module: Fix performance regression on modules with large symbol tables

Looking at /proc/kallsyms, one starts to ponder whether all of the extra
strtab-related complexity in module.c is worth the memory savings.

Instead of making the add_kallsyms() loop even more complex, I tried the
other route of deleting the strmap logic and naively copying each string
into core_strtab with no consideration for consolidating duplicates.

Performance on an "already exists" insmod of nvidia.ko (runs
add_kallsyms() but does not actually initialize the module):

	Original scheme: 1.230s
	With naive copying: 0.058s

Extra space used: 35k (of a 408k module).

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <73defb5e4bca04a6431392cc341112b1@localhost>

diff --git a/kernel/module.c b/kernel/module.c
index cf9f1b6..4928cff 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -138,7 +138,6 @@ struct load_info {
        unsigned long len;
        Elf_Shdr *sechdrs;
        char *secstrings, *strtab;
-       unsigned long *strmap;
        unsigned long symoffs, stroffs;
        struct _ddebug *debug;
        unsigned int num_debug;
@@ -2178,12 +2177,19 @@ static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
        return true;
 }
 
+/*
+ * We only allocate and copy the strings needed by the parts of symtab
+ * we keep.  This is simple, but has the effect of making multiple
+ * copies of duplicates.  We could be more sophisticated, see
+ * linux-kernel thread starting with
+ * <73defb5e4bca04a6431392cc341112b1@localhost>.
+ */
 static void layout_symtab(struct module *mod, struct load_info *info)
 {
        Elf_Shdr *symsect = info->sechdrs + info->index.sym;
        Elf_Shdr *strsect = info->sechdrs + info->index.str;
        const Elf_Sym *src;
-       unsigned int i, nsrc, ndst;
+       unsigned int i, nsrc, ndst, strtab_size;
 
        /* Put symbol section at end of init part of module. */
        symsect->sh_flags |= SHF_ALLOC;
@@ -2194,38 +2200,23 @@ static void layout_symtab(struct module *mod, struct load_info *info)
        src = (void *)info->hdr + symsect->sh_offset;
        nsrc = symsect->sh_size / sizeof(*src);
 
-       /*
-        * info->strmap has a '1' bit for each byte of .strtab we want to
-        * keep resident in mod->core_strtab.  Everything else in .strtab
-        * is unreferenced by the symbols in mod->core_symtab, and will be
-        * discarded when add_kallsyms() compacts the string table.
-        */
-       for (ndst = i = 1; i < nsrc; ++i, ++src)
+       /* Compute total space required for the core symbols' strtab. */
+       for (ndst = i = strtab_size = 1; i < nsrc; ++i, ++src)
                if (is_core_symbol(src, info->sechdrs, info->hdr->e_shnum)) {
-                       unsigned int j = src->st_name;
-
-                       while (!__test_and_set_bit(j, info->strmap)
-                              && info->strtab[j])
-                               ++j;
-                       ++ndst;
+                       strtab_size += strlen(&info->strtab[src->st_name]) + 1;
+                       ndst++;
                }
 
        /* Append room for core symbols at end of core part. */
        info->symoffs = ALIGN(mod->core_size, symsect->sh_addralign ?: 1);
-       mod->core_size = info->symoffs + ndst * sizeof(Elf_Sym);
+       info->stroffs = mod->core_size = info->symoffs + ndst * sizeof(Elf_Sym);
+       mod->core_size += strtab_size;
 
        /* Put string table section at end of init part of module. */
        strsect->sh_flags |= SHF_ALLOC;
        strsect->sh_entsize = get_offset(mod, &mod->init_size, strsect,
                                         info->index.str) | INIT_OFFSET_MASK;
        DEBUGP("\t%s\n", info->secstrings + strsect->sh_name);
-
-       /* Append room for core symbols' strings at end of core part. */
-       info->stroffs = mod->core_size;
-
-       /* First strtab byte (and first symtab entry) are zeroes. */
-       __set_bit(0, info->strmap);
-       mod->core_size += bitmap_weight(info->strmap, strsect->sh_size);
 }
 
 static void add_kallsyms(struct module *mod, const struct load_info *info)
@@ -2246,22 +2237,19 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
                mod->symtab[i].st_info = elf_type(&mod->symtab[i], info);
 
        mod->core_symtab = dst = mod->module_core + info->symoffs;
+       mod->core_strtab = s = mod->module_core + info->stroffs;
        src = mod->symtab;
        *dst = *src;
+       *s++ = 0;
        for (ndst = i = 1; i < mod->num_symtab; ++i, ++src) {
                if (!is_core_symbol(src, info->sechdrs, info->hdr->e_shnum))
                        continue;
+
                dst[ndst] = *src;
-               dst[ndst].st_name = bitmap_weight(info->strmap,
-                                                 dst[ndst].st_name);
-               ++ndst;
+               dst[ndst++].st_name = s - mod->core_strtab;
+               s += strlcpy(s, &mod->strtab[src->st_name], KSYM_NAME_LEN) + 1;
        }
        mod->core_num_syms = ndst;
-
-       mod->core_strtab = s = mod->module_core + info->stroffs;
-       for (*s = 0, i = 1; i < info->sechdrs[info->index.str].sh_size; ++i)
-               if (test_bit(i, info->strmap))
-                       *++s = mod->strtab[i];
 }
 #else
 static inline void layout_symtab(struct module *mod, struct load_info *info)
@@ -2751,27 +2739,18 @@ static struct module *layout_and_allocate(struct load_info *info)
           this is done generically; there doesn't appear to be any
           special cases for the architectures. */
        layout_sections(mod, info);
-
-       info->strmap = kzalloc(BITS_TO_LONGS(info->sechdrs[info->index.str].sh_size)
-                        * sizeof(long), GFP_KERNEL);
-       if (!info->strmap) {
-               err = -ENOMEM;
-               goto free_percpu;
-       }
        layout_symtab(mod, info);
 
        /* Allocate and move to the final place */
        err = move_module(mod, info);
        if (err)
-               goto free_strmap;
+               goto free_percpu;
 
        /* Module has been copied to its final place now: return it. */
        mod = (void *)info->sechdrs[info->index.mod].sh_addr;
        kmemleak_load_module(mod, info);
        return mod;
 
-free_strmap:
-       kfree(info->strmap);
 free_percpu:
        percpu_modfree(mod);
 out:
@@ -2781,7 +2760,6 @@ out:
 /* mod is no longer valid after this! */
 static void module_deallocate(struct module *mod, struct load_info *info)
 {
-       kfree(info->strmap);
        percpu_modfree(mod);
        module_free(mod, mod->module_init);
        module_free(mod, mod->module_core);
@@ -2911,8 +2889,7 @@ static struct module *load_module(void __user *umod,
        if (err < 0)
                goto unlink;
 
-       /* Get rid of temporary copy and strmap. */
-       kfree(info.strmap);
+       /* Get rid of temporary copy. */
        free_copy(&info);
 
        /* Done! */