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03-18-2009 11:23 AM

GFS2: Fix alignment issue and tidy gfs2_bitfit
 
From: Steven Whitehouse <swhiteho@redhat.com>

An alignment issue with the existing bitfit algorithm was reported
on IA64. This patch attempts to fix that, and also to tidy up the
code a bit. There is now more documentation about how this works
and it has survived a number of different tests.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index a068ac9..c0abe69 100644
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -132,81 +132,89 @@ static inline unsigned char gfs2_testbit(struct gfs2_rgrpd *rgd,
}

/**
+ * gfs2_bit_search
+ * @ptr: Pointer to bitmap data
+ * @mask: Mask to use (normally 0x55555.... but adjusted for search start)
+ * @state: The state we are searching for
+ *
+ * We xor the bitmap data with a patter which is the bitwise opposite
+ * of what we are looking for, this gives rise to a pattern of ones
+ * wherever there is a match. Since we have two bits per entry, we
+ * take this pattern, shift it down by one place and then and it with
+ * the original. All the even bit positions (0,2,4, etc) then represent
+ * successful matches, so we mask with 0x55555..... to remove the unwanted
+ * odd bit positions.
+ *
+ * This allows searching of a whole u64 at once (32 blocks) with a
+ * single test (on 64 bit arches).
+ */
+
+static inline u64 gfs2_bit_search(const __le64 *ptr, u64 mask, u8 state)
+{
+ u64 tmp;
+ static const u64 search[] = {
+ [0] = 0xffffffffffffffff,
+ [1] = 0xaaaaaaaaaaaaaaaa,
+ [2] = 0x5555555555555555,
+ [3] = 0x0000000000000000,
+ };
+ tmp = le64_to_cpu(*ptr) ^ search[state];
+ tmp &= (tmp >> 1);
+ tmp &= mask;
+ return tmp;
+}
+
+/**
* gfs2_bitfit - Search an rgrp's bitmap buffer to find a bit-pair representing
* a block in a given allocation state.
* @buffer: the buffer that holds the bitmaps
- * @buflen: the length (in bytes) of the buffer
+ * @len: the length (in bytes) of the buffer
* @goal: start search at this block's bit-pair (within @buffer)
- * @old_state: GFS2_BLKST_XXX the state of the block we're looking for.
+ * @state: GFS2_BLKST_XXX the state of the block we're looking for.
*
* Scope of @goal and returned block number is only within this bitmap buffer,
* not entire rgrp or filesystem. @buffer will be offset from the actual
- * beginning of a bitmap block buffer, skipping any header structures.
+ * beginning of a bitmap block buffer, skipping any header structures, but
+ * headers are always a multiple of 64 bits long so that the buffer is
+ * always aligned to a 64 bit boundary.
+ *
+ * The size of the buffer is in bytes, but is it assumed that it is
+ * always ok to to read a complete multiple of 64 bits at the end
+ * of the block in case the end is no aligned to a natural boundary.
*
* Return: the block number (bitmap buffer scope) that was found
*/

-static u32 gfs2_bitfit(const u8 *buffer, unsigned int buflen, u32 goal,
- u8 old_state)
+u32 gfs2_bitfit(const u8 *buf, const unsigned int len, u32 goal, u8 state)
{
- const u8 *byte, *start, *end;
- int bit, startbit;
- u32 g1, g2, misaligned;
- unsigned long *plong;
- unsigned long lskipval;
-
- lskipval = (old_state & GFS2_BLKST_USED) ? LBITSKIP00 : LBITSKIP55;
- g1 = (goal / GFS2_NBBY);
- start = buffer + g1;
- byte = start;
- end = buffer + buflen;
- g2 = ALIGN(g1, sizeof(unsigned long));
- plong = (unsigned long *)(buffer + g2);
- startbit = bit = (goal % GFS2_NBBY) * GFS2_BIT_SIZE;
- misaligned = g2 - g1;
- if (!misaligned)
- goto ulong_aligned;
-/* parse the bitmap a byte at a time */
-misaligned:
- while (byte < end) {
- if (((*byte >> bit) & GFS2_BIT_MASK) == old_state) {
- return goal +
- (((byte - start) * GFS2_NBBY) +
- ((bit - startbit) >> 1));
- }
- bit += GFS2_BIT_SIZE;
- if (bit >= GFS2_NBBY * GFS2_BIT_SIZE) {
- bit = 0;
- byte++;
- misaligned--;
- if (!misaligned) {
- plong = (unsigned long *)byte;
- goto ulong_aligned;
- }
- }
- }
- return BFITNOENT;
-
-/* parse the bitmap a unsigned long at a time */
-ulong_aligned:
- /* Stop at "end - 1" or else prefetch can go past the end and segfault.
- We could "if" it but we'd lose some of the performance gained.
- This way will only slow down searching the very last 4/8 bytes
- depending on architecture. I've experimented with several ways
- of writing this section such as using an else before the goto
- but this one seems to be the fastest. */
- while ((unsigned char *)plong < end - sizeof(unsigned long)) {
- prefetch(plong + 1);
- if (((*plong) & LBITMASK) != lskipval)
- break;
- plong++;
- }
- if ((unsigned char *)plong < end) {
- byte = (const u8 *)plong;
- misaligned += sizeof(unsigned long) - 1;
- goto misaligned;
+ u32 spoint = (goal << 1) & ((8*sizeof(u64)) - 1);
+ const __le64 *ptr = ((__le64 *)buf) + (goal >> 5);
+ const __le64 *end = (__le64 *)(buf + ALIGN(len, sizeof(u64)));
+ u64 tmp;
+ u64 mask = 0x5555555555555555;
+ u32 bit;
+
+ BUG_ON(state > 3);
+
+ /* Mask off bits we don't care about at the start of the search */
+ mask <<= spoint;
+ tmp = gfs2_bit_search(ptr, mask, state);
+ ptr++;
+ while(tmp == 0 && ptr < end) {
+ tmp = gfs2_bit_search(ptr, 0x5555555555555555, state);
+ ptr++;
}
- return BFITNOENT;
+ /* Mask off any bits which are more than len bytes from the start */
+ if (ptr == end && (len & (sizeof(u64) - 1)))
+ tmp &= (((u64)~0) >> (64 - 8*(len & (sizeof(u64) - 1))));
+ /* Didn't find anything, so return */
+ if (tmp == 0)
+ return BFITNOENT;
+ ptr--;
+ bit = fls64(tmp);
+ bit--; /* fls64 always adds one to the bit count */
+ bit /= 2; /* two bits per entry in the bitmap */
+ return (((const unsigned char *)ptr - buf) * GFS2_NBBY) + bit;
}

/**
--
1.6.0.3


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