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Use hash func to boost file creation and lookup #79

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8cf581b
Optimize the file iteration
RoyWFHuang Nov 10, 2025
94e38bb
Rename variables for better readability
RoyWFHuang Nov 11, 2025
7323d62
Use hash func to boost fiile creation and lookup
RoyWFHuang Apr 3, 2026
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2 changes: 1 addition & 1 deletion Makefile
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
obj-m += simplefs.o
simplefs-objs := fs.o super.o inode.o file.o dir.o extent.o
simplefs-objs := fs.o super.o inode.o file.o dir.o extent.o hash.o

KDIR ?= /lib/modules/$(shell uname -r)/build

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8 changes: 5 additions & 3 deletions dir.c
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Expand Up @@ -62,9 +62,10 @@ static int simplefs_iterate(struct file *dir, struct dir_context *ctx)
for (; remained_nr_files && ei < SIMPLEFS_MAX_EXTENTS; ei++) {
if (eblock->extents[ei].ee_start == 0)
continue;

int ei_nr = eblock->extents[ei].nr_files;
/* Iterate over blocks in one extent */
for (bi = 0; bi < eblock->extents[ei].ee_len && remained_nr_files;
for (bi = 0;
bi < eblock->extents[ei].ee_len && remained_nr_files && ei_nr;
bi++) {
bh2 = sb_bread(sb, eblock->extents[ei].ee_start + bi);
if (!bh2) {
Expand All @@ -80,12 +81,13 @@ static int simplefs_iterate(struct file *dir, struct dir_context *ctx)
continue;
}

for (fi = 0; fi < SIMPLEFS_FILES_PER_BLOCK;) {
for (fi = 0; fi < SIMPLEFS_FILES_PER_BLOCK && dblock->nr_files;) {
if (dblock->files[fi].inode != 0) {
if (offset) {
offset--;
} else {
remained_nr_files--;
ei_nr--;
if (!dir_emit(ctx, dblock->files[fi].filename,
SIMPLEFS_FILENAME_LEN,
dblock->files[fi].inode, DT_UNKNOWN)) {
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15 changes: 15 additions & 0 deletions hash.c
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
#include <linux/dcache.h>
#include <linux/types.h>
#include "simplefs.h"

uint32_t simplefs_hash(struct dentry *dentry)
{
const char *str = dentry->d_name.name;
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@jserv jserv Apr 3, 2026

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Unkeyed FNV-1a is vulnerable to HashDoS. FNV-1a is deterministic and trivially reversible. Attackers can craft filenames that all collide, forcing O(n) linear scans on every directory operation.

Consider using the kernel's full_name_hash() which is SipHash-based with a per-boot random key, or at minimum incorporate a per-superblock random salt.

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@RoyWFHuang RoyWFHuang Apr 3, 2026 •
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Based on our previous discussion

full_name_hash() is designed for VFS dentry caching, not for on-disk indexing. It uses a per-boot salt that changes on reboot. Consider: what happens to hash placement after reboot?

If full_name_hash() returns different values after reboot (it does), lookups will start from wrong positions. The linear probe fallback saves correctness, but destroys the performance benefit.

Use a deterministic hash like FNV-1a or djb2.

I believe we should stick with this approach to ensure consistency across reboots.

uint64_t h = 0xcbf29ce484222325ULL;
while (*str) {
h ^= (unsigned char) (*str++);
h *= 0x100000001b3ULL;
}
/* Fold high 32 bits into low 32 bits to mix the full 64-bit result */
return (uint32_t) (h ^ (h >> 32));
}
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