[PATCH v6 4/5] elf: Don't cache symbol nor relocation tables with mmap
Sam James
sam@gentoo.org
Sun Mar 10 00:44:48 GMT 2024
"H.J. Lu" <hjl.tools@gmail.com> writes:
> 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.
>
> Also rename _bfd_link_keep_memory to _bfd_elf_link_keep_memory. Since
> the --no-keep-memory linker option causes:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=31458
>
> this is opt-in by each backend. Add the --keep-memory linker option to
> always cache the symbol and relocation tables to mitigate --no-keep-memory
> bugs like:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=31466
>
> bfd/
>
> * elf-bfd.h (_bfd_elf_link_keep_memory): New.
> * elf32-i386.c (elf_i386_scan_relocs): Replace
> _bfd_link_keep_memory with _bfd_elf_link_keep_memory.
> * elf64-x86-64.c (elf_x86_64_scan_relocs): Likewise.
> * elflink.c (_bfd_elf_link_iterate_on_relocs): Likewise.
> (elf_link_add_object_symbols): Likewise.
> (init_reloc_cookie): Likewise.
> (init_reloc_cookie_rels): Likewise.
> (_bfd_elf_link_keep_memory): New.
> * libbfd-in.h (_bfd_link_keep_memory): Removed.
> * linker.c (_bfd_link_keep_memory): Likewise.
> * libbfd.h: Regenerated.
>
> include/
>
> * bfdlink.h (bfd_link_info): Add keep_memory_is_set.
>
> ld/
>
> * NEWS: Mention --keep-memory.
> * ld.texi: Document --keep-memory.
> * ldlex.h (option_values): Add OPTION_KEEP_MEMORY.
> * lexsup.c (ld_options): Add --keep-memory.
> (parse_args): Handle OPTION_KEEP_MEMORY and set
> link_info.keep_memory_is_set for OPTION_NO_KEEP_MEMORY.
> ---
> bfd/elf-bfd.h | 3 ++
> bfd/elf32-i386.c | 2 +-
> bfd/elf64-x86-64.c | 2 +-
> bfd/elflink.c | 72 ++++++++++++++++++++++++++++++++++++++--------
> bfd/libbfd-in.h | 3 --
> bfd/libbfd.h | 3 --
> bfd/linker.c | 35 ----------------------
> include/bfdlink.h | 3 ++
> ld/NEWS | 2 ++
> ld/ld.texi | 14 +++++----
> ld/ldlex.h | 3 +-
> ld/lexsup.c | 11 +++++--
> 12 files changed, 90 insertions(+), 63 deletions(-)
>
> diff --git a/bfd/elf-bfd.h b/bfd/elf-bfd.h
> index e7c2cd19bed..b9ec590cd4e 100644
> --- a/bfd/elf-bfd.h
> +++ b/bfd/elf-bfd.h
> @@ -3144,6 +3144,9 @@ extern bool _bfd_elf_mmap_section_contents
> extern void _bfd_elf_munmap_section_contents
> (asection *, void *);
>
> +extern bool _bfd_elf_link_keep_memory
> + (struct bfd_link_info *);
> +
> /* Large common section. */
> extern asection _bfd_elf_large_com_section;
>
> diff --git a/bfd/elf32-i386.c b/bfd/elf32-i386.c
> index 0f963c373be..d1bd11094de 100644
> --- a/bfd/elf32-i386.c
> +++ b/bfd/elf32-i386.c
> @@ -1932,7 +1932,7 @@ elf_i386_scan_relocs (bfd *abfd,
>
> if (elf_section_data (sec)->this_hdr.contents != contents)
> {
> - if (!converted && !_bfd_link_keep_memory (info))
> + if (!converted && !_bfd_elf_link_keep_memory (info))
> _bfd_elf_munmap_section_contents (sec, contents);
> else
> {
> diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
> index 2e3f86840e5..5e1fad069d1 100644
> --- a/bfd/elf64-x86-64.c
> +++ b/bfd/elf64-x86-64.c
> @@ -2590,7 +2590,7 @@ elf_x86_64_scan_relocs (bfd *abfd, struct bfd_link_info *info,
>
> if (elf_section_data (sec)->this_hdr.contents != contents)
> {
> - if (!converted && !_bfd_link_keep_memory (info))
> + if (!converted && !_bfd_elf_link_keep_memory (info))
> _bfd_elf_munmap_section_contents (sec, contents);
> else
> {
> diff --git a/bfd/elflink.c b/bfd/elflink.c
> index 2a0586722e8..e5f2c75cb74 100644
> --- a/bfd/elflink.c
> +++ b/bfd/elflink.c
> @@ -4205,10 +4205,9 @@ _bfd_elf_link_iterate_on_relocs
> || bfd_is_abs_section (o->output_section))
> continue;
>
> - internal_relocs = _bfd_elf_link_info_read_relocs (abfd, info,
> - o, NULL,
> - NULL,
> - _bfd_link_keep_memory (info));
> + internal_relocs = _bfd_elf_link_info_read_relocs
> + (abfd, info, o, NULL, NULL,
> + _bfd_elf_link_keep_memory (info));
> if (internal_relocs == NULL)
> return false;
>
> @@ -5574,10 +5573,9 @@ elf_link_add_object_symbols (bfd *abfd, struct bfd_link_info *info)
> && (s->flags & SEC_DEBUGGING) != 0))
> continue;
>
> - internal_relocs = _bfd_elf_link_info_read_relocs (abfd, info,
> - s, NULL,
> - NULL,
> - _bfd_link_keep_memory (info));
> + internal_relocs = _bfd_elf_link_info_read_relocs
> + (abfd, info, s, NULL, NULL,
> + _bfd_elf_link_keep_memory (info));
> if (internal_relocs == NULL)
> goto error_free_vers;
>
> @@ -13630,7 +13628,7 @@ init_reloc_cookie (struct elf_reloc_cookie *cookie,
> info->callbacks->einfo (_("%P%X: can not read symbols: %E\n"));
> return false;
> }
> - if (_bfd_link_keep_memory (info) )
> + if (_bfd_elf_link_keep_memory (info) )
> {
> symtab_hdr->contents = (bfd_byte *) cookie->locsyms;
> info->cache_size += (cookie->locsymcount
> @@ -13667,9 +13665,9 @@ init_reloc_cookie_rels (struct elf_reloc_cookie *cookie,
> }
> else
> {
> - cookie->rels = _bfd_elf_link_info_read_relocs (abfd, info, sec,
> - NULL, NULL,
> - _bfd_link_keep_memory (info));
> + cookie->rels = _bfd_elf_link_info_read_relocs
> + (abfd, info, sec, NULL, NULL,
> + _bfd_elf_link_keep_memory (info));
> if (cookie->rels == NULL)
> return false;
> cookie->rel = cookie->rels;
> @@ -15599,3 +15597,53 @@ _bfd_elf_add_dynamic_tags (bfd *output_bfd, struct bfd_link_info *info,
>
> return true;
> }
> +
> +/* Return false if linker should avoid caching relocation information
> + and symbol tables of input files in memory. */
> +
> +bool
> +_bfd_elf_link_keep_memory (struct bfd_link_info *info)
> +{
> +#ifdef USE_MMAP
> + /* Don't cache symbol nor relocation tables if they are mapped in.
> + NB: Since the --no-keep-memory linker option causes:
> +
> + https://sourceware.org/bugzilla/show_bug.cgi?id=31458
> +
> + this is opt-in by each backend. */
> + if (!info->keep_memory_is_set)
> + {
> + const struct elf_backend_data *bed
> + = get_elf_backend_data (info->output_bfd);
> + if (bed->use_mmap)
> + return false;
> + }
> +#endif
> + bfd *abfd;
> + bfd_size_type size;
> +
> + if (!info->keep_memory)
> + return false;
> +
> + if (info->max_cache_size == (bfd_size_type) -1)
> + return true;
> +
> + abfd = info->input_bfds;
> + size = info->cache_size;
> + do
> + {
> + if (size >= info->max_cache_size)
> + {
> + /* Over the limit. Reduce the memory usage. */
> + info->keep_memory = false;
> + return false;
> + }
> + if (!abfd)
> + break;
> + size += abfd->alloc_size;
> + abfd = abfd->link.next;
> + }
> + while (1);
> +
> + return true;
> +}
> diff --git a/bfd/libbfd-in.h b/bfd/libbfd-in.h
> index 9d5396a0354..8ad9d36e396 100644
> --- a/bfd/libbfd-in.h
> +++ b/bfd/libbfd-in.h
> @@ -848,9 +848,6 @@ extern bfd_byte * _bfd_write_unsigned_leb128
> extern struct bfd_link_info *_bfd_get_link_info (bfd *)
> ATTRIBUTE_HIDDEN;
>
> -extern bool _bfd_link_keep_memory (struct bfd_link_info *)
> - ATTRIBUTE_HIDDEN;
> -
> extern uintptr_t _bfd_pagesize ATTRIBUTE_HIDDEN;
> extern uintptr_t _bfd_pagesize_m1 ATTRIBUTE_HIDDEN;
> extern uintptr_t _bfd_minimum_mmap_size ATTRIBUTE_HIDDEN;
> diff --git a/bfd/libbfd.h b/bfd/libbfd.h
> index 4b605c4ce6c..17a6095ba00 100644
> --- a/bfd/libbfd.h
> +++ b/bfd/libbfd.h
> @@ -854,9 +854,6 @@ extern bfd_byte * _bfd_write_unsigned_leb128
> extern struct bfd_link_info *_bfd_get_link_info (bfd *)
> ATTRIBUTE_HIDDEN;
>
> -extern bool _bfd_link_keep_memory (struct bfd_link_info *)
> - ATTRIBUTE_HIDDEN;
> -
> extern uintptr_t _bfd_pagesize ATTRIBUTE_HIDDEN;
> extern uintptr_t _bfd_pagesize_m1 ATTRIBUTE_HIDDEN;
> extern uintptr_t _bfd_minimum_mmap_size ATTRIBUTE_HIDDEN;
> diff --git a/bfd/linker.c b/bfd/linker.c
> index 36cca9624c2..eb42a78b622 100644
> --- a/bfd/linker.c
> +++ b/bfd/linker.c
> @@ -3556,38 +3556,3 @@ _bfd_nolink_bfd_define_start_stop (struct bfd_link_info *info ATTRIBUTE_UNUSED,
> {
> return (struct bfd_link_hash_entry *) _bfd_ptr_bfd_null_error (sec->owner);
> }
> -
> -/* Return false if linker should avoid caching relocation infomation
> - and symbol tables of input files in memory. */
> -
> -bool
> -_bfd_link_keep_memory (struct bfd_link_info * info)
> -{
> - bfd *abfd;
> - bfd_size_type size;
> -
> - if (!info->keep_memory)
> - return false;
> -
> - if (info->max_cache_size == (bfd_size_type) -1)
> - return true;
> -
> - abfd = info->input_bfds;
> - size = info->cache_size;
> - do
> - {
> - if (size >= info->max_cache_size)
> - {
> - /* Over the limit. Reduce the memory usage. */
> - info->keep_memory = false;
> - return false;
> - }
> - if (!abfd)
> - break;
> - size += abfd->alloc_size;
> - abfd = abfd->link.next;
> - }
> - while (1);
> -
> - return true;
> -}
> diff --git a/include/bfdlink.h b/include/bfdlink.h
> index eac07d78364..9397032c6dd 100644
> --- a/include/bfdlink.h
> +++ b/include/bfdlink.h
> @@ -580,6 +580,9 @@ struct bfd_link_info
> /* TRUE if commonpagesize is set on command-line. */
> unsigned int commonpagesize_is_set : 1;
>
> + /* TRUE if keep_memory is set on command-line. */
> + unsigned int keep_memory_is_set : 1;
> +
> /* Char that may appear as the first char of a symbol, but should be
> skipped (like symbol_leading_char) when looking up symbols in
> wrap_hash. Used by PowerPC Linux for 'dot' symbols. */
> diff --git a/ld/NEWS b/ld/NEWS
> index f70d2157339..8751b177329 100644
> --- a/ld/NEWS
> +++ b/ld/NEWS
> @@ -1,5 +1,7 @@
> -*- text -*-
>
> +* Add --keep-memory to always cache the symbol and relocation tables.
> +
> * Add -plugin-save-temps to store plugin intermediate files permanently.
>
> Changes in 2.42:
> diff --git a/ld/ld.texi b/ld/ld.texi
> index 6f234752278..792fdf1c2f3 100644
> --- a/ld/ld.texi
> +++ b/ld/ld.texi
> @@ -2140,13 +2140,17 @@ If the map file already exists then it will be overwritten by this
> operation.
>
> @cindex memory usage
> +@kindex --keep-memory
> @kindex --no-keep-memory
> -@item --no-keep-memory
> +@item --keep-memory
> @command{ld} normally optimizes for speed over memory usage by caching the
> -symbol tables of input files in memory. This option tells @command{ld} to
> -instead optimize for memory usage, by rereading the symbol tables as
> -necessary. This may be required if @command{ld} runs out of memory space
> -while linking a large executable.
> +symbol and relocation tables of input files in memory if @samp{mmap} isn't
> +used to map them in. The @option{--no-keep-memory} option tells @command{ld}
> +to instead optimize for memory usage, by rereading the symbol and
> +relocation tables as necessary. This may be required if @command{ld}
> +runs out of memory space while linking a large executable.
> +The @option{--keep-memory} option tells @command{ld} to always cache the
> +symbol and relocation tables.
Sorry, one more thing: this is tri-state now, right?
i.e. there's three different behaviours:
1) --keep-memory -> we cache more
2) nothing/default -> old behaviour
3) --no-keep-memory -> we cache less, as before
If that's right, maybe explicitly say "The default is a compromise
between explicit --no-keep-memory and --keep-memory. --keep-memory
behavior is different." or similar?
Right now, it might sound like --keep-memory does the same as not
passing it at all, unless overridden later.
>
> @kindex --no-undefined
> @kindex -z defs
> diff --git a/ld/ldlex.h b/ld/ldlex.h
> index d575562a357..49ac8552e1f 100644
> --- a/ld/ldlex.h
> +++ b/ld/ldlex.h
> @@ -43,9 +43,10 @@ enum option_values
> OPTION_NO_EXPORT_DYNAMIC,
> OPTION_HELP,
> OPTION_IGNORE,
> + OPTION_KEEP_MEMORY,
> + OPTION_NO_KEEP_MEMORY,
> OPTION_MAP,
> OPTION_NO_DEMANGLE,
> - OPTION_NO_KEEP_MEMORY,
> OPTION_NO_WARN_MISMATCH,
> OPTION_NO_WARN_SEARCH_MISMATCH,
> OPTION_NOINHIBIT_EXEC,
> diff --git a/ld/lexsup.c b/ld/lexsup.c
> index dad3b6059ed..96d37ca3093 100644
> --- a/ld/lexsup.c
> +++ b/ld/lexsup.c
> @@ -383,14 +383,16 @@ static const struct ld_option ld_options[] =
> '\0', NULL, N_("Print option help"), TWO_DASHES },
> { {"init", required_argument, NULL, OPTION_INIT},
> '\0', N_("SYMBOL"), N_("Call SYMBOL at load-time"), ONE_DASH },
> + { {"keep-memory", no_argument, NULL, OPTION_KEEP_MEMORY},
> + '\0', NULL, N_("Use more memory and less disk I/O"), TWO_DASHES },
> + { {"no-keep-memory", no_argument, NULL, OPTION_NO_KEEP_MEMORY},
> + '\0', NULL, N_("Use less memory and more disk I/O"), TWO_DASHES },
> { {"Map", required_argument, NULL, OPTION_MAP},
> '\0', N_("FILE/DIR"), N_("Write a linker map to FILE or DIR/<outputname>.map"), ONE_DASH },
> { {"no-define-common", no_argument, NULL, OPTION_NO_DEFINE_COMMON},
> '\0', NULL, N_("Do not define Common storage"), TWO_DASHES },
> { {"no-demangle", no_argument, NULL, OPTION_NO_DEMANGLE },
> '\0', NULL, N_("Do not demangle symbol names"), TWO_DASHES },
> - { {"no-keep-memory", no_argument, NULL, OPTION_NO_KEEP_MEMORY},
> - '\0', NULL, N_("Use less memory and more disk I/O"), TWO_DASHES },
> { {"no-undefined", no_argument, NULL, OPTION_NO_UNDEFINED},
> '\0', NULL, N_("Do not allow unresolved references in object files"),
> TWO_DASHES },
> @@ -1091,8 +1093,13 @@ parse_args (unsigned argc, char **argv)
> case OPTION_NO_PRINT_GC_SECTIONS:
> link_info.print_gc_sections = false;
> break;
> + case OPTION_KEEP_MEMORY:
> + link_info.keep_memory = true;
> + link_info.keep_memory_is_set = true;
> + break;
> case OPTION_NO_KEEP_MEMORY:
> link_info.keep_memory = false;
> + link_info.keep_memory_is_set = true;
> break;
> case OPTION_NO_UNDEFINED:
> link_info.unresolved_syms_in_objects = RM_DIAGNOSE;
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