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Re: [PATCH] xfs: Make fiemap works with sparse file.

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH] xfs: Make fiemap works with sparse file.
From: Tao Ma <tao.ma@xxxxxxxxxx>
Date: Sat, 12 Jun 2010 07:37:31 +0800
Cc: xfs@xxxxxxxxxxx, Christoph Hellwig <hch@xxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Alex Elder <aelder@xxxxxxx>
In-reply-to: <4C125C0D.4080600@xxxxxxxxxxx>
References: <1276236135-12051-1-git-send-email-tao.ma@xxxxxxxxxx> <4C125C0D.4080600@xxxxxxxxxxx>
User-agent: Mozilla-Thunderbird (X11/20090706)
Eric Sandeen wrote:
Tao Ma wrote:
In xfs_vn_fiemap, we set bvm_count to fi_extent_max + 1 and want
to return fi_extent_max extents, but actually it won't work for
a sparse file. The reason is that in xfs_getbmap we will
calculate holes and set it in 'out', while out is malloced by
bmv_count(fi_extent_max+1) which didn't consider holes. So in the
worst case, if 'out' vector looks like
[hole, extent, hole, extent, hole, ... hole, extent, hole],
we will only return half of fi_extent_max extents.

So in xfs_vn_fiemap, we should consider this worst case. If the
user wants fi_extent_max extents, we need a 'out' with size of
2 *fi_extent_max + 1.

This all seems right to me, though your commit message above (+1)
doesn't match the comment and code in the patch (+2)
oh, yes, I will change the commit log and send a v2.
and can I add your ack for this patch?


Cc: Alex Elder <aelder@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Tao Ma <tao.ma@xxxxxxxxxx>
 fs/xfs/linux-2.6/xfs_iops.c |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_iops.c b/fs/xfs/linux-2.6/xfs_iops.c
index 9c8019c..1db92e3 100644
--- a/fs/xfs/linux-2.6/xfs_iops.c
+++ b/fs/xfs/linux-2.6/xfs_iops.c
@@ -672,9 +672,21 @@ xfs_vn_fiemap(
                bm.bmv_length = BTOBB(length);
- /* We add one because in getbmap world count includes the header */
+       /*
+        * It is a bit tricky for us to calculate the bmv_count from
+        * fi_extent_max.
+        * If we support to return fi_extent_max extents to the user,
+        * we need at most 2 * fi_extent_max + 1 for bmv_count since
+        * in xfs_getbmap we will calculate holes while fi_extent_max
+        * don't have them. So in the worst case, bmv can looks like
+        * [hole, extent, hole, extent, hole, ... hole, extent, hole].
+        * So there will be 2 *fi_extent_max + 1.
+        * What's more, in getbmap world count have to include the
+        * header, so we need another bmv. So the total number will
+        * be 2 * fieinfo->fi_extents_max + 2.
+        */
        bm.bmv_count = !fieinfo->fi_extents_max ? MAXEXTNUM :
-                                       fieinfo->fi_extents_max + 1;
+                       2 * fieinfo->fi_extents_max + 2;
        bm.bmv_count = min_t(__s32, bm.bmv_count,
                             (PAGE_SIZE * 16 / sizeof(struct getbmapx)));
        bm.bmv_iflags = BMV_IF_PREALLOC;

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