RE: [PATCH] xfs: hold xfs_buf locked between shortform->leaf conversion and the addition of an attribute

[Shyam Kaushik <shyam@zadara.com>]

Hi Darrick,

We are trying to port your patch
https://github.com/torvalds/linux/commit/6e643cd094de3bd0f97edcc1db0089afa
24d909f to 4.14 LTS kernel. In 4.14 there is no xfs_defer_bjoin(). Can you
please comment if the below 4.14 LTS kernel patch looks ok to you? Do you
see any issues with it?

Thanks.

--Shyam

PATCH
-----

diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
index ea66f04f46f7..f7316138a8db 100644
--- a/fs/xfs/libxfs/xfs_attr.c
+++ b/fs/xfs/libxfs/xfs_attr.c
@@ -218,6 +218,7 @@ xfs_attr_set(
        xfs_fsblock_t           firstblock;
        int                     rsvd = (flags & ATTR_ROOT) != 0;
        int                     error, err2, local;
+       struct xfs_buf          *leaf_bp = NULL;

        XFS_STATS_INC(mp, xs_attr_set);

@@ -327,9 +328,15 @@ xfs_attr_set(
                 * GROT: another possible req'mt for a double-split btree
op.
                 */
                xfs_defer_init(args.dfops, args.firstblock);
-               error = xfs_attr_shortform_to_leaf(&args);
+               error = xfs_attr_shortform_to_leaf(&args, &leaf_bp);
                if (error)
                        goto out_defer_cancel;
+               /*
+                * Prevent the leaf buffer from being unlocked so that a
+                * concurrent AIL push cannot grab the half-baked leaf
+                * buffer and run into problems with the write verifier.
+                */
+               xfs_trans_bhold(args.trans, leaf_bp);
                xfs_defer_ijoin(args.dfops, dp);
                error = xfs_defer_finish(&args.trans, args.dfops);
                if (error)
@@ -337,13 +344,15 @@ xfs_attr_set(

                /*
                 * Commit the leaf transformation.  We'll need another
(linked)
-                * transaction to add the new attribute to the leaf.
+                * transaction to add the new attribute to the leaf, which
+                * means that we have to hold & join the leaf buffer here
too.
                 */

                error = xfs_trans_roll_inode(&args.trans, dp);
                if (error)
                        goto out;
-
+               xfs_trans_bjoin(args.trans, leaf_bp);
+               leaf_bp = NULL;
        }

        if (xfs_bmap_one_block(dp, XFS_ATTR_FORK))
@@ -374,8 +383,9 @@ xfs_attr_set(

 out_defer_cancel:
        xfs_defer_cancel(&dfops);
-       args.trans = NULL;
 out:
+       if (leaf_bp)
+               xfs_buf_relse(leaf_bp);
        if (args.trans)
                xfs_trans_cancel(args.trans);
        xfs_iunlock(dp, XFS_ILOCK_EXCL);
diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
index 40e53a4fc0a6..92ae04ac413a 100644
--- a/fs/xfs/libxfs/xfs_attr_leaf.c
+++ b/fs/xfs/libxfs/xfs_attr_leaf.c
@@ -739,10 +739,13 @@ xfs_attr_shortform_getvalue(xfs_da_args_t *args)
 }

 /*
- * Convert from using the shortform to the leaf.
+ * Convert from using the shortform to the leaf.  On success, return the
+ * buffer so that we can keep it locked until we're totally done with it.
  */
 int
-xfs_attr_shortform_to_leaf(xfs_da_args_t *args)
+xfs_attr_shortform_to_leaf(
+       xfs_da_args_t *args,
+       struct xfs_buf **leaf_bp)
 {
        xfs_inode_t *dp;
        xfs_attr_shortform_t *sf;
@@ -821,6 +824,7 @@ xfs_attr_shortform_to_leaf(xfs_da_args_t *args)
                sfe = XFS_ATTR_SF_NEXTENTRY(sfe);
        }
        error = 0;
+       *leaf_bp = bp;

 out:
        kmem_free(tmpbuffer);
diff --git a/fs/xfs/libxfs/xfs_attr_leaf.h b/fs/xfs/libxfs/xfs_attr_leaf.h
index f7dda0c237b0..894124efb421 100644
--- a/fs/xfs/libxfs/xfs_attr_leaf.h
+++ b/fs/xfs/libxfs/xfs_attr_leaf.h
@@ -48,7 +48,8 @@ void  xfs_attr_shortform_create(struct xfs_da_args
*args);
 void   xfs_attr_shortform_add(struct xfs_da_args *args, int forkoff);
 int    xfs_attr_shortform_lookup(struct xfs_da_args *args);
 int    xfs_attr_shortform_getvalue(struct xfs_da_args *args);
-int    xfs_attr_shortform_to_leaf(struct xfs_da_args *args);
+int    xfs_attr_shortform_to_leaf(struct xfs_da_args *args,
+                       struct xfs_buf **leaf_bp);
 int    xfs_attr_shortform_remove(struct xfs_da_args *args);
 int    xfs_attr_shortform_allfit(struct xfs_buf *bp, struct xfs_inode
*dp);
 int    xfs_attr_shortform_bytesfit(struct xfs_inode *dp, int bytes);



-----Original Message-----
From: linux-xfs-owner@vger.kernel.org
[mailto:linux-xfs-owner@vger.kernel.org] On Behalf Of Brian Foster
Sent: 14 August 2017 17:52
To: Alex Lyakas
Cc: Dave Chinner; Darrick J. Wong; linux-xfs@vger.kernel.org;
libor.klepac@bcom.cz
Subject: Re: [PATCH] xfs: hold xfs_buf locked between shortform->leaf
conversion and the addition of an attribute

On Mon, Aug 14, 2017 at 11:11:41AM +0300, Alex Lyakas wrote:
> Hello David, Brian,
>
> I was not able to follow the details, unfortunately. Can you confirm
that
> this patch is safe to go into kernel 3.18?
>

This is the open question in the separate subthread (this one is
discussion around designing a solution for the current code):

http://marc.info/?l=linux-xfs&m=150246184413604&w=2

This could use confirmation, but my understanding is that this is safe
because v3.18 doesn't have the more advanced deferred ops
infrastructure. It uses xfs_bmap_finish() which has a max roll count of
one and a transaction with enough reservation for 2 rolls before
blocking reservation is required.

Note that doesn't mean we'd officially post a v3.18 stable patch before
this is fixed in the upstream code. We always fix upstream first and
backport from there to ensure a consistent base going forward (we don't
want to go change v3.18, end up with a slightly different upstream
patch, then have to backport more changes to fix the original patch).
This may be safe enough for you to use locally in the meantime, however.

Brian

> Thanks,
> Alex.
>
>
> -----Original Message----- From: Dave Chinner
> Sent: Monday, August 14, 2017 3:28 AM
> To: Brian Foster
> Cc: Darrick J. Wong ; Alex Lyakas ; linux-xfs@vger.kernel.org ;
> libor.klepac@bcom.cz
> Subject: Re: [PATCH] xfs: hold xfs_buf locked between shortform->leaf
> conversion and the addition of an attribute
>
> On Sat, Aug 12, 2017 at 10:04:34AM -0400, Brian Foster wrote:
> > On Sat, Aug 12, 2017 at 10:16:37AM +1000, Dave Chinner wrote:
> > > On Fri, Aug 11, 2017 at 10:27:43AM -0400, Brian Foster wrote:
> > > > On Fri, Aug 11, 2017 at 12:22:04PM +1000, Dave Chinner wrote:
> > > Using XFS_BLI_ORDERED allows us to log the buffer without recording
> > > a new dirty range on the buffer. IOWs, it retains whatever dirty
range
> > > it already had, and so after joining, marking it ordered and then
> > > logging the buffer, we have a XFS_BLI_DIRTY | XFS_BLI_ORDERED buffer
> > > in the transaction.
> > >
> > > The question is this: what happens when a XFS_BLI_ORDERED buffer
> > > with a pre-existing dirty region is formatted for the CIL? We
> > > haven't done that before, so I'm betting that we don't relog the
> > > dirty region like we should be doing....
> > >
> > > ... and we don't relog the existing dirty range because the
> > > ordered flag takes precedence.
> > >
> >
> > Right.. so it seems that the current implementation for ordered
buffers
> > assumes a buffer is only ever used in one mode or the other.
> > Additionally, the AIL assumes that any reinserted item has been fully
> > relogged and so it moves the LSN forward unconditionally. Current
> > ordered buffer processing violates this constraint for an already
logged
> > buffer.
>
> Right, but it's not been a concern until now because we've only ever
> used ordered buffers on newly allocated buffers that haven't been
> previously logged.
>
> > > Ok, the ordered buffer checks in xfs_buf_item_size() and
> > > xfs_buf_item_format() need to also check for dirty regions. If dirty
> > > regions exist, then we treat it like a normal buffer rather than an
> > > ordered buffer. We can factor the dirty region check out of
> > > xfs_buf_item_unlock() for this...
> > >
> > > Actually, check the case in xfs_buf_item_size() and remove the
> > > ordered flag if there are dirty regions. Then xfs_buf_item_format()
> > > will do the right thing without needing a duplicate check...
> > >
> >
> > I think that would work, assuming we actually check the
> > xfs_buf_log_format for dirty-ness rather than just the log item. As it
> > is, note that ordered buffers are still "logged" in the transaction
> > because otherwise the transaction infrastructure will assume it made
no
> > change to the buf and toss the log item at commit time (we also need
to
> > set up I/O completion on the buf and whatnot).
>
> *nod*
>
> > What concerns me about this approach is that I think we introduce the
> > possibility for subtle bugs. Existing ordered buffer code does this:
> >
> >         xfs_trans_ordered_buf(tp, fbuf);
> >         xfs_trans_log_buf(tp, fbuf, 0,
> >                           BBTOB(fbuf->b_length) - 1);
> >
> > ... which should continue to work fine. Allowing ordered buffers to
> > physically log means that something like this:
> >
> >         xfs_trans_log_buf(tp, fbuf, 0,
> >                           BBTOB(fbuf->b_length) - 1);
> >         xfs_trans_ordered_buf(tp, fbuf);
> >
> > ... is now a bug that is only apparent after scrutiny of xfs_trans_*()
> > and logging internals. Granted, the above already is incorrect, but it
> > technically still works as expected. I don't see the need to turn that
> > into a real problem by actually logging the buffer when we might not
> > expect to.
>
> Well, it's not a "things go bad" bug. It's a "we screwed up an
> optimisation" bug, because logging the buffer contents unnecessarily
> only increases the required log bandwidth. It shouldn't affect
> replay because the buffer is still correctly ordered in the log.
> Hence both the transient and end states of the buffer during replay
> will still be the same...
>
> > So while I agree that this could probably be made to work and I think
it
> > is ideal to doing any kind of logged range tracking in the deferred
ops
> > code, it still seems more tricky than it needs to be. To relog a held
> > buffer in a new transaction, why not just mark the lidp dirty in the
new
> > transaction so it inherits all existing dirty segments? AFAICT, all we
> > really need to do is:
> >
> >         tp->t_flags |= XFS_TRANS_DIRTY;
> >         lidp->lid_flags |= XFS_LID_DIRTY;
> >
> > ... on the new transaction and everything should just work as designed
> > (for a buffer that has been previously logged, held, rolled and
> > rejoined).
>
> We would also need to set:
>
> bip->bli_flags |= XFS_BLI_DIRTY | XFS_BLI_LOGGED;
>
> which means we should....
>
> > To elaborate a bit, I think we could refactor xfs_trans_log_buf() into
a
> > new xfs_trans_dirty_buf() helper that covers all of the relevant bits
> > not related to actually dirtying the bli. xfs_trans_log_buf() would
call
> > xfs_trans_dirty_buf() and thus would not change functionally.
> > xfs_trans_ordered_buf() could now call xfs_trans_dirty_buf() and thus
> > the existing ordered buf users would no longer need to log a range of
> > the buffer (which doesn't make much sense anyways).
>
> ... do this. :)
>
> > Finally, the
> > deferred infrastructure could join/dirty/hold the buffer to the new
> > transaction after each roll without needing to track and relog
specific
> > regions of the buffer. Thoughts?
>
> Yup, that's exactly what I was thinking should be possible by using
> ordered buffers.... :)
>
> And Christoph's rework of the transaction roll and deferred inode
> handling that he just posted should make adding buffer handling
> quite a bit neater and cleaner.
>
> > Unless I'm missing something as to why this is busted, I'll take a
> > closer look at the code and float an rfc next week since otherwise it
> > sounds like this is something we could actually fix up in the ordered
> > buffer code today.
>
> Cool.
>
> > > Nothing in XFS is ever simple, is it? :P
> >
> > There used to be a level of satisfaction at feeling I understood some
> > new corner of XFS. Nowadays I know that just means I'm not yet aware
of
> > whatever dragons remain in that corner (is that paranoia? not if it's
> > true!). :P
>
> Ah, the true signs of expertise: developing a knowledge base and
> insight deep enough to understand that there is always another
> hidden dragon poised to bite your head off. :)
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
>
> --
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