freebsd-dev/module/zfs/dmu_zfetch.c
Brian Behlendorf 79c76d5b65 Change KM_PUSHPAGE -> KM_SLEEP
By marking DMU transaction processing contexts with PF_FSTRANS
we can revert the KM_PUSHPAGE -> KM_SLEEP changes.  This brings
us back in line with upstream.  In some cases this means simply
swapping the flags back.  For others fnvlist_alloc() was replaced
by nvlist_alloc(..., KM_PUSHPAGE) and must be reverted back to
fnvlist_alloc() which assumes KM_SLEEP.

The one place KM_PUSHPAGE is kept is when allocating ARC buffers
which allows us to dip in to reserved memory.  This is again the
same as upstream.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
2015-01-16 14:41:26 -08:00

749 lines
20 KiB
C

/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2009 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/*
* Copyright (c) 2013 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
#include <sys/dnode.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_zfetch.h>
#include <sys/dmu.h>
#include <sys/dbuf.h>
#include <sys/kstat.h>
/*
* I'm against tune-ables, but these should probably exist as tweakable globals
* until we can get this working the way we want it to.
*/
int zfs_prefetch_disable = 0;
/* max # of streams per zfetch */
unsigned int zfetch_max_streams = 8;
/* min time before stream reclaim */
unsigned int zfetch_min_sec_reap = 2;
/* max number of blocks to fetch at a time */
unsigned int zfetch_block_cap = 256;
/* number of bytes in a array_read at which we stop prefetching (1Mb) */
unsigned long zfetch_array_rd_sz = 1024 * 1024;
/* forward decls for static routines */
static boolean_t dmu_zfetch_colinear(zfetch_t *, zstream_t *);
static void dmu_zfetch_dofetch(zfetch_t *, zstream_t *);
static uint64_t dmu_zfetch_fetch(dnode_t *, uint64_t, uint64_t);
static uint64_t dmu_zfetch_fetchsz(dnode_t *, uint64_t, uint64_t);
static boolean_t dmu_zfetch_find(zfetch_t *, zstream_t *, int);
static int dmu_zfetch_stream_insert(zfetch_t *, zstream_t *);
static zstream_t *dmu_zfetch_stream_reclaim(zfetch_t *);
static void dmu_zfetch_stream_remove(zfetch_t *, zstream_t *);
static int dmu_zfetch_streams_equal(zstream_t *, zstream_t *);
typedef struct zfetch_stats {
kstat_named_t zfetchstat_hits;
kstat_named_t zfetchstat_misses;
kstat_named_t zfetchstat_colinear_hits;
kstat_named_t zfetchstat_colinear_misses;
kstat_named_t zfetchstat_stride_hits;
kstat_named_t zfetchstat_stride_misses;
kstat_named_t zfetchstat_reclaim_successes;
kstat_named_t zfetchstat_reclaim_failures;
kstat_named_t zfetchstat_stream_resets;
kstat_named_t zfetchstat_stream_noresets;
kstat_named_t zfetchstat_bogus_streams;
} zfetch_stats_t;
static zfetch_stats_t zfetch_stats = {
{ "hits", KSTAT_DATA_UINT64 },
{ "misses", KSTAT_DATA_UINT64 },
{ "colinear_hits", KSTAT_DATA_UINT64 },
{ "colinear_misses", KSTAT_DATA_UINT64 },
{ "stride_hits", KSTAT_DATA_UINT64 },
{ "stride_misses", KSTAT_DATA_UINT64 },
{ "reclaim_successes", KSTAT_DATA_UINT64 },
{ "reclaim_failures", KSTAT_DATA_UINT64 },
{ "streams_resets", KSTAT_DATA_UINT64 },
{ "streams_noresets", KSTAT_DATA_UINT64 },
{ "bogus_streams", KSTAT_DATA_UINT64 },
};
#define ZFETCHSTAT_INCR(stat, val) \
atomic_add_64(&zfetch_stats.stat.value.ui64, (val));
#define ZFETCHSTAT_BUMP(stat) ZFETCHSTAT_INCR(stat, 1);
kstat_t *zfetch_ksp;
/*
* Given a zfetch structure and a zstream structure, determine whether the
* blocks to be read are part of a co-linear pair of existing prefetch
* streams. If a set is found, coalesce the streams, removing one, and
* configure the prefetch so it looks for a strided access pattern.
*
* In other words: if we find two sequential access streams that are
* the same length and distance N appart, and this read is N from the
* last stream, then we are probably in a strided access pattern. So
* combine the two sequential streams into a single strided stream.
*
* Returns whether co-linear streams were found.
*/
static boolean_t
dmu_zfetch_colinear(zfetch_t *zf, zstream_t *zh)
{
zstream_t *z_walk;
zstream_t *z_comp;
if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
return (0);
if (zh == NULL) {
rw_exit(&zf->zf_rwlock);
return (0);
}
for (z_walk = list_head(&zf->zf_stream); z_walk;
z_walk = list_next(&zf->zf_stream, z_walk)) {
for (z_comp = list_next(&zf->zf_stream, z_walk); z_comp;
z_comp = list_next(&zf->zf_stream, z_comp)) {
int64_t diff;
if (z_walk->zst_len != z_walk->zst_stride ||
z_comp->zst_len != z_comp->zst_stride) {
continue;
}
diff = z_comp->zst_offset - z_walk->zst_offset;
if (z_comp->zst_offset + diff == zh->zst_offset) {
z_walk->zst_offset = zh->zst_offset;
z_walk->zst_direction = diff < 0 ?
ZFETCH_BACKWARD : ZFETCH_FORWARD;
z_walk->zst_stride =
diff * z_walk->zst_direction;
z_walk->zst_ph_offset =
zh->zst_offset + z_walk->zst_stride;
dmu_zfetch_stream_remove(zf, z_comp);
mutex_destroy(&z_comp->zst_lock);
kmem_free(z_comp, sizeof (zstream_t));
dmu_zfetch_dofetch(zf, z_walk);
rw_exit(&zf->zf_rwlock);
return (1);
}
diff = z_walk->zst_offset - z_comp->zst_offset;
if (z_walk->zst_offset + diff == zh->zst_offset) {
z_walk->zst_offset = zh->zst_offset;
z_walk->zst_direction = diff < 0 ?
ZFETCH_BACKWARD : ZFETCH_FORWARD;
z_walk->zst_stride =
diff * z_walk->zst_direction;
z_walk->zst_ph_offset =
zh->zst_offset + z_walk->zst_stride;
dmu_zfetch_stream_remove(zf, z_comp);
mutex_destroy(&z_comp->zst_lock);
kmem_free(z_comp, sizeof (zstream_t));
dmu_zfetch_dofetch(zf, z_walk);
rw_exit(&zf->zf_rwlock);
return (1);
}
}
}
rw_exit(&zf->zf_rwlock);
return (0);
}
/*
* Given a zstream_t, determine the bounds of the prefetch. Then call the
* routine that actually prefetches the individual blocks.
*/
static void
dmu_zfetch_dofetch(zfetch_t *zf, zstream_t *zs)
{
uint64_t prefetch_tail;
uint64_t prefetch_limit;
uint64_t prefetch_ofst;
uint64_t prefetch_len;
uint64_t blocks_fetched;
zs->zst_stride = MAX((int64_t)zs->zst_stride, zs->zst_len);
zs->zst_cap = MIN(zfetch_block_cap, 2 * zs->zst_cap);
prefetch_tail = MAX((int64_t)zs->zst_ph_offset,
(int64_t)(zs->zst_offset + zs->zst_stride));
/*
* XXX: use a faster division method?
*/
prefetch_limit = zs->zst_offset + zs->zst_len +
(zs->zst_cap * zs->zst_stride) / zs->zst_len;
while (prefetch_tail < prefetch_limit) {
prefetch_ofst = zs->zst_offset + zs->zst_direction *
(prefetch_tail - zs->zst_offset);
prefetch_len = zs->zst_len;
/*
* Don't prefetch beyond the end of the file, if working
* backwards.
*/
if ((zs->zst_direction == ZFETCH_BACKWARD) &&
(prefetch_ofst > prefetch_tail)) {
prefetch_len += prefetch_ofst;
prefetch_ofst = 0;
}
/* don't prefetch more than we're supposed to */
if (prefetch_len > zs->zst_len)
break;
blocks_fetched = dmu_zfetch_fetch(zf->zf_dnode,
prefetch_ofst, zs->zst_len);
prefetch_tail += zs->zst_stride;
/* stop if we've run out of stuff to prefetch */
if (blocks_fetched < zs->zst_len)
break;
}
zs->zst_ph_offset = prefetch_tail;
zs->zst_last = ddi_get_lbolt();
}
void
zfetch_init(void)
{
zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc",
KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t),
KSTAT_FLAG_VIRTUAL);
if (zfetch_ksp != NULL) {
zfetch_ksp->ks_data = &zfetch_stats;
kstat_install(zfetch_ksp);
}
}
void
zfetch_fini(void)
{
if (zfetch_ksp != NULL) {
kstat_delete(zfetch_ksp);
zfetch_ksp = NULL;
}
}
/*
* This takes a pointer to a zfetch structure and a dnode. It performs the
* necessary setup for the zfetch structure, grokking data from the
* associated dnode.
*/
void
dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
{
if (zf == NULL) {
return;
}
zf->zf_dnode = dno;
zf->zf_stream_cnt = 0;
zf->zf_alloc_fail = 0;
list_create(&zf->zf_stream, sizeof (zstream_t),
offsetof(zstream_t, zst_node));
rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
}
/*
* This function computes the actual size, in blocks, that can be prefetched,
* and fetches it.
*/
static uint64_t
dmu_zfetch_fetch(dnode_t *dn, uint64_t blkid, uint64_t nblks)
{
uint64_t fetchsz;
uint64_t i;
fetchsz = dmu_zfetch_fetchsz(dn, blkid, nblks);
for (i = 0; i < fetchsz; i++) {
dbuf_prefetch(dn, blkid + i, ZIO_PRIORITY_ASYNC_READ);
}
return (fetchsz);
}
/*
* this function returns the number of blocks that would be prefetched, based
* upon the supplied dnode, blockid, and nblks. This is used so that we can
* update streams in place, and then prefetch with their old value after the
* fact. This way, we can delay the prefetch, but subsequent accesses to the
* stream won't result in the same data being prefetched multiple times.
*/
static uint64_t
dmu_zfetch_fetchsz(dnode_t *dn, uint64_t blkid, uint64_t nblks)
{
uint64_t fetchsz;
if (blkid > dn->dn_maxblkid) {
return (0);
}
/* compute fetch size */
if (blkid + nblks + 1 > dn->dn_maxblkid) {
fetchsz = (dn->dn_maxblkid - blkid) + 1;
ASSERT(blkid + fetchsz - 1 <= dn->dn_maxblkid);
} else {
fetchsz = nblks;
}
return (fetchsz);
}
/*
* given a zfetch and a zstream structure, see if there is an associated zstream
* for this block read. If so, it starts a prefetch for the stream it
* located and returns true, otherwise it returns false
*/
static boolean_t
dmu_zfetch_find(zfetch_t *zf, zstream_t *zh, int prefetched)
{
zstream_t *zs;
int64_t diff;
int reset = !prefetched;
int rc = 0;
if (zh == NULL)
return (0);
/*
* XXX: This locking strategy is a bit coarse; however, it's impact has
* yet to be tested. If this turns out to be an issue, it can be
* modified in a number of different ways.
*/
rw_enter(&zf->zf_rwlock, RW_READER);
top:
for (zs = list_head(&zf->zf_stream); zs;
zs = list_next(&zf->zf_stream, zs)) {
/*
* XXX - should this be an assert?
*/
if (zs->zst_len == 0) {
/* bogus stream */
ZFETCHSTAT_BUMP(zfetchstat_bogus_streams);
continue;
}
/*
* We hit this case when we are in a strided prefetch stream:
* we will read "len" blocks before "striding".
*/
if (zh->zst_offset >= zs->zst_offset &&
zh->zst_offset < zs->zst_offset + zs->zst_len) {
if (prefetched) {
/* already fetched */
ZFETCHSTAT_BUMP(zfetchstat_stride_hits);
rc = 1;
goto out;
} else {
ZFETCHSTAT_BUMP(zfetchstat_stride_misses);
}
}
/*
* This is the forward sequential read case: we increment
* len by one each time we hit here, so we will enter this
* case on every read.
*/
if (zh->zst_offset == zs->zst_offset + zs->zst_len) {
reset = !prefetched && zs->zst_len > 1;
mutex_enter(&zs->zst_lock);
if (zh->zst_offset != zs->zst_offset + zs->zst_len) {
mutex_exit(&zs->zst_lock);
goto top;
}
zs->zst_len += zh->zst_len;
diff = zs->zst_len - zfetch_block_cap;
if (diff > 0) {
zs->zst_offset += diff;
zs->zst_len = zs->zst_len > diff ?
zs->zst_len - diff : 0;
}
zs->zst_direction = ZFETCH_FORWARD;
break;
/*
* Same as above, but reading backwards through the file.
*/
} else if (zh->zst_offset == zs->zst_offset - zh->zst_len) {
/* backwards sequential access */
reset = !prefetched && zs->zst_len > 1;
mutex_enter(&zs->zst_lock);
if (zh->zst_offset != zs->zst_offset - zh->zst_len) {
mutex_exit(&zs->zst_lock);
goto top;
}
zs->zst_offset = zs->zst_offset > zh->zst_len ?
zs->zst_offset - zh->zst_len : 0;
zs->zst_ph_offset = zs->zst_ph_offset > zh->zst_len ?
zs->zst_ph_offset - zh->zst_len : 0;
zs->zst_len += zh->zst_len;
diff = zs->zst_len - zfetch_block_cap;
if (diff > 0) {
zs->zst_ph_offset = zs->zst_ph_offset > diff ?
zs->zst_ph_offset - diff : 0;
zs->zst_len = zs->zst_len > diff ?
zs->zst_len - diff : zs->zst_len;
}
zs->zst_direction = ZFETCH_BACKWARD;
break;
} else if ((zh->zst_offset - zs->zst_offset - zs->zst_stride <
zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
/* strided forward access */
mutex_enter(&zs->zst_lock);
if ((zh->zst_offset - zs->zst_offset - zs->zst_stride >=
zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
mutex_exit(&zs->zst_lock);
goto top;
}
zs->zst_offset += zs->zst_stride;
zs->zst_direction = ZFETCH_FORWARD;
break;
} else if ((zh->zst_offset - zs->zst_offset + zs->zst_stride <
zs->zst_len) && (zs->zst_len != zs->zst_stride)) {
/* strided reverse access */
mutex_enter(&zs->zst_lock);
if ((zh->zst_offset - zs->zst_offset + zs->zst_stride >=
zs->zst_len) || (zs->zst_len == zs->zst_stride)) {
mutex_exit(&zs->zst_lock);
goto top;
}
zs->zst_offset = zs->zst_offset > zs->zst_stride ?
zs->zst_offset - zs->zst_stride : 0;
zs->zst_ph_offset = (zs->zst_ph_offset >
(2 * zs->zst_stride)) ?
(zs->zst_ph_offset - (2 * zs->zst_stride)) : 0;
zs->zst_direction = ZFETCH_BACKWARD;
break;
}
}
if (zs) {
if (reset) {
zstream_t *remove = zs;
ZFETCHSTAT_BUMP(zfetchstat_stream_resets);
rc = 0;
mutex_exit(&zs->zst_lock);
rw_exit(&zf->zf_rwlock);
rw_enter(&zf->zf_rwlock, RW_WRITER);
/*
* Relocate the stream, in case someone removes
* it while we were acquiring the WRITER lock.
*/
for (zs = list_head(&zf->zf_stream); zs;
zs = list_next(&zf->zf_stream, zs)) {
if (zs == remove) {
dmu_zfetch_stream_remove(zf, zs);
mutex_destroy(&zs->zst_lock);
kmem_free(zs, sizeof (zstream_t));
break;
}
}
} else {
ZFETCHSTAT_BUMP(zfetchstat_stream_noresets);
rc = 1;
dmu_zfetch_dofetch(zf, zs);
mutex_exit(&zs->zst_lock);
}
}
out:
rw_exit(&zf->zf_rwlock);
return (rc);
}
/*
* Clean-up state associated with a zfetch structure. This frees allocated
* structure members, empties the zf_stream tree, and generally makes things
* nice. This doesn't free the zfetch_t itself, that's left to the caller.
*/
void
dmu_zfetch_rele(zfetch_t *zf)
{
zstream_t *zs;
zstream_t *zs_next;
ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));
for (zs = list_head(&zf->zf_stream); zs; zs = zs_next) {
zs_next = list_next(&zf->zf_stream, zs);
list_remove(&zf->zf_stream, zs);
mutex_destroy(&zs->zst_lock);
kmem_free(zs, sizeof (zstream_t));
}
list_destroy(&zf->zf_stream);
rw_destroy(&zf->zf_rwlock);
zf->zf_dnode = NULL;
}
/*
* Given a zfetch and zstream structure, insert the zstream structure into the
* AVL tree contained within the zfetch structure. Peform the appropriate
* book-keeping. It is possible that another thread has inserted a stream which
* matches one that we are about to insert, so we must be sure to check for this
* case. If one is found, return failure, and let the caller cleanup the
* duplicates.
*/
static int
dmu_zfetch_stream_insert(zfetch_t *zf, zstream_t *zs)
{
zstream_t *zs_walk;
zstream_t *zs_next;
ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
for (zs_walk = list_head(&zf->zf_stream); zs_walk; zs_walk = zs_next) {
zs_next = list_next(&zf->zf_stream, zs_walk);
if (dmu_zfetch_streams_equal(zs_walk, zs)) {
return (0);
}
}
list_insert_head(&zf->zf_stream, zs);
zf->zf_stream_cnt++;
return (1);
}
/*
* Walk the list of zstreams in the given zfetch, find an old one (by time), and
* reclaim it for use by the caller.
*/
static zstream_t *
dmu_zfetch_stream_reclaim(zfetch_t *zf)
{
zstream_t *zs;
if (! rw_tryenter(&zf->zf_rwlock, RW_WRITER))
return (0);
for (zs = list_head(&zf->zf_stream); zs;
zs = list_next(&zf->zf_stream, zs)) {
if (((ddi_get_lbolt() - zs->zst_last)/hz) > zfetch_min_sec_reap)
break;
}
if (zs) {
dmu_zfetch_stream_remove(zf, zs);
mutex_destroy(&zs->zst_lock);
bzero(zs, sizeof (zstream_t));
} else {
zf->zf_alloc_fail++;
}
rw_exit(&zf->zf_rwlock);
return (zs);
}
/*
* Given a zfetch and zstream structure, remove the zstream structure from its
* container in the zfetch structure. Perform the appropriate book-keeping.
*/
static void
dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
{
ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
list_remove(&zf->zf_stream, zs);
zf->zf_stream_cnt--;
}
static int
dmu_zfetch_streams_equal(zstream_t *zs1, zstream_t *zs2)
{
if (zs1->zst_offset != zs2->zst_offset)
return (0);
if (zs1->zst_len != zs2->zst_len)
return (0);
if (zs1->zst_stride != zs2->zst_stride)
return (0);
if (zs1->zst_ph_offset != zs2->zst_ph_offset)
return (0);
if (zs1->zst_cap != zs2->zst_cap)
return (0);
if (zs1->zst_direction != zs2->zst_direction)
return (0);
return (1);
}
/*
* This is the prefetch entry point. It calls all of the other dmu_zfetch
* routines to create, delete, find, or operate upon prefetch streams.
*/
void
dmu_zfetch(zfetch_t *zf, uint64_t offset, uint64_t size, int prefetched)
{
zstream_t zst;
zstream_t *newstream;
boolean_t fetched;
int inserted;
unsigned int blkshft;
uint64_t blksz;
if (zfs_prefetch_disable)
return;
/* files that aren't ln2 blocksz are only one block -- nothing to do */
if (!zf->zf_dnode->dn_datablkshift)
return;
/* convert offset and size, into blockid and nblocks */
blkshft = zf->zf_dnode->dn_datablkshift;
blksz = (1 << blkshft);
bzero(&zst, sizeof (zstream_t));
zst.zst_offset = offset >> blkshft;
zst.zst_len = (P2ROUNDUP(offset + size, blksz) -
P2ALIGN(offset, blksz)) >> blkshft;
fetched = dmu_zfetch_find(zf, &zst, prefetched);
if (fetched) {
ZFETCHSTAT_BUMP(zfetchstat_hits);
} else {
ZFETCHSTAT_BUMP(zfetchstat_misses);
if ((fetched = dmu_zfetch_colinear(zf, &zst))) {
ZFETCHSTAT_BUMP(zfetchstat_colinear_hits);
} else {
ZFETCHSTAT_BUMP(zfetchstat_colinear_misses);
}
}
if (!fetched) {
newstream = dmu_zfetch_stream_reclaim(zf);
/*
* we still couldn't find a stream, drop the lock, and allocate
* one if possible. Otherwise, give up and go home.
*/
if (newstream) {
ZFETCHSTAT_BUMP(zfetchstat_reclaim_successes);
} else {
uint64_t maxblocks;
uint32_t max_streams;
uint32_t cur_streams;
ZFETCHSTAT_BUMP(zfetchstat_reclaim_failures);
cur_streams = zf->zf_stream_cnt;
maxblocks = zf->zf_dnode->dn_maxblkid;
max_streams = MIN(zfetch_max_streams,
(maxblocks / zfetch_block_cap));
if (max_streams == 0) {
max_streams++;
}
if (cur_streams >= max_streams) {
return;
}
newstream =
kmem_zalloc(sizeof (zstream_t), KM_SLEEP);
}
newstream->zst_offset = zst.zst_offset;
newstream->zst_len = zst.zst_len;
newstream->zst_stride = zst.zst_len;
newstream->zst_ph_offset = zst.zst_len + zst.zst_offset;
newstream->zst_cap = zst.zst_len;
newstream->zst_direction = ZFETCH_FORWARD;
newstream->zst_last = ddi_get_lbolt();
mutex_init(&newstream->zst_lock, NULL, MUTEX_DEFAULT, NULL);
rw_enter(&zf->zf_rwlock, RW_WRITER);
inserted = dmu_zfetch_stream_insert(zf, newstream);
rw_exit(&zf->zf_rwlock);
if (!inserted) {
mutex_destroy(&newstream->zst_lock);
kmem_free(newstream, sizeof (zstream_t));
}
}
}
#if defined(_KERNEL) && defined(HAVE_SPL)
module_param(zfs_prefetch_disable, int, 0644);
MODULE_PARM_DESC(zfs_prefetch_disable, "Disable all ZFS prefetching");
module_param(zfetch_max_streams, uint, 0644);
MODULE_PARM_DESC(zfetch_max_streams, "Max number of streams per zfetch");
module_param(zfetch_min_sec_reap, uint, 0644);
MODULE_PARM_DESC(zfetch_min_sec_reap, "Min time before stream reclaim");
module_param(zfetch_block_cap, uint, 0644);
MODULE_PARM_DESC(zfetch_block_cap, "Max number of blocks to fetch at a time");
module_param(zfetch_array_rd_sz, ulong, 0644);
MODULE_PARM_DESC(zfetch_array_rd_sz, "Number of bytes in a array_read");
#endif