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linux/fs/xfs/xfs_extent_busy.c
Brian Foster 06058bc405 xfs: don't reuse busy extents on extent trim 
Freed extents are marked busy from the point the freeing transaction
commits until the associated CIL context is checkpointed to the log.
This prevents reuse and overwrite of recently freed blocks before
the changes are committed to disk, which can lead to corruption
after a crash. The exception to this rule is that metadata
allocation is allowed to reuse busy extents because metadata changes
are also logged.

As of commit 97d3ac75e5 ("xfs: exact busy extent tracking"), XFS
has allowed modification or complete invalidation of outstanding
busy extents for metadata allocations. This implementation assumes
that use of the associated extent is imminent, which is not always
the case. For example, the trimmed extent might not satisfy the
minimum length of the allocation request, or the allocation
algorithm might be involved in a search for the optimal result based
on locality.

generic/019 reproduces a corruption caused by this scenario. First,
a metadata block (usually a bmbt or symlink block) is freed from an
inode. A subsequent bmbt split on an unrelated inode attempts a near
mode allocation request that invalidates the busy block during the
search, but does not ultimately allocate it. Due to the busy state
invalidation, the block is no longer considered busy to subsequent
allocation. A direct I/O write request immediately allocates the
block and writes to it. Finally, the filesystem crashes while in a
state where the initial metadata block free had not committed to the
on-disk log. After recovery, the original metadata block is in its
original location as expected, but has been corrupted by the
aforementioned dio.

This demonstrates that it is fundamentally unsafe to modify busy
extent state for extents that are not guaranteed to be allocated.
This applies to pretty much all of the code paths that currently
trim busy extents for one reason or another. Therefore to address
this problem, drop the reuse mechanism from the busy extent trim
path. This code already knows how to return partial non-busy ranges
of the targeted free extent and higher level code tracks the busy
state of the allocation attempt. If a block allocation fails where
one or more candidate extents is busy, we force the log and retry
the allocation.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
2021-02-25 07:58:46 -08:00

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// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (c) 2000-2002,2005 Silicon Graphics, Inc.
* Copyright (c) 2010 David Chinner.
* Copyright (c) 2011 Christoph Hellwig.
* All Rights Reserved.
*/
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_shared.h"
#include "xfs_trans_resv.h"
#include "xfs_sb.h"
#include "xfs_mount.h"
#include "xfs_alloc.h"
#include "xfs_extent_busy.h"
#include "xfs_trace.h"
#include "xfs_trans.h"
#include "xfs_log.h"
void
xfs_extent_busy_insert(
struct xfs_trans *tp,
xfs_agnumber_t agno,
xfs_agblock_t bno,
xfs_extlen_t len,
unsigned int flags)
{
struct xfs_extent_busy *new;
struct xfs_extent_busy *busyp;
struct xfs_perag *pag;
struct rb_node **rbp;
struct rb_node *parent = NULL;
new = kmem_zalloc(sizeof(struct xfs_extent_busy), 0);
new->agno = agno;
new->bno = bno;
new->length = len;
INIT_LIST_HEAD(&new->list);
new->flags = flags;
/* trace before insert to be able to see failed inserts */
trace_xfs_extent_busy(tp->t_mountp, agno, bno, len);
pag = xfs_perag_get(tp->t_mountp, new->agno);
spin_lock(&pag->pagb_lock);
rbp = &pag->pagb_tree.rb_node;
while (*rbp) {
parent = *rbp;
busyp = rb_entry(parent, struct xfs_extent_busy, rb_node);
if (new->bno < busyp->bno) {
rbp = &(*rbp)->rb_left;
ASSERT(new->bno + new->length <= busyp->bno);
} else if (new->bno > busyp->bno) {
rbp = &(*rbp)->rb_right;
ASSERT(bno >= busyp->bno + busyp->length);
} else {
ASSERT(0);
}
}
rb_link_node(&new->rb_node, parent, rbp);
rb_insert_color(&new->rb_node, &pag->pagb_tree);
list_add(&new->list, &tp->t_busy);
spin_unlock(&pag->pagb_lock);
xfs_perag_put(pag);
}
/*
* Search for a busy extent within the range of the extent we are about to
* allocate. You need to be holding the busy extent tree lock when calling
* xfs_extent_busy_search(). This function returns 0 for no overlapping busy
* extent, -1 for an overlapping but not exact busy extent, and 1 for an exact
* match. This is done so that a non-zero return indicates an overlap that
* will require a synchronous transaction, but it can still be
* used to distinguish between a partial or exact match.
*/
int
xfs_extent_busy_search(
struct xfs_mount *mp,
xfs_agnumber_t agno,
xfs_agblock_t bno,
xfs_extlen_t len)
{
struct xfs_perag *pag;
struct rb_node *rbp;
struct xfs_extent_busy *busyp;
int match = 0;
pag = xfs_perag_get(mp, agno);
spin_lock(&pag->pagb_lock);
rbp = pag->pagb_tree.rb_node;
/* find closest start bno overlap */
while (rbp) {
busyp = rb_entry(rbp, struct xfs_extent_busy, rb_node);
if (bno < busyp->bno) {
/* may overlap, but exact start block is lower */
if (bno + len > busyp->bno)
match = -1;
rbp = rbp->rb_left;
} else if (bno > busyp->bno) {
/* may overlap, but exact start block is higher */
if (bno < busyp->bno + busyp->length)
match = -1;
rbp = rbp->rb_right;
} else {
/* bno matches busyp, length determines exact match */
match = (busyp->length == len) ? 1 : -1;
break;
}
}
spin_unlock(&pag->pagb_lock);
xfs_perag_put(pag);
return match;
}
/*
* The found free extent [fbno, fend] overlaps part or all of the given busy
* extent. If the overlap covers the beginning, the end, or all of the busy
* extent, the overlapping portion can be made unbusy and used for the
* allocation. We can't split a busy extent because we can't modify a
* transaction/CIL context busy list, but we can update an entry's block
* number or length.
*
* Returns true if the extent can safely be reused, or false if the search
* needs to be restarted.
*/
STATIC bool
xfs_extent_busy_update_extent(
struct xfs_mount *mp,
struct xfs_perag *pag,
struct xfs_extent_busy *busyp,
xfs_agblock_t fbno,
xfs_extlen_t flen,
bool userdata) __releases(&pag->pagb_lock)
__acquires(&pag->pagb_lock)
{
xfs_agblock_t fend = fbno + flen;
xfs_agblock_t bbno = busyp->bno;
xfs_agblock_t bend = bbno + busyp->length;
/*
* This extent is currently being discarded. Give the thread
* performing the discard a chance to mark the extent unbusy
* and retry.
*/
if (busyp->flags & XFS_EXTENT_BUSY_DISCARDED) {
spin_unlock(&pag->pagb_lock);
delay(1);
spin_lock(&pag->pagb_lock);
return false;
}
/*
* If there is a busy extent overlapping a user allocation, we have
* no choice but to force the log and retry the search.
*
* Fortunately this does not happen during normal operation, but
* only if the filesystem is very low on space and has to dip into
* the AGFL for normal allocations.
*/
if (userdata)
goto out_force_log;
if (bbno < fbno && bend > fend) {
/*
* Case 1:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +---------+
* fbno fend
*/
/*
* We would have to split the busy extent to be able to track
* it correct, which we cannot do because we would have to
* modify the list of busy extents attached to the transaction
* or CIL context, which is immutable.
*
* Force out the log to clear the busy extent and retry the
* search.
*/
goto out_force_log;
} else if (bbno >= fbno && bend <= fend) {
/*
* Case 2:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-----------------+
* fbno fend
*
* Case 3:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +--------------------------+
* fbno fend
*
* Case 4:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +--------------------------+
* fbno fend
*
* Case 5:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-----------------------------------+
* fbno fend
*
*/
/*
* The busy extent is fully covered by the extent we are
* allocating, and can simply be removed from the rbtree.
* However we cannot remove it from the immutable list
* tracking busy extents in the transaction or CIL context,
* so set the length to zero to mark it invalid.
*
* We also need to restart the busy extent search from the
* tree root, because erasing the node can rearrange the
* tree topology.
*/
rb_erase(&busyp->rb_node, &pag->pagb_tree);
busyp->length = 0;
return false;
} else if (fend < bend) {
/*
* Case 6:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +---------+
* fbno fend
*
* Case 7:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +------------------+
* fbno fend
*
*/
busyp->bno = fend;
} else if (bbno < fbno) {
/*
* Case 8:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-------------+
* fbno fend
*
* Case 9:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +----------------------+
* fbno fend
*/
busyp->length = fbno - busyp->bno;
} else {
ASSERT(0);
}
trace_xfs_extent_busy_reuse(mp, pag->pag_agno, fbno, flen);
return true;
out_force_log:
spin_unlock(&pag->pagb_lock);
xfs_log_force(mp, XFS_LOG_SYNC);
trace_xfs_extent_busy_force(mp, pag->pag_agno, fbno, flen);
spin_lock(&pag->pagb_lock);
return false;
}
/*
* For a given extent [fbno, flen], make sure we can reuse it safely.
*/
void
xfs_extent_busy_reuse(
struct xfs_mount *mp,
xfs_agnumber_t agno,
xfs_agblock_t fbno,
xfs_extlen_t flen,
bool userdata)
{
struct xfs_perag *pag;
struct rb_node *rbp;
ASSERT(flen > 0);
pag = xfs_perag_get(mp, agno);
spin_lock(&pag->pagb_lock);
restart:
rbp = pag->pagb_tree.rb_node;
while (rbp) {
struct xfs_extent_busy *busyp =
rb_entry(rbp, struct xfs_extent_busy, rb_node);
xfs_agblock_t bbno = busyp->bno;
xfs_agblock_t bend = bbno + busyp->length;
if (fbno + flen <= bbno) {
rbp = rbp->rb_left;
continue;
} else if (fbno >= bend) {
rbp = rbp->rb_right;
continue;
}
if (!xfs_extent_busy_update_extent(mp, pag, busyp, fbno, flen,
userdata))
goto restart;
}
spin_unlock(&pag->pagb_lock);
xfs_perag_put(pag);
}
/*
* For a given extent [fbno, flen], search the busy extent list to find a
* subset of the extent that is not busy. If *rlen is smaller than
* args->minlen no suitable extent could be found, and the higher level
* code needs to force out the log and retry the allocation.
*
* Return the current busy generation for the AG if the extent is busy. This
* value can be used to wait for at least one of the currently busy extents
* to be cleared. Note that the busy list is not guaranteed to be empty after
* the gen is woken. The state of a specific extent must always be confirmed
* with another call to xfs_extent_busy_trim() before it can be used.
*/
bool
xfs_extent_busy_trim(
struct xfs_alloc_arg *args,
xfs_agblock_t *bno,
xfs_extlen_t *len,
unsigned *busy_gen)
{
xfs_agblock_t fbno;
xfs_extlen_t flen;
struct rb_node *rbp;
bool ret = false;
ASSERT(*len > 0);
spin_lock(&args->pag->pagb_lock);
fbno = *bno;
flen = *len;
rbp = args->pag->pagb_tree.rb_node;
while (rbp && flen >= args->minlen) {
struct xfs_extent_busy *busyp =
rb_entry(rbp, struct xfs_extent_busy, rb_node);
xfs_agblock_t fend = fbno + flen;
xfs_agblock_t bbno = busyp->bno;
xfs_agblock_t bend = bbno + busyp->length;
if (fend <= bbno) {
rbp = rbp->rb_left;
continue;
} else if (fbno >= bend) {
rbp = rbp->rb_right;
continue;
}
if (bbno <= fbno) {
/* start overlap */
/*
* Case 1:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +---------+
* fbno fend
*
* Case 2:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-------------+
* fbno fend
*
* Case 3:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-------------+
* fbno fend
*
* Case 4:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-----------------+
* fbno fend
*
* No unbusy region in extent, return failure.
*/
if (fend <= bend)
goto fail;
/*
* Case 5:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +----------------------+
* fbno fend
*
* Case 6:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +--------------------------+
* fbno fend
*
* Needs to be trimmed to:
* +-------+
* fbno fend
*/
fbno = bend;
} else if (bend >= fend) {
/* end overlap */
/*
* Case 7:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +------------------+
* fbno fend
*
* Case 8:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +--------------------------+
* fbno fend
*
* Needs to be trimmed to:
* +-------+
* fbno fend
*/
fend = bbno;
} else {
/* middle overlap */
/*
* Case 9:
* bbno bend
* +BBBBBBBBBBBBBBBBB+
* +-----------------------------------+
* fbno fend
*
* Can be trimmed to:
* +-------+ OR +-------+
* fbno fend fbno fend
*
* Backward allocation leads to significant
* fragmentation of directories, which degrades
* directory performance, therefore we always want to
* choose the option that produces forward allocation
* patterns.
* Preferring the lower bno extent will make the next
* request use "fend" as the start of the next
* allocation; if the segment is no longer busy at
* that point, we'll get a contiguous allocation, but
* even if it is still busy, we will get a forward
* allocation.
* We try to avoid choosing the segment at "bend",
* because that can lead to the next allocation
* taking the segment at "fbno", which would be a
* backward allocation. We only use the segment at
* "fbno" if it is much larger than the current
* requested size, because in that case there's a
* good chance subsequent allocations will be
* contiguous.
*/
if (bbno - fbno >= args->maxlen) {
/* left candidate fits perfect */
fend = bbno;
} else if (fend - bend >= args->maxlen * 4) {
/* right candidate has enough free space */
fbno = bend;
} else if (bbno - fbno >= args->minlen) {
/* left candidate fits minimum requirement */
fend = bbno;
} else {
goto fail;
}
}
flen = fend - fbno;
}
out:
if (fbno != *bno || flen != *len) {
trace_xfs_extent_busy_trim(args->mp, args->agno, *bno, *len,
fbno, flen);
*bno = fbno;
*len = flen;
*busy_gen = args->pag->pagb_gen;
ret = true;
}
spin_unlock(&args->pag->pagb_lock);
return ret;
fail:
/*
* Return a zero extent length as failure indications. All callers
* re-check if the trimmed extent satisfies the minlen requirement.
*/
flen = 0;
goto out;
}
STATIC void
xfs_extent_busy_clear_one(
struct xfs_mount *mp,
struct xfs_perag *pag,
struct xfs_extent_busy *busyp)
{
if (busyp->length) {
trace_xfs_extent_busy_clear(mp, busyp->agno, busyp->bno,
busyp->length);
rb_erase(&busyp->rb_node, &pag->pagb_tree);
}
list_del_init(&busyp->list);
kmem_free(busyp);
}
static void
xfs_extent_busy_put_pag(
struct xfs_perag *pag,
bool wakeup)
__releases(pag->pagb_lock)
{
if (wakeup) {
pag->pagb_gen++;
wake_up_all(&pag->pagb_wait);
}
spin_unlock(&pag->pagb_lock);
xfs_perag_put(pag);
}
/*
* Remove all extents on the passed in list from the busy extents tree.
* If do_discard is set skip extents that need to be discarded, and mark
* these as undergoing a discard operation instead.
*/
void
xfs_extent_busy_clear(
struct xfs_mount *mp,
struct list_head *list,
bool do_discard)
{
struct xfs_extent_busy *busyp, *n;
struct xfs_perag *pag = NULL;
xfs_agnumber_t agno = NULLAGNUMBER;
bool wakeup = false;
list_for_each_entry_safe(busyp, n, list, list) {
if (busyp->agno != agno) {
if (pag)
xfs_extent_busy_put_pag(pag, wakeup);
agno = busyp->agno;
pag = xfs_perag_get(mp, agno);
spin_lock(&pag->pagb_lock);
wakeup = false;
}
if (do_discard && busyp->length &&
!(busyp->flags & XFS_EXTENT_BUSY_SKIP_DISCARD)) {
busyp->flags = XFS_EXTENT_BUSY_DISCARDED;
} else {
xfs_extent_busy_clear_one(mp, pag, busyp);
wakeup = true;
}
}
if (pag)
xfs_extent_busy_put_pag(pag, wakeup);
}
/*
* Flush out all busy extents for this AG.
*/
void
xfs_extent_busy_flush(
struct xfs_mount *mp,
struct xfs_perag *pag,
unsigned busy_gen)
{
DEFINE_WAIT (wait);
int error;
error = xfs_log_force(mp, XFS_LOG_SYNC);
if (error)
return;
do {
prepare_to_wait(&pag->pagb_wait, &wait, TASK_KILLABLE);
if (busy_gen != READ_ONCE(pag->pagb_gen))
break;
schedule();
} while (1);
finish_wait(&pag->pagb_wait, &wait);
}
void
xfs_extent_busy_wait_all(
struct xfs_mount *mp)
{
DEFINE_WAIT (wait);
xfs_agnumber_t agno;
for (agno = 0; agno < mp->m_sb.sb_agcount; agno++) {
struct xfs_perag *pag = xfs_perag_get(mp, agno);
do {
prepare_to_wait(&pag->pagb_wait, &wait, TASK_KILLABLE);
if (RB_EMPTY_ROOT(&pag->pagb_tree))
break;
schedule();
} while (1);
finish_wait(&pag->pagb_wait, &wait);
xfs_perag_put(pag);
}
}
/*
* Callback for list_sort to sort busy extents by the AG they reside in.
*/
int
xfs_extent_busy_ag_cmp(
void *priv,
struct list_head *l1,
struct list_head *l2)
{
struct xfs_extent_busy *b1 =
container_of(l1, struct xfs_extent_busy, list);
struct xfs_extent_busy *b2 =
container_of(l2, struct xfs_extent_busy, list);
s32 diff;
diff = b1->agno - b2->agno;
if (!diff)
diff = b1->bno - b2->bno;
return diff;
}
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