Re: [PATCH 5/5] xfs: fix agfl wrapping

[Brian Foster <bfoster@redhat.com>]

On Fri, Mar 02, 2018 at 08:12:07AM -0500, Brian Foster wrote:
> On Thu, Mar 01, 2018 at 12:55:41PM -0800, Darrick J. Wong wrote:
> > On Thu, Mar 01, 2018 at 12:28:33PM -0500, Brian Foster wrote:
> > > On Wed, Feb 28, 2018 at 03:20:32PM -0800, Darrick J. Wong wrote:
> > > > On Wed, Feb 28, 2018 at 05:43:51PM -0500, Brian Foster wrote:
> > > > > On Tue, Feb 27, 2018 at 01:03:13PM -0800, Darrick J. Wong wrote:
> > > > > > On Tue, Feb 27, 2018 at 02:35:49PM -0500, Brian Foster wrote:
> > > > > > > On Thu, Feb 22, 2018 at 06:00:15PM -0800, Darrick J. Wong wrote:
> > > > > > > > From: Darrick J. Wong <darrick.wong@oracle.com>
...
> > Going forward, I want the number of unpacked kernels to decrease as
> > quickly as possible.  I understand that distro kernel maintainers are
> > not willing to apply the packing patch to their kernel until we come up
> > with a smooth transition path.
> > 
> 
> I agree wrt to upstream, but note that I don't anticipate including the
> padding fix downstream any time soon.
> 
> > I don't want to support fixing agfls to be 118 units long on 64-bit
> > unpacked kernels and 119 units long on 32-bit unpacked kernels, and I
> > only want to support the packed kernels with their 119 unit long agfls.
> > An AGFL that starts at 0 and ends at flcount-1 is compatible with packed
> > and unpacked kernels, so the v2 patch I sent last night removes the
> > delicate per-case surgery in favor of a new strategy where the mount
> > time and unmount time helpers both look for agfl configurationss that
> > are known to cause problems, and solves them all with the same solution:
> > moving the agfl list towards the start of the block.
> > 
> 
> The purpose of the patch I sent was not for upstream unpacked support
> going forward. Upstream has clearly moved forward with the packed
> format. The goal of the patch was to explore a single/generic patch that
> could be merged upstream/downstream and handle compatibility cleanly.
> 

FWIW, here's a new variant of a bidirectional fixup. It's refactored and
polished up a bit into a patch. It basically inspects the agf when first
read for any evidence that the on-disk fields reflect a size mismatch
with the current kernel and sets a flag if so. agfl gets/puts check the
flag and thus the first transaction that attempts to modify a mismatched
agfl swaps a block into or out of the gap slot appropriately.

This avoids the need for any new transactions or mount time scan and the
(downstream motivated) packed -> unpacked case is only 10 or so lines of
additional code. Only spot tested, but I _think_ it covers all of the
cases. Hm?

Brian

--- 8< ---

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index c02781a4c091..1cbf80d8481f 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -2053,6 +2053,115 @@ xfs_alloc_space_available(
 	return true;
 }
 
+static int
+xfs_agfl_ondisk_size(
+	struct xfs_mount	*mp,
+	int			first,
+	int			last,
+	int			count)
+{
+	int			active = count;
+	int			agfl_size = XFS_AGFL_SIZE(mp);
+
+	if (count && last >= first)
+		active = last - first + 1;
+	else if (count)
+		active = agfl_size - first + last + 1;
+
+	if (active == count + 1)
+		return agfl_size - 1;
+	if (active == count - 1 || first == agfl_size || last == agfl_size)
+		return agfl_size + 1;
+
+	ASSERT(active == count);
+	return agfl_size;
+}
+
+static bool
+xfs_agfl_need_padfix(
+	struct xfs_mount	*mp,
+	struct xfs_agf		*agf)
+{
+	int			f = be32_to_cpu(agf->agf_flfirst);
+	int			l = be32_to_cpu(agf->agf_fllast);
+	int			c = be32_to_cpu(agf->agf_flcount);
+
+	return xfs_agfl_ondisk_size(mp, f, l, c) != XFS_AGFL_SIZE(mp);
+}
+
+static int
+xfs_agfl_check_padfix(
+	struct xfs_trans	*tp,
+	struct xfs_buf		*agbp,
+	struct xfs_buf		*agflbp,
+	struct xfs_perag	*pag)
+{
+	struct xfs_mount	*mp = tp->t_mountp;
+	struct xfs_agf		*agf = XFS_BUF_TO_AGF(agbp);
+	__be32			*agfl_bno = XFS_BUF_TO_AGFL_BNO(mp, agflbp);
+	int			agfl_size = XFS_AGFL_SIZE(mp);
+	int			ofirst, olast, osize;
+	int			nfirst, nlast;
+	int			logflags = 0;
+	int			startoff = 0;
+
+	if (!pag->pagf_needpadfix)
+		return 0;
+
+	ofirst = nfirst = be32_to_cpu(agf->agf_flfirst);
+	olast = nlast = be32_to_cpu(agf->agf_fllast);
+	osize = xfs_agfl_ondisk_size(mp, ofirst, olast, pag->pagf_flcount);
+
+	/*
+	 * If the on-disk agfl is smaller than what the kernel expects, the
+	 * last slot of the on-disk agfl is a gap with bogus data. Move the
+	 * first valid block into the gap and bump the pointer.
+	 */
+	if (osize < agfl_size) {
+		ASSERT(pag->pagf_flcount != 0);
+		agfl_bno[agfl_size - 1] = agfl_bno[ofirst];
+		startoff = (char *) &agfl_bno[agfl_size - 1] - (char *) agflbp->b_addr;
+		nfirst++;
+		goto done;
+	}
+
+	/*
+	 * Otherwise, the on-disk agfl is larger than what the current kernel
+	 * expects. If empty, just fix up the first and last pointers. If not,
+	 * move the inaccessible block to the end of the valid range.
+	 */
+	nfirst = do_mod(nfirst, agfl_size);
+	if (pag->pagf_flcount == 0) {
+		nlast = (nfirst == 0 ? agfl_size - 1 : nfirst - 1);
+		goto done;
+	}
+	if (nlast != agfl_size)
+		nlast++;
+	nlast = do_mod(nlast, agfl_size);
+	agfl_bno[nlast] = agfl_bno[osize - 1];
+	startoff = (char *) &agfl_bno[nlast] - (char *) agflbp->b_addr;
+
+done:
+	if (nfirst != ofirst) {
+		agf->agf_flfirst = cpu_to_be32(nfirst);
+		logflags |= XFS_AGF_FLFIRST;
+	}
+	if (nlast != olast) {
+		agf->agf_fllast = cpu_to_be32(nlast);
+		logflags |= XFS_AGF_FLLAST;
+	}
+	if (startoff) {
+		xfs_trans_buf_set_type(tp, agflbp, XFS_BLFT_AGFL_BUF);
+		xfs_trans_log_buf(tp, agflbp, startoff,
+				  startoff + sizeof(xfs_agblock_t) - 1);
+	}
+	if (logflags)
+		xfs_alloc_log_agf(tp, agbp, logflags);
+
+	pag->pagf_needpadfix = false;
+	return 0;
+}
+
 /*
  * Decide whether to use this allocation group for this allocation.
  * If so, fix up the btree freelist's size.
@@ -2258,6 +2367,12 @@ xfs_alloc_get_freelist(
 	if (error)
 		return error;
 
+	pag = xfs_perag_get(mp, be32_to_cpu(agf->agf_seqno));
+	error = xfs_agfl_check_padfix(tp, agbp, agflbp, pag);
+	if (error) {
+		xfs_perag_put(pag);
+		return error;
+	}
 
 	/*
 	 * Get the block number and update the data structures.
@@ -2269,7 +2384,6 @@ xfs_alloc_get_freelist(
 	if (be32_to_cpu(agf->agf_flfirst) == XFS_AGFL_SIZE(mp))
 		agf->agf_flfirst = 0;
 
-	pag = xfs_perag_get(mp, be32_to_cpu(agf->agf_seqno));
 	be32_add_cpu(&agf->agf_flcount, -1);
 	xfs_trans_agflist_delta(tp, -1);
 	pag->pagf_flcount--;
@@ -2376,11 +2490,18 @@ xfs_alloc_put_freelist(
 	if (!agflbp && (error = xfs_alloc_read_agfl(mp, tp,
 			be32_to_cpu(agf->agf_seqno), &agflbp)))
 		return error;
+
+	pag = xfs_perag_get(mp, be32_to_cpu(agf->agf_seqno));
+	error = xfs_agfl_check_padfix(tp, agbp, agflbp, pag);
+	if (error) {
+		xfs_perag_put(pag);
+		return error;
+	}
+
 	be32_add_cpu(&agf->agf_fllast, 1);
 	if (be32_to_cpu(agf->agf_fllast) == XFS_AGFL_SIZE(mp))
 		agf->agf_fllast = 0;
 
-	pag = xfs_perag_get(mp, be32_to_cpu(agf->agf_seqno));
 	be32_add_cpu(&agf->agf_flcount, 1);
 	xfs_trans_agflist_delta(tp, 1);
 	pag->pagf_flcount++;
@@ -2588,6 +2709,7 @@ xfs_alloc_read_agf(
 		pag->pagb_count = 0;
 		pag->pagb_tree = RB_ROOT;
 		pag->pagf_init = 1;
+		pag->pagf_needpadfix = xfs_agfl_need_padfix(mp, agf);
 	}
 #ifdef DEBUG
 	else if (!XFS_FORCED_SHUTDOWN(mp)) {
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index e0792d036be2..78a6377a9b38 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -353,6 +353,7 @@ typedef struct xfs_perag {
 	char		pagi_inodeok;	/* The agi is ok for inodes */
 	uint8_t		pagf_levels[XFS_BTNUM_AGF];
 					/* # of levels in bno & cnt btree */
+	bool		pagf_needpadfix;
 	uint32_t	pagf_flcount;	/* count of blocks in freelist */
 	xfs_extlen_t	pagf_freeblks;	/* total free blocks */
 	xfs_extlen_t	pagf_longest;	/* longest free space */