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Fix an overflow bug in the blist allocator that needlessly capped max

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swap size by dividing a value, which was always a multiple of 64, by
64.  Remove the code that reduced max swap size down to that cap.

Eliminate the distinction between BLIST_BMAP_RADIX and
BLIST_META_RADIX.  Call them both BLIST_RADIX.

Make improvments to the blist self-test code to silence compiler
warnings and to test larger blists.

Reported by:	jmallett
Reviewed by:	alc
Discussed with:	kib
Tested by:	pho
Differential Revision:	https://reviews.freebsd.org/D25736
This commit is contained in:
Doug Moore 2020-07-25 18:29:10 +00:00
parent 46da523a2b
commit 00fd73d2da
3 changed files with 115 additions and 126 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

221
sys/kern/subr_blist.c
View File

@ -36,15 +36,14 @@
*
* A radix tree controls access to pieces of the bitmap, and includes
* auxiliary information at each interior node about the availabilty of
* contiguous free blocks in the subtree rooted at that node. Two radix
* constants are involved: one for the size of the bitmaps contained in the
* leaf nodes (BLIST_BMAP_RADIX), and one for the number of descendents of
* each of the meta (interior) nodes (BLIST_META_RADIX). Each subtree is
* associated with a range of blocks. The root of any subtree stores a
* hint field that defines an upper bound on the size of the largest
* allocation that can begin in the associated block range. A hint is an
* upper bound on a potential allocation, but not necessarily a tight upper
* bound.
* contiguous free blocks in the subtree rooted at that node. A radix
* constant defines the size of the bitmaps contained in a leaf node
* and the number of descendents of each of the meta (interior) nodes.
* Each subtree is associated with a range of blocks. The root of any
* subtree stores a hint field that defines an upper bound on the size
* of the largest allocation that can begin in the associated block
* range. A hint is an upper bound on a potential allocation, but not
* necessarily a tight upper bound.
*
* The bitmap field in each node directs the search for available blocks.
* For a leaf node, a bit is set if the corresponding block is free. For a
@ -64,17 +63,16 @@
*
* LAYOUT: The radix tree is laid out recursively using a linear array.
* Each meta node is immediately followed (laid out sequentially in
* memory) by BLIST_META_RADIX lower level nodes. This is a recursive
* memory) by BLIST_RADIX lower-level nodes. This is a recursive
* structure but one that can be easily scanned through a very simple
* 'skip' calculation. The memory allocation is only large enough to
* cover the number of blocks requested at creation time. Nodes that
* represent blocks beyond that limit, nodes that would never be read
* or written, are not allocated, so that the last of the
* BLIST_META_RADIX lower level nodes of a some nodes may not be
* allocated.
* BLIST_RADIX lower-level nodes of a some nodes may not be allocated.
*
* NOTE: the allocator cannot currently allocate more than
* BLIST_BMAP_RADIX blocks per call. It will panic with 'allocation too
* BLIST_RADIX blocks per call. It will panic with 'allocation too
* large' if you try. This is an area that could use improvement. The
* radix is large enough that this restriction does not effect the swap
* system, though. Currently only the allocation code is affected by
@ -152,24 +150,19 @@ static void blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix,
static MALLOC_DEFINE(M_SWAP, "SWAP", "Swap space");
#endif
_Static_assert(BLIST_BMAP_RADIX % BLIST_META_RADIX == 0,
"radix divisibility error");
#define BLIST_BMAP_MASK (BLIST_BMAP_RADIX - 1)
#define BLIST_META_MASK (BLIST_META_RADIX - 1)
#define BLIST_MASK (BLIST_RADIX - 1)
/*
* For a subtree that can represent the state of up to 'radix' blocks, the
* number of leaf nodes of the subtree is L=radix/BLIST_BMAP_RADIX. If 'm'
* is short for BLIST_META_RADIX, then for a tree of height h with L=m**h
* number of leaf nodes of the subtree is L=radix/BLIST_RADIX. If 'm'
* is short for BLIST_RADIX, then for a tree of height h with L=m**h
* leaf nodes, the total number of tree nodes is 1 + m + m**2 + ... + m**h,
* or, equivalently, (m**(h+1)-1)/(m-1). This quantity is called 'skip'
* in the 'meta' functions that process subtrees. Since integer division
* discards remainders, we can express this computation as
* skip = (m * m**h) / (m - 1)
* skip = (m * (radix / BLIST_BMAP_RADIX)) / (m - 1)
* and since m divides BLIST_BMAP_RADIX, we can simplify further to
* skip = (radix / (BLIST_BMAP_RADIX / m)) / (m - 1)
* skip = radix / ((BLIST_BMAP_RADIX / m) * (m - 1))
* skip = (m * (radix / m)) / (m - 1)
* skip = radix / (m - 1)
* so that simple integer division by a constant can safely be used for the
* calculation.
*/
@ -177,8 +170,7 @@ static inline daddr_t
radix_to_skip(daddr_t radix)
{
return (radix /
((BLIST_BMAP_RADIX / BLIST_META_RADIX) * BLIST_META_MASK));
return (radix / BLIST_MASK);
}
/*
@ -189,7 +181,7 @@ bitrange(int n, int count)
{
return (((u_daddr_t)-1 << n) &
((u_daddr_t)-1 >> (BLIST_BMAP_RADIX - (n + count))));
((u_daddr_t)-1 >> (BLIST_RADIX - (n + count))));
}
/*
@ -201,7 +193,7 @@ generic_bitpos(u_daddr_t mask)
int hi, lo, mid;
lo = 0;
hi = BLIST_BMAP_RADIX;
hi = BLIST_RADIX;
while (lo + 1 < hi) {
mid = (lo + hi) >> 1;
if (mask & bitrange(0, mid))
@ -238,7 +230,7 @@ bitpos(u_daddr_t mask)
* flags - malloc flags
*
* The smallest blist consists of a single leaf node capable of
* managing BLIST_BMAP_RADIX blocks.
* managing BLIST_RADIX blocks.
*/
blist_t
blist_create(daddr_t blocks, int flags)
@ -252,11 +244,8 @@ blist_create(daddr_t blocks, int flags)
* Calculate the radix and node count used for scanning.
*/
nodes = 1;
radix = BLIST_BMAP_RADIX;
while (radix <= blocks) {
nodes += 1 + (blocks - 1) / radix;
radix *= BLIST_META_RADIX;
}
for (radix = 1; radix <= blocks / BLIST_RADIX; radix *= BLIST_RADIX)
nodes += 1 + (blocks - 1) / radix / BLIST_RADIX;
bl = malloc(offsetof(struct blist, bl_root[nodes]), M_SWAP, flags |
M_ZERO);
@ -549,19 +538,12 @@ blist_stats(blist_t bl, struct sbuf *s)
init_gap_stats(stats);
nodes = 0;
i = bl->bl_radix;
while (i < bl->bl_radix + bl->bl_blocks) {
/*
* Find max size subtree starting at i.
*/
radix = BLIST_BMAP_RADIX;
while (((i / radix) & BLIST_META_MASK) == 0)
radix *= BLIST_META_RADIX;
radix = bl->bl_radix;
for (i = 0; i < bl->bl_blocks; ) {
/*
* Check for skippable subtrees starting at i.
*/
while (radix > BLIST_BMAP_RADIX) {
while (radix != 1) {
if (bl->bl_root[nodes].bm_bitmap == 0) {
if (gap_stats_counting(stats))
update_gap_stats(stats, i);
@ -572,9 +554,9 @@ blist_stats(blist_t bl, struct sbuf *s)
* Skip subtree root.
*/
nodes++;
radix /= BLIST_META_RADIX;
radix /= BLIST_RADIX;
}
if (radix == BLIST_BMAP_RADIX) {
if (radix == 1) {
/*
* Scan leaf.
*/
@ -588,8 +570,16 @@ blist_stats(blist_t bl, struct sbuf *s)
diff ^= bitrange(digit, 1);
}
}
nodes += radix_to_skip(radix);
i += radix;
nodes += radix_to_skip(radix * BLIST_RADIX);
i += radix * BLIST_RADIX;
/*
* Find max size subtree starting at i.
*/
for (radix = 1;
((i / BLIST_RADIX / radix) & BLIST_MASK) == 0;
radix *= BLIST_RADIX)
;
}
update_gap_stats(stats, i);
dump_gap_stats(stats, s);
@ -623,13 +613,13 @@ blst_next_leaf_alloc(blmeta_t *scan, daddr_t start, int count, int maxcount)
daddr_t blk;
int avail, digit;
start += BLIST_BMAP_RADIX;
for (blk = start; blk - start < maxcount; blk += BLIST_BMAP_RADIX) {
start += BLIST_RADIX;
for (blk = start; blk - start < maxcount; blk += BLIST_RADIX) {
/* Skip meta-nodes, as long as they promise more free blocks. */
radix = BLIST_BMAP_RADIX;
radix = BLIST_RADIX;
while (((++scan)->bm_bitmap & 1) == 1 &&
((blk / radix) & BLIST_META_MASK) == 0)
radix *= BLIST_META_RADIX;
((blk / radix) & BLIST_MASK) == 0)
radix *= BLIST_RADIX;
if (~scan->bm_bitmap != 0) {
/*
* Either there is no next leaf with any free blocks,
@ -647,39 +637,37 @@ blst_next_leaf_alloc(blmeta_t *scan, daddr_t start, int count, int maxcount)
return (avail);
}
maxcount = imin(avail, maxcount);
if (maxcount % BLIST_BMAP_RADIX == 0) {
if (maxcount % BLIST_RADIX == 0) {
/*
* There was no next leaf. Back scan up to
* last leaf.
*/
--scan;
while (radix != BLIST_BMAP_RADIX) {
radix /= BLIST_META_RADIX;
do {
radix /= BLIST_RADIX;
--scan;
}
blk -= BLIST_BMAP_RADIX;
} while (radix != 1);
blk -= BLIST_RADIX;
}
}
}
/*
* 'scan' is the last leaf that provides blocks. Clear from 1 to
* BLIST_BMAP_RADIX bits to represent the allocation of those last
* blocks.
* BLIST_RADIX bits to represent the allocation of those last blocks.
*/
if (maxcount % BLIST_BMAP_RADIX != 0)
scan->bm_bitmap &= ~bitrange(0, maxcount % BLIST_BMAP_RADIX);
if (maxcount % BLIST_RADIX != 0)
scan->bm_bitmap &= ~bitrange(0, maxcount % BLIST_RADIX);
else
scan->bm_bitmap = 0;
for (;;) {
/* Back up over meta-nodes, clearing bits if necessary. */
blk -= BLIST_BMAP_RADIX;
radix = BLIST_BMAP_RADIX;
while ((digit = ((blk / radix) & BLIST_META_MASK)) == 0) {
blk -= BLIST_RADIX;
for (radix = BLIST_RADIX;
(digit = ((blk / radix) & BLIST_MASK)) == 0;
radix *= BLIST_RADIX) {
if ((scan--)->bm_bitmap == 0)
scan->bm_bitmap ^= 1;
radix *= BLIST_META_RADIX;
}
if ((scan--)->bm_bitmap == 0)
scan[-digit * radix_to_skip(radix)].bm_bitmap ^=
@ -734,17 +722,17 @@ blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int *count, int maxcount)
}
/* Discard any candidates that appear before blk. */
if ((blk & BLIST_BMAP_MASK) != 0) {
if ((~mask & bitrange(0, blk & BLIST_BMAP_MASK)) != 0) {
if ((blk & BLIST_MASK) != 0) {
if ((~mask & bitrange(0, blk & BLIST_MASK)) != 0) {
/* Grow bighint in case all discarded bits are set. */
bighint += blk & BLIST_BMAP_MASK;
mask |= bitrange(0, blk & BLIST_BMAP_MASK);
bighint += blk & BLIST_MASK;
mask |= bitrange(0, blk & BLIST_MASK);
if (~mask == 0) {
scan->bm_bighint = bighint;
return (SWAPBLK_NONE);
}
}
blk -= blk & BLIST_BMAP_MASK;
blk -= blk & BLIST_MASK;
}
/*
@ -763,17 +751,17 @@ blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int *count, int maxcount)
hi = lo + maxcount;
*count = hi - lo;
mask = ~bitrange(lo, *count);
} else if (maxcount <= BLIST_BMAP_RADIX - lo) {
} else if (maxcount <= BLIST_RADIX - lo) {
/* All the blocks we can use are available here. */
hi = lo + maxcount;
*count = maxcount;
mask = ~bitrange(lo, *count);
if (hi == BLIST_BMAP_RADIX)
if (hi == BLIST_RADIX)
scan->bm_bighint = bighint;
} else {
/* Check next leaf for some of the blocks we want or need. */
count1 = *count - (BLIST_BMAP_RADIX - lo);
maxcount -= BLIST_BMAP_RADIX - lo;
count1 = *count - (BLIST_RADIX - lo);
maxcount -= BLIST_RADIX - lo;
hi = blst_next_leaf_alloc(scan, blk, count1, maxcount);
if (hi < count1)
/*
@ -785,7 +773,7 @@ blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int *count, int maxcount)
* this leaf.
*/
return (SWAPBLK_NONE);
*count = BLIST_BMAP_RADIX - lo + hi;
*count = BLIST_RADIX - lo + hi;
scan->bm_bighint = bighint;
}
@ -811,16 +799,15 @@ blst_meta_alloc(blmeta_t *scan, daddr_t cursor, int *count,
bool scan_from_start;
int digit;
if (radix == BLIST_BMAP_RADIX)
if (radix == 1)
return (blst_leaf_alloc(scan, cursor, count, maxcount));
blk = cursor & -radix;
blk = cursor & -(radix * BLIST_RADIX);
scan_from_start = (cursor == blk);
radix /= BLIST_META_RADIX;
skip = radix_to_skip(radix);
mask = scan->bm_bitmap;
/* Discard any candidates that appear before cursor. */
digit = (cursor / radix) & BLIST_META_MASK;
digit = (cursor / radix) & BLIST_MASK;
mask &= (u_daddr_t)-1 << digit;
if (mask == 0)
return (SWAPBLK_NONE);
@ -846,7 +833,7 @@ blst_meta_alloc(blmeta_t *scan, daddr_t cursor, int *count,
* The allocation might fit beginning in the i'th subtree.
*/
r = blst_meta_alloc(&scan[i], cursor + digit * radix,
count, maxcount, radix);
count, maxcount, radix / BLIST_RADIX);
if (r != SWAPBLK_NONE) {
if (scan[i].bm_bitmap == 0)
scan->bm_bitmap ^= bitrange(digit, 1);
@ -860,7 +847,7 @@ blst_meta_alloc(blmeta_t *scan, daddr_t cursor, int *count,
* We couldn't allocate count in this subtree. If the whole tree was
* scanned, and the last tree node is allocated, update bighint.
*/
if (scan_from_start && !(digit == BLIST_META_RADIX - 1 &&
if (scan_from_start && !(digit == BLIST_RADIX - 1 &&
scan[i].bm_bighint == BLIST_MAX_ALLOC))
scan->bm_bighint = *count - 1;
@ -882,7 +869,7 @@ blst_leaf_free(blmeta_t *scan, daddr_t blk, int count)
* \_________/\__/
* count n
*/
mask = bitrange(blk & BLIST_BMAP_MASK, count);
mask = bitrange(blk & BLIST_MASK, count);
KASSERT((scan->bm_bitmap & mask) == 0,
("freeing free block: %jx, size %d, mask %jx",
(uintmax_t)blk, count, (uintmax_t)scan->bm_bitmap & mask));
@ -913,20 +900,26 @@ blst_meta_free(blmeta_t *scan, daddr_t freeBlk, daddr_t count, u_daddr_t radix)
*/
scan->bm_bighint = BLIST_MAX_ALLOC;
if (radix == BLIST_BMAP_RADIX)
if (radix == 1)
return (blst_leaf_free(scan, freeBlk, count));
endBlk = ummin(freeBlk + count, (freeBlk + radix) & -radix);
radix /= BLIST_META_RADIX;
endBlk = freeBlk + count;
blk = (freeBlk + radix * BLIST_RADIX) & -(radix * BLIST_RADIX);
/*
* blk is first block past the end of the range of this meta node,
* or 0 in case of overflow.
*/
if (blk != 0)
endBlk = ummin(endBlk, blk);
skip = radix_to_skip(radix);
blk = freeBlk & -radix;
digit = (blk / radix) & BLIST_META_MASK;
endDigit = 1 + (((endBlk - 1) / radix) & BLIST_META_MASK);
digit = (blk / radix) & BLIST_MASK;
endDigit = 1 + (((endBlk - 1) / radix) & BLIST_MASK);
scan->bm_bitmap |= bitrange(digit, endDigit - digit);
for (i = 1 + digit * skip; blk < endBlk; i += skip) {
blk += radix;
count = ummin(blk, endBlk) - freeBlk;
blst_meta_free(&scan[i], freeBlk, count, radix);
blst_meta_free(&scan[i], freeBlk, count, radix / BLIST_RADIX);
freeBlk = blk;
}
}
@ -947,7 +940,7 @@ blst_copy(blmeta_t *scan, daddr_t blk, daddr_t radix, blist_t dest,
* Leaf node
*/
if (radix == BLIST_BMAP_RADIX) {
if (radix == 1) {
u_daddr_t v = scan->bm_bitmap;
if (v == (u_daddr_t)-1) {
@ -975,14 +968,14 @@ blst_copy(blmeta_t *scan, daddr_t blk, daddr_t radix, blist_t dest,
}
endBlk = blk + count;
radix /= BLIST_META_RADIX;
skip = radix_to_skip(radix);
for (i = 1; blk < endBlk; i += skip) {
blk += radix;
count = radix;
if (blk >= endBlk)
count -= blk - endBlk;
blst_copy(&scan[i], blk - radix, radix, dest, count);
blst_copy(&scan[i], blk - radix,
radix / BLIST_RADIX, dest, count);
}
}
@ -999,7 +992,7 @@ blst_leaf_fill(blmeta_t *scan, daddr_t blk, int count)
daddr_t nblks;
u_daddr_t mask;
mask = bitrange(blk & BLIST_BMAP_MASK, count);
mask = bitrange(blk & BLIST_MASK, count);
/* Count the number of blocks that we are allocating. */
nblks = bitcount64(scan->bm_bitmap & mask);
@ -1022,20 +1015,27 @@ blst_meta_fill(blmeta_t *scan, daddr_t allocBlk, daddr_t count, u_daddr_t radix)
daddr_t blk, endBlk, i, nblks, skip;
int digit;
if (radix == BLIST_BMAP_RADIX)
if (radix == 1)
return (blst_leaf_fill(scan, allocBlk, count));
endBlk = ummin(allocBlk + count, (allocBlk + radix) & -radix);
radix /= BLIST_META_RADIX;
endBlk = allocBlk + count;
blk = (allocBlk + radix * BLIST_RADIX) & -(radix * BLIST_RADIX);
/*
* blk is first block past the end of the range of this meta node,
* or 0 in case of overflow.
*/
if (blk != 0)
endBlk = ummin(endBlk, blk);
skip = radix_to_skip(radix);
blk = allocBlk & -radix;
nblks = 0;
while (blk < endBlk) {
digit = (blk / radix) & BLIST_META_MASK;
digit = (blk / radix) & BLIST_MASK;
i = 1 + digit * skip;
blk += radix;
count = ummin(blk, endBlk) - allocBlk;
nblks += blst_meta_fill(&scan[i], allocBlk, count, radix);
nblks += blst_meta_fill(&scan[i], allocBlk, count,
radix / BLIST_RADIX);
if (scan[i].bm_bitmap == 0)
scan->bm_bitmap &= ~((u_daddr_t)1 << digit);
allocBlk = blk;
@ -1052,12 +1052,12 @@ blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
u_daddr_t mask;
int digit;
if (radix == BLIST_BMAP_RADIX) {
if (radix == 1) {
printf(
"%*.*s(%08llx,%lld): bitmap %0*llx big=%lld\n",
tab, tab, "",
(long long)blk, (long long)radix,
1 + (BLIST_BMAP_RADIX - 1) / 4,
(long long)blk, (long long)BLIST_RADIX,
(int)(1 + (BLIST_RADIX - 1) / 4),
(long long)scan->bm_bitmap,
(long long)scan->bm_bighint
);
@ -1067,14 +1067,13 @@ blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
printf(
"%*.*s(%08llx): subtree (%lld/%lld) bitmap %0*llx big=%lld {\n",
tab, tab, "",
(long long)blk, (long long)radix,
(long long)radix,
1 + (BLIST_META_RADIX - 1) / 4,
(long long)blk, (long long)radix * BLIST_RADIX,
(long long)radix * BLIST_RADIX,
(int)(1 + (BLIST_RADIX - 1) / 4),
(long long)scan->bm_bitmap,
(long long)scan->bm_bighint
);
radix /= BLIST_META_RADIX;
skip = radix_to_skip(radix);
tab += 4;
@ -1083,7 +1082,7 @@ blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
do {
digit = bitpos(mask);
blst_radix_print(&scan[1 + digit * skip], blk + digit * radix,
radix, tab);
radix / BLIST_RADIX, tab);
} while ((mask ^= bitrange(digit, 1)) != 0);
tab -= 4;
@ -1100,7 +1099,7 @@ blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
int
main(int ac, char **av)
{
int size = BLIST_META_RADIX * BLIST_BMAP_RADIX;
daddr_t size = BLIST_RADIX * BLIST_RADIX;
int i;
blist_t bl;
struct sbuf *s;
@ -1108,7 +1107,7 @@ main(int ac, char **av)
for (i = 1; i < ac; ++i) {
const char *ptr = av[i];
if (*ptr != '-') {
size = strtol(ptr, NULL, 0);
size = strtoll(ptr, NULL, 0);
continue;
}
ptr += 2;
@ -1116,6 +1115,10 @@ main(int ac, char **av)
exit(1);
}
bl = blist_create(size, M_WAITOK);
if (bl == NULL) {
fprintf(stderr, "blist_create failed\n");
exit(1);
}
blist_free(bl, 0, size);
for (;;) {
@ -1124,7 +1127,7 @@ main(int ac, char **av)
int count = 0, maxcount = 0;
printf("%lld/%lld/%lld> ", (long long)blist_avail(bl),
(long long)size, (long long)bl->bl_radix);
(long long)size, (long long)bl->bl_radix * BLIST_RADIX);
fflush(stdout);
if (fgets(buf, sizeof(buf), stdin) == NULL)
break;

5
sys/sys/blist.h
View File

@ -85,10 +85,9 @@ typedef struct blist {
blmeta_t bl_root[1]; /* root of radix tree */
} *blist_t;
#define BLIST_BMAP_RADIX (sizeof(u_daddr_t)*8)
#define BLIST_META_RADIX BLIST_BMAP_RADIX
#define BLIST_RADIX (sizeof(u_daddr_t) * 8)
#define BLIST_MAX_ALLOC BLIST_BMAP_RADIX
#define BLIST_MAX_ALLOC BLIST_RADIX
struct sbuf;

15
sys/vm/swap_pager.c
View File

@ -2369,7 +2369,6 @@ swaponsomething(struct vnode *vp, void *id, u_long nblks,
{
struct swdevt *sp, *tsp;
daddr_t dvbase;
u_long mblocks;
/*
* nblks is in DEV_BSIZE'd chunks, convert to PAGE_SIZE'd chunks.
@ -2380,19 +2379,8 @@ swaponsomething(struct vnode *vp, void *id, u_long nblks,
nblks &= ~(ctodb(1) - 1);
nblks = dbtoc(nblks);
/*
* If we go beyond this, we get overflows in the radix
* tree bitmap code.
*/
mblocks = 0x40000000 / BLIST_META_RADIX;
if (nblks > mblocks) {
printf(
"WARNING: reducing swap size to maximum of %luMB per unit\n",
mblocks / 1024 / 1024 * PAGE_SIZE);
nblks = mblocks;
}
sp = malloc(sizeof *sp, M_VMPGDATA, M_WAITOK | M_ZERO);
sp->sw_blist = blist_create(nblks, M_WAITOK);
sp->sw_vp = vp;
sp->sw_id = id;
sp->sw_dev = dev;
@ -2402,7 +2390,6 @@ swaponsomething(struct vnode *vp, void *id, u_long nblks,
sp->sw_close = close;
sp->sw_flags = flags;
sp->sw_blist = blist_create(nblks, M_WAITOK);
/*
* Do not free the first blocks in order to avoid overwriting
* any bsd label at the front of the partition