Re: [PATCH 016/119] xfs: move deferred operations into a separate file

Brian Foster
2016-06-28T12:32:40Z

On Mon, Jun 27, 2016 at 12:14:01PM -0700, Darrick J. Wong wrote:
> On Mon, Jun 27, 2016 at 09:14:54AM -0400, Brian Foster wrote:
> > On Thu, Jun 16, 2016 at 06:19:34PM -0700, Darrick J. Wong wrote:
> > > All the code around struct xfs_bmap_free basically implements a
> > > deferred operation framework through which we can roll transactions
> > > (to unlock buffers and avoid violating lock order rules) while
> > > managing all the necessary log redo items. Previously we only used
> > > this code to free extents after some sort of mapping operation, but
> > > with the advent of rmap and reflink, we suddenly need to do more than
> > > that.
> > >
> > > With that in mind, xfs_bmap_free really becomes a deferred ops control
> > > structure. Rename the structure and move the deferred ops into their
> > > own file to avoid further bloating of the bmap code.
> > >
> > > v2: actually sort the work items by AG to avoid deadlocks
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> >
> > So if I'm following this correctly, we 1.) abstract the bmap freeing
> > infrastructure into a generic mechanism and 2.) enhance it a bit to
> > provide things like partial intent completion, etc.
>
> [Back from vacation]
>
> Yup. The partial intent completion code is for use by the refcount adjust
> function because in the worst case an adjustment of N blocks could require
> N record updates.
>

Ok, technically those bits could be punted off to the reflink series.

> > If so and for future
> > reference, this would probably be easier to review if the abstraction
> > and enhancement were done separately. It's probably not worth that at
> > this point, however...
>
> It wouldn't be difficult to separate them; the partial intent completion
> are the two code blocks below that handle the -EAGAIN case.
>

That's kind of what I figured, since otherwise most of the rest of the
code maps to the xfs_bmap_*() stuff.

> (On the other hand it's so little code that I figured I might as well
> just do the whole file all at once.)
>

It's more a matter of simplifying review when a change is explicitly
refactoring vs. having to read through and identify where the
enhancements actually are. It leaves a cleaner git history and tends to
simplify backporting as well, fwiw.

That said, I don't mind leaving this one as is at this point.

> > > fs/xfs/Makefile | 2
> > > fs/xfs/libxfs/xfs_defer.c | 471 +++++++++++++++++++++++++++++++++++++++++++++
> > > fs/xfs/libxfs/xfs_defer.h | 96 +++++++++
> > > fs/xfs/xfs_defer_item.c | 36 +++
> > > fs/xfs/xfs_super.c | 2
> > > 5 files changed, 607 insertions(+)
> > > create mode 100644 fs/xfs/libxfs/xfs_defer.c
> > > create mode 100644 fs/xfs/libxfs/xfs_defer.h
> > > create mode 100644 fs/xfs/xfs_defer_item.c
> > >
> > >
> > > diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
> > > index 3542d94..ad46a2d 100644
> > > --- a/fs/xfs/Makefile
> > > +++ b/fs/xfs/Makefile
> > > @@ -39,6 +39,7 @@ xfs-y += $(addprefix libxfs/, \
> > > xfs_btree.o \
> > > xfs_da_btree.o \
> > > xfs_da_format.o \
> > > + xfs_defer.o \
> > > xfs_dir2.o \
> > > xfs_dir2_block.o \
> > > xfs_dir2_data.o \
> > > @@ -66,6 +67,7 @@ xfs-y += xfs_aops.o \
> > > xfs_attr_list.o \
> > > xfs_bmap_util.o \
> > > xfs_buf.o \
> > > + xfs_defer_item.o \
> > > xfs_dir2_readdir.o \
> > > xfs_discard.o \
> > > xfs_error.o \
> > > diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> > > new file mode 100644
> > > index 0000000..ad14e33e
> > > --- /dev/null
> > > +++ b/fs/xfs/libxfs/xfs_defer.c
...
> > > +int
> > > +xfs_defer_finish(
> > > + struct xfs_trans **tp,
> > > + struct xfs_defer_ops *dop,
> > > + struct xfs_inode *ip)
> > > +{
> > > + struct xfs_defer_pending *dfp;
> > > + struct list_head *li;
> > > + struct list_head *n;
> > > + void *done_item = NULL;
> > > + void *state;
> > > + int error = 0;
> > > + void (*cleanup_fn)(struct xfs_trans *, void *, int);
> > > +
> > > + ASSERT((*tp)->t_flags & XFS_TRANS_PERM_LOG_RES);
> > > +
> > > + /* Until we run out of pending work to finish... */
> > > + while (xfs_defer_has_unfinished_work(dop)) {
> > > + /* Log intents for work items sitting in the intake. */
> > > + xfs_defer_intake_work(*tp, dop);
> > > +
> > > + /* Roll the transaction. */
> > > + error = xfs_defer_trans_roll(tp, dop, ip);
> > > + if (error)
> > > + goto out;
> > > +
> > > + /* Mark all pending intents as committed. */
> > > + list_for_each_entry_reverse(dfp, &dop->dop_pending, dfp_list) {
> > > + if (dfp->dfp_committed)
> > > + break;
> > > + dfp->dfp_committed = true;
> > > + }
> > > +
> > > + /* Log an intent-done item for the first pending item. */
> > > + dfp = list_first_entry(&dop->dop_pending,
> > > + struct xfs_defer_pending, dfp_list);
> > > + done_item = dfp->dfp_type->create_done(*tp, dfp->dfp_intent,
> > > + dfp->dfp_count);
> > > + cleanup_fn = dfp->dfp_type->finish_cleanup;
> > > +
> > > + /* Finish the work items. */
> > > + state = NULL;
> > > + list_for_each_safe(li, n, &dfp->dfp_work) {
> > > + list_del(li);
> > > + dfp->dfp_count--;
> > > + error = dfp->dfp_type->finish_item(*tp, dop, li,
> > > + done_item, &state);
> > > + if (error == -EAGAIN) {
> > > + /*
> > > + * If the caller needs to try again, put the
> > > + * item back on the pending list and jump out
> > > + * for further processing.
> >
> > A little confused by the terminology here. Perhaps better to say "back
> > on the work list" rather than "pending list?"
>
> Yes.
>
> > Also, what is the meaning/purpose of -EAGAIN here? This isn't used by
> > the extent free bits so I'm missing some context.
>
> Generally, the ->finish_item() uses -EAGAIN to signal that it couldn't finish
> the work item and that it's necessary to log a new redo item and try again.
>

Ah, Ok. So it is explicitly part of the dfops interface/infrastructure.
I think that is worth documenting above with a comment (i.e., "certain
callers might require many transactions, use -EAGAIN to indicate ...
blah blah").

> Practically, the only user of this mechanism is the refcountbt adjust function.
> It might be the case that we want to adjust N blocks, but some pathological
> user has creatively used reflink to create many refcount records. In that
> case we could blow out the transaction reservation logging all the updates.
>
> To avoid that, the refcount code tries to guess (conservatively) when it
> might be getting close and returns a short *adjusted. See the call sites of
> xfs_refcount_still_have_space(). Next, xfs_trans_log_finish_refcount_update()
> will notice the short adjust returned and fixes up the CUD item to have a
> reduced cud_nextents and to reflect where the operation stopped. Then,
> xfs_refcount_update_finish_item() notices the short return, updates the work
> item list, and returns -EAGAIN. Finally, xfs_defer_finish() sees the -EAGAIN
> and requeues the work item so that we resume refcount adjusting after the
> transaction rolls.
>

Hmm, this makes me think that maybe it is better to split this up into
two patches for now after all. I'm expecting this is going to be merged
along with the rmap bits before the refcount stuff and I'm not a huge
fan of putting in infrastructure code without users, moreso without
fully understanding how/why said code is going to be used (and I'm not
really looking to jump ahead into the refcount stuff yet).

> > For example, is there
> > an issue with carrying a done_item with an unexpected list count?
>
> AFAICT, nothing in log recovery ever checks that the list counts of the
> intent and done items actually match, let alone the extents logged with
> them. It only seems to care if there's an efd such that efd->efd_efi_id ==
> efi->efi_id, in which case it won't replay the efi.
>

Yeah, I didn't notice any issues with respect to EFI/EFD handling,
though I didn't look too hard because it doesn't use this -EAGAIN
mechanism. If it did, I think you might hit the odd ASSERT() check here
or there (see xfs_efd_item_format()), but that's probably not
catastrophic. I think it also affects the size of the transaction
written to the log, fwiw.

I ask more because it's unexpected to have a structure with a list count
that doesn't match the actual number of items and I don't see it called
out anywhere. This might be another good reason to punt this part off to
the reflink series...

> I don't know if that was a deliberate part of the log design, but the
> lack of checking helps us here.
>
> > Is it
> > expected that xfs_defer_finish() will not return until -EAGAIN is
> > "cleared" (does relogging below and rolling somehow address this)?
>
> Yes, relogging and rolling gives us a fresh transaction with which to
> continue updating.
>
> > > + */
> > > + list_add(li, &dfp->dfp_work);
> > > + dfp->dfp_count++;
> > > + break;
> > > + } else if (error) {
> > > + /*
> > > + * Clean up after ourselves and jump out.
> > > + * xfs_defer_cancel will take care of freeing
> > > + * all these lists and stuff.
> > > + */
> > > + if (cleanup_fn)
> > > + cleanup_fn(*tp, state, error);
> > > + xfs_defer_trans_abort(*tp, dop, error);
> > > + goto out;
> > > + }
> > > + }
> > > + if (error == -EAGAIN) {
> > > + /*
> > > + * Log a new intent, relog all the remaining work
> > > + * item to the new intent, attach the new intent to
> > > + * the dfp, and leave the dfp at the head of the list
> > > + * for further processing.
> > > + */
> >
> > Similar to the above, could you elaborate on the mechanics of this with
> > respect to the log? E.g., the comment kind of just repeats what the
> > code does as opposed to explain why it's here. Is the point here to log
> > a new intent in the same transaction as the done item to ensure that we
> > (atomically) indicate that certain operations need to be replayed if
> > this transaction hits the log and then we crash?
>
> Yes.
>
> "This effectively replaces the old intent item with a new one listing only
> the work items that were not completed when ->finish_item() returned -EAGAIN.
> After the subsequent transaction roll, we'll resume where we left off with a
> fresh transaction."
>

I'd point out the relevance of doing so in the same transaction,
otherwise sounds good.

Brian

> Thank you for the review!
>
> --D
>
> > Brian
> >
> > > + dfp->dfp_intent = dfp->dfp_type->create_intent(*tp,
> > > + dfp->dfp_count);
> > > + list_for_each(li, &dfp->dfp_work)
> > > + dfp->dfp_type->log_item(*tp, dfp->dfp_intent,
> > > + li);
> > > + } else {
> > > + /* Done with the dfp, free it. */
> > > + list_del(&dfp->dfp_list);
> > > + kmem_free(dfp);
> > > + }
> > > +
> > > + if (cleanup_fn)
> > > + cleanup_fn(*tp, state, error);
> > > + }
> > > +
> > > +out:
> > > + return error;
> > > +}
> > > +
> > > +/*
> > > + * Free up any items left in the list.
> > > + */
> > > +void
> > > +xfs_defer_cancel(
> > > + struct xfs_defer_ops *dop)
> > > +{
> > > + struct xfs_defer_pending *dfp;
> > > + struct xfs_defer_pending *pli;
> > > + struct list_head *pwi;
> > > + struct list_head *n;
> > > +
> > > + /*
> > > + * Free the pending items. Caller should already have arranged
> > > + * for the intent items to be released.
> > > + */
> > > + list_for_each_entry_safe(dfp, pli, &dop->dop_intake, dfp_list) {
> > > + list_del(&dfp->dfp_list);
> > > + list_for_each_safe(pwi, n, &dfp->dfp_work) {
> > > + list_del(pwi);
> > > + dfp->dfp_count--;
> > > + dfp->dfp_type->cancel_item(pwi);
> > > + }
> > > + ASSERT(dfp->dfp_count == 0);
> > > + kmem_free(dfp);
> > > + }
> > > + list_for_each_entry_safe(dfp, pli, &dop->dop_pending, dfp_list) {
> > > + list_del(&dfp->dfp_list);
> > > + list_for_each_safe(pwi, n, &dfp->dfp_work) {
> > > + list_del(pwi);
> > > + dfp->dfp_count--;
> > > + dfp->dfp_type->cancel_item(pwi);
> > > + }
> > > + ASSERT(dfp->dfp_count == 0);
> > > + kmem_free(dfp);
> > > + }
> > > +}
> > > +
> > > +/* Add an item for later deferred processing. */
> > > +void
> > > +xfs_defer_add(
> > > + struct xfs_defer_ops *dop,
> > > + enum xfs_defer_ops_type type,
> > > + struct list_head *li)
> > > +{
> > > + struct xfs_defer_pending *dfp = NULL;
> > > +
> > > + /*
> > > + * Add the item to a pending item at the end of the intake list.
> > > + * If the last pending item has the same type, reuse it. Else,
> > > + * create a new pending item at the end of the intake list.
> > > + */
> > > + if (!list_empty(&dop->dop_intake)) {
> > > + dfp = list_last_entry(&dop->dop_intake,
> > > + struct xfs_defer_pending, dfp_list);
> > > + if (dfp->dfp_type->type != type ||
> > > + (dfp->dfp_type->max_items &&
> > > + dfp->dfp_count >= dfp->dfp_type->max_items))
> > > + dfp = NULL;
> > > + }
> > > + if (!dfp) {
> > > + dfp = kmem_alloc(sizeof(struct xfs_defer_pending),
> > > + KM_SLEEP | KM_NOFS);
> > > + dfp->dfp_type = defer_op_types[type];
> > > + dfp->dfp_committed = false;
> > > + dfp->dfp_intent = NULL;
> > > + dfp->dfp_count = 0;
> > > + INIT_LIST_HEAD(&dfp->dfp_work);
> > > + list_add_tail(&dfp->dfp_list, &dop->dop_intake);
> > > + }
> > > +
> > > + list_add_tail(li, &dfp->dfp_work);
> > > + dfp->dfp_count++;
> > > +}
> > > +
> > > +/* Initialize a deferred operation list. */
> > > +void
> > > +xfs_defer_init_op_type(
> > > + const struct xfs_defer_op_type *type)
> > > +{
> > > + defer_op_types[type->type] = type;
> > > +}
> > > +
> > > +/* Initialize a deferred operation. */
> > > +void
> > > +xfs_defer_init(
> > > + struct xfs_defer_ops *dop,
> > > + xfs_fsblock_t *fbp)
> > > +{
> > > + dop->dop_committed = false;
> > > + dop->dop_low = false;
> > > + memset(&dop->dop_inodes, 0, sizeof(dop->dop_inodes));
> > > + *fbp = NULLFSBLOCK;
> > > + INIT_LIST_HEAD(&dop->dop_intake);
> > > + INIT_LIST_HEAD(&dop->dop_pending);
> > > +}
> > > diff --git a/fs/xfs/libxfs/xfs_defer.h b/fs/xfs/libxfs/xfs_defer.h
> > > new file mode 100644
> > > index 0000000..85c7a3a
> > > --- /dev/null
> > > +++ b/fs/xfs/libxfs/xfs_defer.h
> > > @@ -0,0 +1,96 @@
> > > +/*
> > > + * Copyright (C) 2016 Oracle. All Rights Reserved.
> > > + *
> > > + * Author: Darrick J. Wong <darrick.wong@oracle.com>
> > > + *
> > > + * This program is free software; you can redistribute it and/or
> > > + * modify it under the terms of the GNU General Public License
> > > + * as published by the Free Software Foundation; either version 2
> > > + * of the License, or (at your option) any later version.
> > > + *
> > > + * This program is distributed in the hope that it would be useful,
> > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > > + * GNU General Public License for more details.
> > > + *
> > > + * You should have received a copy of the GNU General Public License
> > > + * along with this program; if not, write the Free Software Foundation,
> > > + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
> > > + */
> > > +#ifndef __XFS_DEFER_H__
> > > +#define __XFS_DEFER_H__
> > > +
> > > +struct xfs_defer_op_type;
> > > +
> > > +/*
> > > + * Save a log intent item and a list of extents, so that we can replay
> > > + * whatever action had to happen to the extent list and file the log done
> > > + * item.
> > > + */
> > > +struct xfs_defer_pending {
> > > + const struct xfs_defer_op_type *dfp_type; /* function pointers */
> > > + struct list_head dfp_list; /* pending items */
> > > + bool dfp_committed; /* committed trans? */
> > > + void *dfp_intent; /* log intent item */
> > > + struct list_head dfp_work; /* work items */
> > > + unsigned int dfp_count; /* # extent items */
> > > +};
> > > +
> > > +/*
> > > + * Header for deferred operation list.
> > > + *
> > > + * dop_low is used by the allocator to activate the lowspace algorithm -
> > > + * when free space is running low the extent allocator may choose to
> > > + * allocate an extent from an AG without leaving sufficient space for
> > > + * a btree split when inserting the new extent. In this case the allocator
> > > + * will enable the lowspace algorithm which is supposed to allow further
> > > + * allocations (such as btree splits and newroots) to allocate from
> > > + * sequential AGs. In order to avoid locking AGs out of order the lowspace
> > > + * algorithm will start searching for free space from AG 0. If the correct
> > > + * transaction reservations have been made then this algorithm will eventually
> > > + * find all the space it needs.
> > > + */
> > > +enum xfs_defer_ops_type {
> > > + XFS_DEFER_OPS_TYPE_MAX,
> > > +};
> > > +
> > > +#define XFS_DEFER_OPS_NR_INODES 2 /* join up to two inodes */
> > > +
> > > +struct xfs_defer_ops {
> > > + bool dop_committed; /* did any trans commit? */
> > > + bool dop_low; /* alloc in low mode */
> > > + struct list_head dop_intake; /* unlogged pending work */
> > > + struct list_head dop_pending; /* logged pending work */
> > > +
> > > + /* relog these inodes with each roll */
> > > + struct xfs_inode *dop_inodes[XFS_DEFER_OPS_NR_INODES];
> > > +};
> > > +
> > > +void xfs_defer_add(struct xfs_defer_ops *dop, enum xfs_defer_ops_type type,
> > > + struct list_head *h);
> > > +int xfs_defer_finish(struct xfs_trans **tp, struct xfs_defer_ops *dop,
> > > + struct xfs_inode *ip);
> > > +void xfs_defer_cancel(struct xfs_defer_ops *dop);
> > > +void xfs_defer_init(struct xfs_defer_ops *dop, xfs_fsblock_t *fbp);
> > > +bool xfs_defer_has_unfinished_work(struct xfs_defer_ops *dop);
> > > +int xfs_defer_join(struct xfs_defer_ops *dop, struct xfs_inode *ip);
> > > +
> > > +/* Description of a deferred type. */
> > > +struct xfs_defer_op_type {
> > > + enum xfs_defer_ops_type type;
> > > + unsigned int max_items;
> > > + void (*abort_intent)(void *);
> > > + void *(*create_done)(struct xfs_trans *, void *, unsigned int);
> > > + int (*finish_item)(struct xfs_trans *, struct xfs_defer_ops *,
> > > + struct list_head *, void *, void **);
> > > + void (*finish_cleanup)(struct xfs_trans *, void *, int);
> > > + void (*cancel_item)(struct list_head *);
> > > + int (*diff_items)(void *, struct list_head *, struct list_head *);
> > > + void *(*create_intent)(struct xfs_trans *, uint);
> > > + void (*log_item)(struct xfs_trans *, void *, struct list_head *);
> > > +};
> > > +
> > > +void xfs_defer_init_op_type(const struct xfs_defer_op_type *type);
> > > +void xfs_defer_init_types(void);
> > > +
> > > +#endif /* __XFS_DEFER_H__ */
> > > diff --git a/fs/xfs/xfs_defer_item.c b/fs/xfs/xfs_defer_item.c
> > > new file mode 100644
> > > index 0000000..849088d
> > > --- /dev/null
> > > +++ b/fs/xfs/xfs_defer_item.c
> > > @@ -0,0 +1,36 @@
> > > +/*
> > > + * Copyright (C) 2016 Oracle. All Rights Reserved.
> > > + *
> > > + * Author: Darrick J. Wong <darrick.wong@oracle.com>
> > > + *
> > > + * This program is free software; you can redistribute it and/or
> > > + * modify it under the terms of the GNU General Public License
> > > + * as published by the Free Software Foundation; either version 2
> > > + * of the License, or (at your option) any later version.
> > > + *
> > > + * This program is distributed in the hope that it would be useful,
> > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > > + * GNU General Public License for more details.
> > > + *
> > > + * You should have received a copy of the GNU General Public License
> > > + * along with this program; if not, write the Free Software Foundation,
> > > + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
> > > + */
> > > +#include "xfs.h"
> > > +#include "xfs_fs.h"
> > > +#include "xfs_shared.h"
> > > +#include "xfs_format.h"
> > > +#include "xfs_log_format.h"
> > > +#include "xfs_trans_resv.h"
> > > +#include "xfs_bit.h"
> > > +#include "xfs_sb.h"
> > > +#include "xfs_mount.h"
> > > +#include "xfs_defer.h"
> > > +#include "xfs_trans.h"
> > > +
> > > +/* Initialize the deferred operation types. */
> > > +void
> > > +xfs_defer_init_types(void)
> > > +{
> > > +}
> > > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > > index 09722a7..bf63f6d 100644
> > > --- a/fs/xfs/xfs_super.c
> > > +++ b/fs/xfs/xfs_super.c
> > > @@ -46,6 +46,7 @@
> > > #include "xfs_quota.h"
> > > #include "xfs_sysfs.h"
> > > #include "xfs_ondisk.h"
> > > +#include "xfs_defer.h"
> > >
> > > #include <linux/namei.h>
> > > #include <linux/init.h>
> > > @@ -1850,6 +1851,7 @@ init_xfs_fs(void)
> > > printk(KERN_INFO XFS_VERSION_STRING " with "
> > > XFS_BUILD_OPTIONS " enabled\n");
> > >
> > > + xfs_defer_init_types();
> > > xfs_dir_startup();
> > >
> > > error = xfs_init_zones();
> > >
> > > _______________________________________________
> > > xfs mailing list
> > > xfs@oss.sgi.com
> > > http://oss.sgi.com/mailman/listinfo/xfs
>
> _______________________________________________
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