lu wrote:
> > Changes in v12:
> >
> > 1. Don't call _bfd_elf_link_keep_memory for x86 since it always returns
> > false.
> > 2. Make _bfd_elf_link_keep_memory static.
> >
> > Changes in v11:
> >
> > 1. Update _bfd_mmap_read_temporary to allocate buffer as needed.
> > 2. Don't allocate buffer when _bfd_mmap_read_temporary is called.
> >
> > Changes in v10:
> >
> > 1. Malloc a 4 * page size buffer for external relocations for the final
> > link to avoid mmap/munmap on small relocation sections.
> > 2. Malloc a buffer for input section contents which are smaller than
> > 4 * page size or can't be mapped for the final link.
> >
> > Changes in v9:
> >
> > 1. Use MAP_FAILED for mmap failure.
> >
> > Changes in v8:
> >
> > 1. Rebase against master branch.
> > 2. Add _bfd_elf_link_mmap_section_contents and
> > _bfd_elf_link_munmap_section_contents.
> >
> > Changes in v7:
> >
> > 1. Don't add the --keep-memory linker option.
> >
> > Changes in v6:
> >
> > 1. Add the --keep-memory linker option and always cache symbol and
> > relocation tables for --keep-memory.
> > 2. Always keep symbol table and relocation info for eh_frame to speedup
> > the Rust binary build by ~300x:
> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=31466
> >
> > Changes in v5:
> >
> > 1. Drop 2 patches which have been merged onto master branch.
> > 2. Rename _bfd_elf_mmap_section to _bfd_elf_mmap_section_contents.
> > 3. Rename _bfd_mmap_readonly_tracked, _bfd_mmap_readonly_untracked,
> > _bfd_munmap_readonly_untracked, _bfd_mmap_read_untracked to
> > _bfd_mmap_readonly_persistent, _bfd_mmap_readonly_temporary,
> > _bfd_munmap_readonly_temporary and _bfd_mmap_read_temporary.
> > 4. Drop the setup_group change.
> > 5. Fix a typo.
> > 6. Update comments.
> >
> > Changes in v4:
> >
> > 1. Change don't cache symbol nor relocation tables with mmap to opt-in.
> >
> > Changes in v3:
> >
> > 1. Fix non-mmap build.
> > 2. Change the argument name of bfd_mmap_local from flags to prot since
> > its values are PROT_XXX.
> >
> > Changes in v2:
> >
> > 1. Don't hard-code BFD_JUMP_TABLE_COPY in bfd so that elf-bfd.h can be
> > included in libbfd.c.
> > 2. Change the --with-mmap default to true.
> > 3. Check USE_MMAP instead of HAVE_MMAP.
> > 4. Remove the asize parameter to _bfd_mmap_readonly_tracked.
> > 5. Add contents_addr and contents_size to bfd_elf_section_data.
> > 6. Rename _bfd_link_keep_memory to _bfd_elf_link_keep_memory.
> >
> > ---
> > We can use mmap to map in ELF section contents, instead of copying them
> > into memory by hand.  We don't need to 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%
> >
> > Data shows that these won't improve the single copy linker performance.
> > But they improve the overall system performance when linker is used by
> > reducing linker memory usage and page faults.  They allow more parallel
> > linker jobs on LLVM debug build.
> >
> > Here is a quote from Noah Goldstein: "on a large project they are an
> > extremely large speedup".
> >
> > H.J. Lu (6):
> >   elf: Use mmap to map in read-only sections
> >   elf: Add _bfd_elf_m[un]map_section_contents
> >   elf: Use mmap to map in symbol and relocation tables
> >   elf: Don't cache symbol nor relocation tables with mmap
> >   elf: Always keep symbol table and relocation info for eh_frame
> >   elf: Add _bfd_elf_link_m[un]map_section_contents
> >
> >  bfd/bfd-in2.h      |  24 ++++-
> >  bfd/bfd.c          |  17 +++
> >  bfd/bfdwin.c       |   8 +-
> >  bfd/cache.c        |   7 +-
> >  bfd/compress.c     |   2 +-
> >  bfd/elf-bfd.h      |  24 +++++
> >  bfd/elf-eh-frame.c |   4 +-
> >  bfd/elf-sframe.c   |   4 +-
> >  bfd/elf.c          | 246 ++++++++++++++++++++++++++++++++++--------
> >  bfd/elf32-i386.c   |   8 +-
> >  bfd/elf64-x86-64.c |  12 +--
> >  bfd/elfcode.h      |   7 +-
> >  bfd/elflink.c      | 213 ++++++++++++++++++++++++------------
> >  bfd/elfxx-target.h |   6 +-
> >  bfd/elfxx-x86.c    |   8 +-
> >  bfd/elfxx-x86.h    |   1 +
> >  bfd/libbfd-in.h    |  33 +++++-
> >  bfd/libbfd.c       | 262 ++++++++++++++++++++++++++++++++++++++++++++-
> >  bfd/libbfd.h       |  33 +++++-
> >  bfd/linker.c       |  35 ------
> >  bfd/lynx-core.c    |   2 +-
> >  bfd/opncls.c       |  21 ++++
> >  bfd/section.c      |   9 +-
> >  bfd/sysdep.h       |   4 +
> >  24 files changed, 795 insertions(+), 195 deletions(-)
> >
>
> Probably known, judging from the e-mail reworking _bfd_mmap_read_temporary, but I thought I'd mention anyway.
>
> Looks like this might be causing gdb crashes of the following nature:
>
> Starting program: /home/builder/worker/gdb-fedora-arm64/gdb-build/gdb/testsuite/outputs/gdb.base/break-idempotent/break-idempotent-pie
> BFD: BFD (GNU Binutils) 2.42.50.20240404 internal error, aborting at ../../binutils-gdb/bfd/libbfd.c:1211 in _bfd_mmap_read_temporary
> Please report this bug.

The patch is at

https://sourceware.org/pipermail/binutils/2024-April/133374.html

-- 
H.J.