[packages/kernel] - xfs hang fix

arekm arekm at pld-linux.org
Sat Aug 9 00:13:13 CEST 2014


commit fc3fc2cf178dbaef089a75182333d8c707a6a32f
Author: Arkadiusz Miśkiewicz <arekm at maven.pl>
Date:   Sat Aug 9 00:12:58 2014 +0200

    - xfs hang fix

 kernel-small_fixes.patch | 138 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 138 insertions(+)
---
diff --git a/kernel-small_fixes.patch b/kernel-small_fixes.patch
index 392d121..59f4a4b 100644
--- a/kernel-small_fixes.patch
+++ b/kernel-small_fixes.patch
@@ -46,3 +46,141 @@ index 7a0c800..ec5ebbb 100644
  	SET_ETHTOOL_OPS(dev, &rtl8169_ethtool_ops);
  	dev->watchdog_timeo = RTL8169_TX_TIMEOUT;
  
+commit 110dc24ad2ae4e9b94b08632fe1eb2fcdff83045
+Author: Dave Chinner <dchinner at redhat.com>
+Date:   Tue May 20 08:18:09 2014 +1000
+
+    xfs: log vector rounding leaks log space
+    
+    The addition of direct formatting of log items into the CIL
+    linear buffer added alignment restrictions that the start of each
+    vector needed to be 64 bit aligned. Hence padding was added in
+    xlog_finish_iovec() to round up the vector length to ensure the next
+    vector started with the correct alignment.
+    
+    This adds a small number of bytes to the size of
+    the linear buffer that is otherwise unused. The issue is that we
+    then use the linear buffer size to determine the log space used by
+    the log item, and this includes the unused space. Hence when we
+    account for space used by the log item, it's more than is actually
+    written into the iclogs, and hence we slowly leak this space.
+    
+    This results on log hangs when reserving space, with threads getting
+    stuck with these stack traces:
+    
+    Call Trace:
+    [<ffffffff81d15989>] schedule+0x29/0x70
+    [<ffffffff8150d3a2>] xlog_grant_head_wait+0xa2/0x1a0
+    [<ffffffff8150d55d>] xlog_grant_head_check+0xbd/0x140
+    [<ffffffff8150ee33>] xfs_log_reserve+0x103/0x220
+    [<ffffffff814b7f05>] xfs_trans_reserve+0x2f5/0x310
+    .....
+    
+    The 4 bytes is significant. Brain Foster did all the hard work in
+    tracking down a reproducable leak to inode chunk allocation (it went
+    away with the ikeep mount option). His rough numbers were that
+    creating 50,000 inodes leaked 11 log blocks. This turns out to be
+    roughly 800 inode chunks or 1600 inode cluster buffers. That
+    works out at roughly 4 bytes per cluster buffer logged, and at that
+    I started looking for a 4 byte leak in the buffer logging code.
+    
+    What I found was that a struct xfs_buf_log_format structure for an
+    inode cluster buffer is 28 bytes in length. This gets rounded up to
+    32 bytes, but the vector length remains 28 bytes. Hence the CIL
+    ticket reservation is decremented by 32 bytes (via lv->lv_buf_len)
+    for that vector rather than 28 bytes which are written into the log.
+    
+    The fix for this problem is to separately track the bytes used by
+    the log vectors in the item and use that instead of the buffer
+    length when accounting for the log space that will be used by the
+    formatted log item.
+    
+    Again, thanks to Brian Foster for doing all the hard work and long
+    hours to isolate this leak and make finding the bug relatively
+    simple.
+    
+    Signed-off-by: Dave Chinner <dchinner at redhat.com>
+    Reviewed-by: Christoph Hellwig <hch at lst.de>
+    Reviewed-by: Brian Foster <bfoster at redhat.com>
+    Signed-off-by: Dave Chinner <david at fromorbit.com>
+
+diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
+index 2c40044..84e0deb 100644
+--- a/fs/xfs/xfs_log.h
++++ b/fs/xfs/xfs_log.h
+@@ -24,7 +24,8 @@ struct xfs_log_vec {
+ 	struct xfs_log_iovec	*lv_iovecp;	/* iovec array */
+ 	struct xfs_log_item	*lv_item;	/* owner */
+ 	char			*lv_buf;	/* formatted buffer */
+-	int			lv_buf_len;	/* size of formatted buffer */
++	int			lv_bytes;	/* accounted space in buffer */
++	int			lv_buf_len;	/* aligned size of buffer */
+ 	int			lv_size;	/* size of allocated lv */
+ };
+ 
+@@ -52,15 +53,21 @@ xlog_prepare_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
+ 	return vec->i_addr;
+ }
+ 
++/*
++ * We need to make sure the next buffer is naturally aligned for the biggest
++ * basic data type we put into it.  We already accounted for this padding when
++ * sizing the buffer.
++ *
++ * However, this padding does not get written into the log, and hence we have to
++ * track the space used by the log vectors separately to prevent log space hangs
++ * due to inaccurate accounting (i.e. a leak) of the used log space through the
++ * CIL context ticket.
++ */
+ static inline void
+ xlog_finish_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec *vec, int len)
+ {
+-	/*
+-	 * We need to make sure the next buffer is naturally aligned for the
+-	 * biggest basic data type we put into it.  We already accounted for
+-	 * this when sizing the buffer.
+-	 */
+ 	lv->lv_buf_len += round_up(len, sizeof(uint64_t));
++	lv->lv_bytes += len;
+ 	vec->i_len = len;
+ }
+ 
+diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
+index 7e54553..de835da 100644
+--- a/fs/xfs/xfs_log_cil.c
++++ b/fs/xfs/xfs_log_cil.c
+@@ -97,7 +97,7 @@ xfs_cil_prepare_item(
+ {
+ 	/* Account for the new LV being passed in */
+ 	if (lv->lv_buf_len != XFS_LOG_VEC_ORDERED) {
+-		*diff_len += lv->lv_buf_len;
++		*diff_len += lv->lv_bytes;
+ 		*diff_iovecs += lv->lv_niovecs;
+ 	}
+ 
+@@ -111,7 +111,7 @@ xfs_cil_prepare_item(
+ 	else if (old_lv != lv) {
+ 		ASSERT(lv->lv_buf_len != XFS_LOG_VEC_ORDERED);
+ 
+-		*diff_len -= old_lv->lv_buf_len;
++		*diff_len -= old_lv->lv_bytes;
+ 		*diff_iovecs -= old_lv->lv_niovecs;
+ 		kmem_free(old_lv);
+ 	}
+@@ -239,7 +239,7 @@ xlog_cil_insert_format_items(
+ 			 * that the space reservation accounting is correct.
+ 			 */
+ 			*diff_iovecs -= lv->lv_niovecs;
+-			*diff_len -= lv->lv_buf_len;
++			*diff_len -= lv->lv_bytes;
+ 		} else {
+ 			/* allocate new data chunk */
+ 			lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
+@@ -259,6 +259,7 @@ xlog_cil_insert_format_items(
+ 
+ 		/* The allocated data region lies beyond the iovec region */
+ 		lv->lv_buf_len = 0;
++		lv->lv_bytes = 0;
+ 		lv->lv_buf = (char *)lv + buf_size - nbytes;
+ 		ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
+ 
================================================================

---- gitweb:

http://git.pld-linux.org/gitweb.cgi/packages/kernel.git/commitdiff/fc3fc2cf178dbaef089a75182333d8c707a6a32f



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