Skip to main content

Browse the archive

https://github.com/torvalds/linux
08 September 2024, 02:26:05 UTC
Revision 3a7e02c040b130b5545e4b115aada7bacd80a2b6 authored by Linus Torvalds on 26 July 2024, 22:32:27 UTC, committed by Linus Torvalds on 26 July 2024, 22:32:27 UTC 
minmax: avoid overly complicated constant expressions in VM code
 
The minmax infrastructure is overkill for simple constants, and can
cause huge expansions because those simple constants are then used by
other things.

For example, 'pageblock_order' is a core VM constant, but because it was
implemented using 'min_t()' and all the type-checking that involves, it
actually expanded to something like 2.5kB of preprocessor noise.

And when that simple constant was then used inside other expansions:

  #define pageblock_nr_pages      (1UL << pageblock_order)
  #define pageblock_start_pfn(pfn)  ALIGN_DOWN((pfn), pageblock_nr_pages)

and we then use that inside a 'max()' macro:

	case ISOLATE_SUCCESS:
		update_cached = false;
		last_migrated_pfn = max(cc->zone->zone_start_pfn,
			pageblock_start_pfn(cc->migrate_pfn - 1));

the end result was that one statement expanding to 253kB in size.

There are probably other cases of this, but this one case certainly
stood out.

I've added 'MIN_T()' and 'MAX_T()' macros for this kind of "core simple
constant with specific type" use.  These macros skip the type checking,
and as such need to be very sparingly used only for obvious cases that
have active issues like this.

Reported-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/all/36aa2cad-1db1-4abf-8dd2-fb20484aabc3@lucifer.local/
Cc: David Laight <David.Laight@aculab.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent e8432ac
Raw File
Tip revision: 3a7e02c040b130b5545e4b115aada7bacd80a2b6 authored by Linus Torvalds on 26 July 2024, 22:32:27 UTC
minmax: avoid overly complicated constant expressions in VM code
Tip revision: 3a7e02c
xxhash_generic.c 
// SPDX-License-Identifier: GPL-2.0

#include <crypto/internal/hash.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/xxhash.h>
#include <asm/unaligned.h>

#define XXHASH64_BLOCK_SIZE	32
#define XXHASH64_DIGEST_SIZE	8

struct xxhash64_tfm_ctx {
	u64 seed;
};

struct xxhash64_desc_ctx {
	struct xxh64_state xxhstate;
};

static int xxhash64_setkey(struct crypto_shash *tfm, const u8 *key,
			 unsigned int keylen)
{
	struct xxhash64_tfm_ctx *tctx = crypto_shash_ctx(tfm);

	if (keylen != sizeof(tctx->seed))
		return -EINVAL;
	tctx->seed = get_unaligned_le64(key);
	return 0;
}

static int xxhash64_init(struct shash_desc *desc)
{
	struct xxhash64_tfm_ctx *tctx = crypto_shash_ctx(desc->tfm);
	struct xxhash64_desc_ctx *dctx = shash_desc_ctx(desc);

	xxh64_reset(&dctx->xxhstate, tctx->seed);

	return 0;
}

static int xxhash64_update(struct shash_desc *desc, const u8 *data,
			 unsigned int length)
{
	struct xxhash64_desc_ctx *dctx = shash_desc_ctx(desc);

	xxh64_update(&dctx->xxhstate, data, length);

	return 0;
}

static int xxhash64_final(struct shash_desc *desc, u8 *out)
{
	struct xxhash64_desc_ctx *dctx = shash_desc_ctx(desc);

	put_unaligned_le64(xxh64_digest(&dctx->xxhstate), out);

	return 0;
}

static int xxhash64_digest(struct shash_desc *desc, const u8 *data,
			 unsigned int length, u8 *out)
{
	struct xxhash64_tfm_ctx *tctx = crypto_shash_ctx(desc->tfm);

	put_unaligned_le64(xxh64(data, length, tctx->seed), out);

	return 0;
}

static struct shash_alg alg = {
	.digestsize	= XXHASH64_DIGEST_SIZE,
	.setkey		= xxhash64_setkey,
	.init		= xxhash64_init,
	.update		= xxhash64_update,
	.final		= xxhash64_final,
	.digest		= xxhash64_digest,
	.descsize	= sizeof(struct xxhash64_desc_ctx),
	.base		= {
		.cra_name	 = "xxhash64",
		.cra_driver_name = "xxhash64-generic",
		.cra_priority	 = 100,
		.cra_flags	 = CRYPTO_ALG_OPTIONAL_KEY,
		.cra_blocksize	 = XXHASH64_BLOCK_SIZE,
		.cra_ctxsize	 = sizeof(struct xxhash64_tfm_ctx),
		.cra_module	 = THIS_MODULE,
	}
};

static int __init xxhash_mod_init(void)
{
	return crypto_register_shash(&alg);
}

static void __exit xxhash_mod_fini(void)
{
	crypto_unregister_shash(&alg);
}

subsys_initcall(xxhash_mod_init);
module_exit(xxhash_mod_fini);

MODULE_AUTHOR("Nikolay Borisov <nborisov@suse.com>");
MODULE_DESCRIPTION("xxhash calculations wrapper for lib/xxhash.c");
MODULE_LICENSE("GPL");
MODULE_ALIAS_CRYPTO("xxhash64");
MODULE_ALIAS_CRYPTO("xxhash64-generic");
back to top