Re: [PATCH 7/9] xfs: Add attr context to log item

Darrick J. Wong
2019-04-24T04:10:32Z

Sorry I'm late back to the party...

On Tue, Apr 23, 2019 at 07:24:40PM -0700, Allison Henderson wrote:
>
> On 4/23/19 6:20 AM, Brian Foster wrote:
> > On Mon, Apr 22, 2019 at 03:01:27PM -0700, Allison Henderson wrote:
> > >
> > >
> > > On 4/22/19 6:03 AM, Brian Foster wrote:
> > > > On Fri, Apr 12, 2019 at 03:50:34PM -0700, Allison Henderson wrote:
> > > > > This patch modifies xfs_attr_item to store a xfs_da_args, a xfs_buf pointer
> > > > > and a new state type. We will use these in the next patch when
> > > > > we modify xfs_set_attr_args to roll transactions by returning EAGAIN.
> > > > > Because the subroutines of this function modify the contents of these
> > > > > structures, we need to find a place to store them where they remain
> > > > > instantiated across multiple calls to xfs_set_attr_args.
> > > > >
> > > > > Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> > > > > ---
> > > >
> > > > I see Darrick has already commented on the whole state thing. I'll
> > > > probably have to grok the next patch to comment further, but just a
> > > > couple initial thoughts:
> > > >
> > > > First, I hit a build failure with this patch. It looks like there's a
> > > > missed include in the scrub code:
> > > >
> > > > ...
> > > > CC [M] fs/xfs/scrub/repair.o
> > > > In file included from fs/xfs/scrub/repair.c:32:
> > > > fs/xfs/libxfs/xfs_attr.h:105:21: error: field ‘xattri_args’ has incomplete type
> > > > struct xfs_da_args xattri_args; /* args context */
> > > Hmm, ok. I'll get that corrected, I probably need to clean out my workspace
> > > and build from scratch.
> > >
> > > > ...
> > > >
> > > > Second, the commit log suggests that the states will reflect the current
> > > > transaction roll points (i.e., establishing re-entry points down in
> > > > xfs_attr_set_args(). I'm kind of wondering if we should break these
> > > > xattr set sub-sequences down into smaller helper functions (refactoring
> > > > the existing code as we go) such that the mechanism could technically be

I had had the thought of "why not just give each step of setting an
attribute its own log item, so we don't have to have this STATE_NNN
business?" but then realized that will generate an insane amount of
boilerplate, and you're already close to a better solution, so I shut up
to think harder. :)

> > > > used deferred or not. Re: the previous thought on whether to defer xattr
> > > > removes or not, there might also be cases where there's not a need to
> > > > defer xattr sets.
> > > >
> > > > E.g., taking a quick peek into the next patch, the state 1 case in
> > > > xfs_attr_try_sf_addname() is actually a transaction commit, which I
> > > > think means we're done. We'd have done an attr memory allocation,
> > > > deferred op and transaction roll where none was necessary so it might
> > > > not be worth it to defer in that scenario. Hmm, it also looks like we
> > > > return -EAGAIN in places where we've not actually done any work, like if
> > > > a shortform add attempt returns -ENOSPC (or the -EAGAIN return before we
> > > > even attempt the sf add). That kind of looks like a waste of transaction
> > > > rolls and further suggests it might be cleaner to break this whole path
> > > > down into helpers and put it back together in a way more conducive to
> > > > deferred operations.

Er, agreed:

> > > Yes, this area is a bit of a wart the way it is right now. I think you're
> > > right in that ultimately we may end up having to do a lot of refactoring in
> > > order to have more efficient "re-entry points". The state machine is hard
> > > to get into subroutines, so it's limited in use in the top level function.

So my current understanding of the problem is that we have this big old
xfs_attr_set_args function that does multiple responsibilities requiring
transaction rolls, which we can't do directly inside a ->finish_item
handler:

1. If no attr fork, add one.
2. If shortform attr fork, try to put it in the sf area.
3. If shortform attr fork and out of space, convert to leaf format.
4. Add attr to leaf/node attr tree.

So how about this: refactor each of these pieces into a separate
function, then add a separate XFS_ATTR_OP_FLAGS_* value for each of
these little pieces. xfs_trans_attr() can call the appropriate little
function for the OP_FLAG and xfs_attr_finish_item can figure out which
state comes next based on the return value.

By directly mapping distinct OP_FLAGS to each piece of the attr setting
puzzle, you can use the existing "roll and come back" part of the defer
ops machinery.

If _finish_item thinks we're done then we just exit. Otherwise, store
the new state in the (struct xfs_attr_item *) parameter passed into
_finish_item and return -EAGAIN, which puts the defer item back on the
defer op list, logs a new xattr intent with the new state, rolls the
transaction, and tries to finish the attr again. I think you've already
done this last part.

xfs_attri_recover then becomes much simpler -- we're passed in the
reconstructed log item from which we figure out which step we need to
do. We call xfs_trans_attr() to do that one step, but unlike
_finish_item, we use the new state to construct a *new* attr intent and
attach it to the transaction, then call xfs_defer_move at the end to
move all the queued defer_ops to the parent_tp because log recovery
requires us to recover all the incomplete log intent items before
finishing any new ones that were created as part of recovery.

This does mean that we end up with dramatically separate code paths for
defer ops attr setting vs. regular attr setting, but as you point out
the parent pointer feature will give the new code paths plenty of exercise.
Tying the new log intent items to a new feature bit is key to preventing
old kernels from stumbling across our new intent items, so we needed to
preserve the old attr set paths anyway.

Anyway, if this all seems confusing, you can track me down, because I
wrote most of this system and therefore have forgotten all of
it^W^W^W^W^Wam available to help. :)

> > >
> > > I was also starting to wonder if maybe I could do some refactoring in
> > > xfs_defer_finish_noroll to capture the common code associated with the
> > > -EAGAIN handling. Then maybe we could make a function pointer that we can
> > > pass through the finish_item interface. The idea being that subroutines
> > > could use the function pointer to cycle out the transaction when needed
> > > instead of having to record states and back out like this. It'd be a new

The state tracking and rolling is already built into xfs_defer.c. :)

> > > parameter to pipe around, but it'd be more efficient than the state machine,
> > > and less surgery in the refactor. And maybe a blessing to any other
> > > operations that might need to go through this transition in the future.
> > > Thoughts?
> > >
> >
> > That's an interesting idea. It still strikes me as a bit of a
> > fallback/hack as opposed to organizing the code to properly fit into the
> > dfops infrastructure, but it could be useful as a transient solution.
> > From a high level, it looks like we'd have to create a new intent, relog
> > this item and all remaining items associated with the dfp to it, roll
> > the tx, and finally create a done item associated with the intent in the
> > new tx. You'd need access to the dfp for some of that, so it's not
> > immediately clear to me that this ends up much easier than fixing up
> > the xattr code.

(I think the code that handles EAGAIN being returned from finish_item
does this for you....)

> >
> > BTW, if we did end up with something like that I'd probably prefer to
> > see it as an exported dfops helper function as opposed to a function
> > pointer being passed around, if possible.
> >
>
> Alrighty, I think for now I may try to pursue something more like what you
> proposed in the next patch and see where I get first. Maybe I'll come back
> to this later if for some reason it doesn't work out, but I think what you
> have there is reasonable.

<nod>

--D

>
> Thanks again for the reviews!
> Allison
>
> > Brian
> >
> > > Thanks again for the reviews!
> > >
> > > Allison
> > >
> > > >
> > > > Brian
> > > >
> > > >
> > > > > fs/xfs/libxfs/xfs_attr.h | 18 +++++++++++++++++-
> > > > > fs/xfs/scrub/common.c | 2 ++
> > > > > fs/xfs/xfs_acl.c | 2 ++
> > > > > fs/xfs/xfs_attr_item.c | 2 +-
> > > > > fs/xfs/xfs_ioctl.c | 2 ++
> > > > > fs/xfs/xfs_ioctl32.c | 2 ++
> > > > > fs/xfs/xfs_iops.c | 1 +
> > > > > fs/xfs/xfs_xattr.c | 1 +
> > > > > 8 files changed, 28 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/fs/xfs/libxfs/xfs_attr.h b/fs/xfs/libxfs/xfs_attr.h
> > > > > index 974c963..4ce3b0a 100644
> > > > > --- a/fs/xfs/libxfs/xfs_attr.h
> > > > > +++ b/fs/xfs/libxfs/xfs_attr.h
> > > > > @@ -77,6 +77,13 @@ typedef struct attrlist_ent { /* data from attr_list() */
> > > > > char a_name[1]; /* attr name (NULL terminated) */
> > > > > } attrlist_ent_t;
> > > > > +/* Attr state machine types */
> > > > > +enum xfs_attr_state {
> > > > > + XFS_ATTR_STATE1 = 1,
> > > > > + XFS_ATTR_STATE2 = 2,
> > > > > + XFS_ATTR_STATE3 = 3,
> > > > > +};
> > > > > +
> > > > > /*
> > > > > * List of attrs to commit later.
> > > > > */
> > > > > @@ -88,7 +95,16 @@ struct xfs_attr_item {
> > > > > void *xattri_name; /* attr name */
> > > > > uint32_t xattri_name_len; /* length of name */
> > > > > uint32_t xattri_flags; /* attr flags */
> > > > > - struct list_head xattri_list;
> > > > > +
> > > > > + /*
> > > > > + * Delayed attr parameters that need to remain instantiated
> > > > > + * across transaction rolls during the defer finish
> > > > > + */
> > > > > + struct xfs_buf *xattri_leaf_bp; /* Leaf buf to release */
> > > > > + enum xfs_attr_state xattri_state; /* state machine marker */
> > > > > + struct xfs_da_args xattri_args; /* args context */
> > > > > +
> > > > > + struct list_head xattri_list;
> > > > > /*
> > > > > * A byte array follows the header containing the file name and
> > > > > diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
> > > > > index 0c54ff5..270c32e 100644
> > > > > --- a/fs/xfs/scrub/common.c
> > > > > +++ b/fs/xfs/scrub/common.c
> > > > > @@ -30,6 +30,8 @@
> > > > > #include "xfs_rmap_btree.h"
> > > > > #include "xfs_log.h"
> > > > > #include "xfs_trans_priv.h"
> > > > > +#include "xfs_da_format.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_reflink.h"
> > > > > #include "scrub/xfs_scrub.h"
> > > > > diff --git a/fs/xfs/xfs_acl.c b/fs/xfs/xfs_acl.c
> > > > > index 142de8d..9b1b93e 100644
> > > > > --- a/fs/xfs/xfs_acl.c
> > > > > +++ b/fs/xfs/xfs_acl.c
> > > > > @@ -10,6 +10,8 @@
> > > > > #include "xfs_mount.h"
> > > > > #include "xfs_inode.h"
> > > > > #include "xfs_acl.h"
> > > > > +#include "xfs_da_format.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_trace.h"
> > > > > #include <linux/slab.h>
> > > > > diff --git a/fs/xfs/xfs_attr_item.c b/fs/xfs/xfs_attr_item.c
> > > > > index 0ea19b4..36e6d1e 100644
> > > > > --- a/fs/xfs/xfs_attr_item.c
> > > > > +++ b/fs/xfs/xfs_attr_item.c
> > > > > @@ -19,10 +19,10 @@
> > > > > #include "xfs_rmap.h"
> > > > > #include "xfs_inode.h"
> > > > > #include "xfs_icache.h"
> > > > > -#include "xfs_attr.h"
> > > > > #include "xfs_shared.h"
> > > > > #include "xfs_da_format.h"
> > > > > #include "xfs_da_btree.h"
> > > > > +#include "xfs_attr.h"
> > > > > static inline struct xfs_attri_log_item *ATTRI_ITEM(struct xfs_log_item *lip)
> > > > > {
> > > > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > > > > index ab341d6..c8728ca 100644
> > > > > --- a/fs/xfs/xfs_ioctl.c
> > > > > +++ b/fs/xfs/xfs_ioctl.c
> > > > > @@ -16,6 +16,8 @@
> > > > > #include "xfs_rtalloc.h"
> > > > > #include "xfs_itable.h"
> > > > > #include "xfs_error.h"
> > > > > +#include "xfs_da_format.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_bmap.h"
> > > > > #include "xfs_bmap_util.h"
> > > > > diff --git a/fs/xfs/xfs_ioctl32.c b/fs/xfs/xfs_ioctl32.c
> > > > > index 5001dca..23f6990 100644
> > > > > --- a/fs/xfs/xfs_ioctl32.c
> > > > > +++ b/fs/xfs/xfs_ioctl32.c
> > > > > @@ -21,6 +21,8 @@
> > > > > #include "xfs_fsops.h"
> > > > > #include "xfs_alloc.h"
> > > > > #include "xfs_rtalloc.h"
> > > > > +#include "xfs_da_format.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_ioctl.h"
> > > > > #include "xfs_ioctl32.h"
> > > > > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> > > > > index e73c21a..561c467 100644
> > > > > --- a/fs/xfs/xfs_iops.c
> > > > > +++ b/fs/xfs/xfs_iops.c
> > > > > @@ -17,6 +17,7 @@
> > > > > #include "xfs_acl.h"
> > > > > #include "xfs_quota.h"
> > > > > #include "xfs_error.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_trans.h"
> > > > > #include "xfs_trace.h"
> > > > > diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
> > > > > index 3013746..938e81d 100644
> > > > > --- a/fs/xfs/xfs_xattr.c
> > > > > +++ b/fs/xfs/xfs_xattr.c
> > > > > @@ -11,6 +11,7 @@
> > > > > #include "xfs_mount.h"
> > > > > #include "xfs_da_format.h"
> > > > > #include "xfs_inode.h"
> > > > > +#include "xfs_da_btree.h"
> > > > > #include "xfs_attr.h"
> > > > > #include "xfs_attr_leaf.h"
> > > > > #include "xfs_acl.h"
> > > > > --
> > > > > 2.7.4
> > > > >