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linux/fs/xfs/xfs_log.h
Darrick J. Wong 5302a5c8be xfs: only clear log incompat flags at clean unmount 
While reviewing the online fsck patchset, someone spied the
xfs_swapext_can_use_without_log_assistance function and wondered why we
go through this inverted-bitmask dance to avoid setting the
XFS_SB_FEAT_INCOMPAT_LOG_SWAPEXT feature.

(The same principles apply to the logged extended attribute update
feature bit in the since-merged LARP series.)

The reason for this dance is that xfs_add_incompat_log_feature is an
expensive operation -- it forces the log, pushes the AIL, and then if
nobody's beaten us to it, sets the feature bit and issues a synchronous
write of the primary superblock.  That could be a one-time cost
amortized over the life of the filesystem, but the log quiesce and cover
operations call xfs_clear_incompat_log_features to remove feature bits
opportunistically.  On a moderately loaded filesystem this leads to us
cycling those bits on and off over and over, which hurts performance.

Why do we clear the log incompat bits?  Back in ~2020 I think Dave and I
had a conversation on IRC[2] about what the log incompat bits represent.
IIRC in that conversation we decided that the log incompat bits protect
unrecovered log items so that old kernels won't try to recover them and
barf.  Since a clean log has no protected log items, we could clear the
bits at cover/quiesce time.

As Dave Chinner pointed out in the thread, clearing log incompat bits at
unmount time has positive effects for golden root disk image generator
setups, since the generator could be running a newer kernel than what
gets written to the golden image -- if there are log incompat fields set
in the golden image that was generated by a newer kernel/OS image
builder then the provisioning host cannot mount the filesystem even
though the log is clean and recovery is unnecessary to mount the
filesystem.

Given that it's expensive to set log incompat bits, we really only want
to do that once per bit per mount.  Therefore, I propose that we only
clear log incompat bits as part of writing a clean unmount record.  Do
this by adding an operational state flag to the xfs mount that guards
whether or not the feature bit clearing can actually take place.

This eliminates the l_incompat_users rwsem that we use to protect a log
cleaning operation from clearing a feature bit that a frontend thread is
trying to set -- this lock adds another way to fail w.r.t. locking.  For
the swapext series, I shard that into multiple locks just to work around
the lockdep complaints, and that's fugly.

Link: https://lore.kernel.org/linux-xfs/20240131230043.GA6180@frogsfrogsfrogs/
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
2024-04-15 14:54:06 -07:00

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// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (c) 2000-2003,2005 Silicon Graphics, Inc.
* All Rights Reserved.
*/
#ifndef __XFS_LOG_H__
#define __XFS_LOG_H__
struct xfs_cil_ctx;
struct xfs_log_vec {
struct list_head lv_list; /* CIL lv chain ptrs */
uint32_t lv_order_id; /* chain ordering info */
int lv_niovecs; /* number of iovecs in lv */
struct xfs_log_iovec *lv_iovecp; /* iovec array */
struct xfs_log_item *lv_item; /* owner */
char *lv_buf; /* formatted buffer */
int lv_bytes; /* accounted space in buffer */
int lv_buf_len; /* aligned size of buffer */
int lv_size; /* size of allocated lv */
};
#define XFS_LOG_VEC_ORDERED (-1)
/*
* Calculate the log iovec length for a given user buffer length. Intended to be
* used by ->iop_size implementations when sizing buffers of arbitrary
* alignments.
*/
static inline int
xlog_calc_iovec_len(int len)
{
return roundup(len, sizeof(uint32_t));
}
void *xlog_prepare_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
uint type);
static inline void
xlog_finish_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec *vec,
int data_len)
{
struct xlog_op_header *oph = vec->i_addr;
int len;
/*
* Always round up the length to the correct alignment so callers don't
* need to know anything about this log vec layout requirement. This
* means we have to zero the area the data to be written does not cover.
* This is complicated by fact the payload region is offset into the
* logvec region by the opheader that tracks the payload.
*/
len = xlog_calc_iovec_len(data_len);
if (len - data_len != 0) {
char *buf = vec->i_addr + sizeof(struct xlog_op_header);
memset(buf + data_len, 0, len - data_len);
}
/*
* The opheader tracks aligned payload length, whilst the logvec tracks
* the overall region length.
*/
oph->oh_len = cpu_to_be32(len);
len += sizeof(struct xlog_op_header);
lv->lv_buf_len += len;
lv->lv_bytes += len;
vec->i_len = len;
/* Catch buffer overruns */
ASSERT((void *)lv->lv_buf + lv->lv_bytes <= (void *)lv + lv->lv_size);
}
/*
* Copy the amount of data requested by the caller into a new log iovec.
*/
static inline void *
xlog_copy_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
uint type, void *data, int len)
{
void *buf;
buf = xlog_prepare_iovec(lv, vecp, type);
memcpy(buf, data, len);
xlog_finish_iovec(lv, *vecp, len);
return buf;
}
static inline void *
xlog_copy_from_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
const struct xfs_log_iovec *src)
{
return xlog_copy_iovec(lv, vecp, src->i_type, src->i_addr, src->i_len);
}
/*
* By comparing each component, we don't have to worry about extra
* endian issues in treating two 32 bit numbers as one 64 bit number
*/
static inline xfs_lsn_t _lsn_cmp(xfs_lsn_t lsn1, xfs_lsn_t lsn2)
{
if (CYCLE_LSN(lsn1) != CYCLE_LSN(lsn2))
return (CYCLE_LSN(lsn1)<CYCLE_LSN(lsn2))? -999 : 999;
if (BLOCK_LSN(lsn1) != BLOCK_LSN(lsn2))
return (BLOCK_LSN(lsn1)<BLOCK_LSN(lsn2))? -999 : 999;
return 0;
}
#define XFS_LSN_CMP(x,y) _lsn_cmp(x,y)
/*
* Flags to xfs_log_force()
*
* XFS_LOG_SYNC: Synchronous force in-core log to disk
*/
#define XFS_LOG_SYNC 0x1
/* Log manager interfaces */
struct xfs_mount;
struct xlog_in_core;
struct xlog_ticket;
struct xfs_log_item;
struct xfs_item_ops;
struct xfs_trans;
struct xlog;
int xfs_log_force(struct xfs_mount *mp, uint flags);
int xfs_log_force_seq(struct xfs_mount *mp, xfs_csn_t seq, uint flags,
int *log_forced);
int xfs_log_mount(struct xfs_mount *mp,
struct xfs_buftarg *log_target,
xfs_daddr_t start_block,
int num_bblocks);
int xfs_log_mount_finish(struct xfs_mount *mp);
void xfs_log_mount_cancel(struct xfs_mount *);
xfs_lsn_t xlog_assign_tail_lsn(struct xfs_mount *mp);
xfs_lsn_t xlog_assign_tail_lsn_locked(struct xfs_mount *mp);
void xfs_log_space_wake(struct xfs_mount *mp);
int xfs_log_reserve(struct xfs_mount *mp, int length, int count,
struct xlog_ticket **ticket, bool permanent);
int xfs_log_regrant(struct xfs_mount *mp, struct xlog_ticket *tic);
void xfs_log_unmount(struct xfs_mount *mp);
bool xfs_log_writable(struct xfs_mount *mp);
struct xlog_ticket *xfs_log_ticket_get(struct xlog_ticket *ticket);
void xfs_log_ticket_put(struct xlog_ticket *ticket);
void xlog_cil_process_committed(struct list_head *list);
bool xfs_log_item_in_current_chkpt(struct xfs_log_item *lip);
void xfs_log_work_queue(struct xfs_mount *mp);
int xfs_log_quiesce(struct xfs_mount *mp);
void xfs_log_clean(struct xfs_mount *mp);
bool xfs_log_check_lsn(struct xfs_mount *, xfs_lsn_t);
xfs_lsn_t xlog_grant_push_threshold(struct xlog *log, int need_bytes);
bool xlog_force_shutdown(struct xlog *log, uint32_t shutdown_flags);
int xfs_attr_use_log_assist(struct xfs_mount *mp);
#endif /* __XFS_LOG_H__ */
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