linux-kernel - [PATCH 1/3] fs/ntfs3: Limit binary search table size

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Message-Id: <20210902154050.5075-2-kari.argillander@gmail.com>
Date:   Thu,  2 Sep 2021 18:40:48 +0300
From:   Kari Argillander <kari.argillander@...il.com>
To:     Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
        ntfs3@...ts.linux.dev
Cc:     Kari Argillander <kari.argillander@...il.com>,
        linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>
Subject: [PATCH 1/3] fs/ntfs3: Limit binary search table size

Current binary search allocates memory for table and fill whole table
before we start actual binary search. This is quite inefficient because
table fill will always be O(n). Also if table is huge we need to
reallocate memory which is costly.

This implementation use just stack memory and always when table is full
we will check if last element is <= and if not start table fill again.
The idea was that it would be same cost as table reallocation.

Signed-off-by: Kari Argillander <kari.argillander@...il.com>
---
 fs/ntfs3/index.c | 110 ++++++++++++++++++-----------------------------
 1 file changed, 41 insertions(+), 69 deletions(-)

diff --git a/fs/ntfs3/index.c b/fs/ntfs3/index.c
index 0daca9adc54c..8bac9d20e7e3 100644
--- a/fs/ntfs3/index.c
+++ b/fs/ntfs3/index.c
@@ -678,98 +678,70 @@ static struct NTFS_DE *hdr_find_e(const struct ntfs_index *indx,
 	u32 off = le32_to_cpu(hdr->de_off);
 
 #ifdef NTFS3_INDEX_BINARY_SEARCH
-	int max_idx = 0, fnd, min_idx;
-	int nslots = 64;
-	u16 *offs;
+	struct NTFS_DE *found = NULL;
+	int min_idx = 0, mid_idx, max_idx = 0;
+	int diff2;
+	u16 offs[64];
 
 	if (end > 0x10000)
 		goto next;
 
-	offs = kmalloc(sizeof(u16) * nslots, GFP_NOFS);
-	if (!offs)
-		goto next;
+fill_table:
+	if (off + sizeof(struct NTFS_DE) > end)
+		return NULL;
 
-	/* Use binary search algorithm. */
-next1:
-	if (off + sizeof(struct NTFS_DE) > end) {
-		e = NULL;
-		goto out1;
-	}
 	e = Add2Ptr(hdr, off);
 	e_size = le16_to_cpu(e->size);
 
-	if (e_size < sizeof(struct NTFS_DE) || off + e_size > end) {
-		e = NULL;
-		goto out1;
-	}
-
-	if (max_idx >= nslots) {
-		u16 *ptr;
-		int new_slots = ALIGN(2 * nslots, 8);
-
-		ptr = kmalloc(sizeof(u16) * new_slots, GFP_NOFS);
-		if (ptr)
-			memcpy(ptr, offs, sizeof(u16) * max_idx);
-		kfree(offs);
-		offs = ptr;
-		nslots = new_slots;
-		if (!ptr)
-			goto next;
-	}
-
-	/* Store entry table. */
-	offs[max_idx] = off;
+	if (e_size < sizeof(struct NTFS_DE) || off + e_size > end)
+		return NULL;
 
 	if (!de_is_last(e)) {
+		offs[max_idx] = off;
 		off += e_size;
-		max_idx += 1;
-		goto next1;
-	}
 
-	/*
-	 * Table of pointers is created.
-	 * Use binary search to find entry that is <= to the search value.
-	 */
-	fnd = -1;
-	min_idx = 0;
+		max_idx++;
+		if (max_idx < ARRAY_SIZE(offs))
+			goto fill_table;
 
-	while (min_idx <= max_idx) {
-		int mid_idx = min_idx + ((max_idx - min_idx) >> 1);
-		int diff2;
-
-		e = Add2Ptr(hdr, offs[mid_idx]);
+		max_idx--;
+	}
 
-		e_key_len = le16_to_cpu(e->key_size);
+binary_search:
+	e_key_len = le16_to_cpu(e->key_size);
 
-		diff2 = (*cmp)(key, key_len, e + 1, e_key_len, ctx);
+	diff2 = (*cmp)(key, key_len, e + 1, e_key_len, ctx);
+	if (diff2 > 0) {
+		if (found) {
+			min_idx = mid_idx + 1;
+		} else {
+			if (de_is_last(e))
+				return NULL;
 
-		if (!diff2) {
-			*diff = 0;
-			goto out1;
+			max_idx = 0;
+			goto fill_table;
 		}
-
-		if (diff2 < 0) {
+	} else if (diff2 < 0) {
+		if (found)
 			max_idx = mid_idx - 1;
-			fnd = mid_idx;
-			if (!fnd)
-				break;
-		} else {
-			min_idx = mid_idx + 1;
-		}
-	}
+		else
+			max_idx--;
 
-	if (fnd == -1) {
-		e = NULL;
-		goto out1;
+		found = e;
+	} else {
+		*diff = 0;
+		return e;
 	}
 
-	*diff = -1;
-	e = Add2Ptr(hdr, offs[fnd]);
+	if (min_idx > max_idx) {
+		*diff = -1;
+		return found;
+	}
 
-out1:
-	kfree(offs);
+	mid_idx = (min_idx + max_idx) >> 1;
+	e = Add2Ptr(hdr, offs[mid_idx]);
 
-	return e;
+	goto binary_search;
 #endif
 
 next:
-- 
2.25.1