On Tue, Apr 11, 2017 at 08:07:06PM +0530, Chandan Rajendra wrote:
> On Sunday, April 09, 2017 08:39:44 PM Chandan Rajendra wrote:
> > On Friday, April 07, 2017 09:25:28 AM Darrick J. Wong wrote:
> > > On Fri, Apr 07, 2017 at 03:02:43PM +0530, Chandan Rajendra wrote:
> > > > On a ppc64 machine executing overlayfs/019 with xfs as the lower and
> > > > upper filesystem causes the following call trace,
> > > >
> > > > WARNING: CPU: 2 PID: 8034 at /root/repos/linux/fs/iomap.c:765 .iomap_dio_actor+0xcc/0x420
> > > > Modules linked in:
> > > > CPU: 2 PID: 8034 Comm: fsstress Tainted: G L 4.11.0-rc5-next-20170405 #100
> > > > task: c000000631314880 task.stack: c0000003915d4000
> > > > NIP: c00000000035a72c LR: c00000000035a6f4 CTR: c00000000035a660
> > > > REGS: c0000003915d7570 TRAP: 0700 Tainted: G L (4.11.0-rc5-next-20170405)
> > > > MSR: 800000000282b032 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI>
> > > > CR: 24004284 XER: 00000000
> > > > CFAR: c0000000006f7190 SOFTE: 1
> > > > GPR00: c00000000035a6f4 c0000003915d77f0 c0000000015a3f00 000000007c22f600
> > > > GPR04: 000000000022d000 0000000000002600 c0000003b2d56360 c0000003915d7960
> > > > GPR08: c0000003915d7cd0 0000000000000002 0000000000002600 c000000000521cc0
> > > > GPR12: 0000000024004284 c00000000fd80a00 000000004b04ae64 ffffffffffffffff
> > > > GPR16: 000000001000ca70 0000000000000000 c0000003b2d56380 c00000000153d2b8
> > > > GPR20: 0000000000000010 c0000003bc87bac8 0000000000223000 000000000022f5ff
> > > > GPR24: c0000003b2d56360 000000000000000c 0000000000002600 000000000022d000
> > > > GPR28: 0000000000000000 c0000003915d7960 c0000003b2d56360 00000000000001ff
> > > > NIP [c00000000035a72c] .iomap_dio_actor+0xcc/0x420
> > > > LR [c00000000035a6f4] .iomap_dio_actor+0x94/0x420
> > > > Call Trace:
> > > > [c0000003915d77f0] [c00000000035a6f4] .iomap_dio_actor+0x94/0x420 (unreliable)
> > > > [c0000003915d78f0] [c00000000035b9f4] .iomap_apply+0xf4/0x1f0
> > > > [c0000003915d79d0] [c00000000035c320] .iomap_dio_rw+0x230/0x420
> > > > [c0000003915d7ae0] [c000000000512a14] .xfs_file_dio_aio_read+0x84/0x160
> > > > [c0000003915d7b80] [c000000000512d24] .xfs_file_read_iter+0x104/0x130
> > > > [c0000003915d7c10] [c0000000002d6234] .__vfs_read+0x114/0x1a0
> > > > [c0000003915d7cf0] [c0000000002d7a8c] .vfs_read+0xac/0x1a0
> > > > [c0000003915d7d90] [c0000000002d96b8] .SyS_read+0x58/0x100
> > > > [c0000003915d7e30] [c00000000000b8e0] system_call+0x38/0xfc
> > > > Instruction dump:
> > > > 78630020 7f831b78 7ffc07b4 7c7ce039 40820360 a13d0018 2f890003 419e0288
> > > > 2f890004 419e00a0 2f890001 419e02a8 <0fe00000> 3b80fffb 38210100 7f83e378
> > > >
> > > > The above problem can also be recreated on a regular xfs filesystem
> > > > using the command,
> > > >
> > > > $ fsstress -d /mnt -l 1000 -n 1000 -p 1000
> > > >
> > > > The reason for the call trace is,
> > > > 1. When 'reserving' blocks for delayed allocation , XFS reserves more
> > > > blocks (i.e. past file's current EOF) than required. This is done
> > > > because XFS assumes that userspace might write more data and hence
> > > > 'reserving' more blocks might lead to the file's new data being
> > > > stored contiguously on disk.
> > > > 2. The in-memory 'struct xfs_bmbt_irec' mapping the file's last extent would
> > > > then cover the prealloc-ed EOF blocks in addition to the regular blocks.
> > > > 3. When flushing the dirty blocks to disk, we only flush data till the
> > > > file's EOF. But before writing out the dirty data, we allocate blocks
> > > > on the disk for holding the file's new data. This allocation includes
> > > > the blocks that are part of the 'prealloc EOF blocks'.
> > > > 4. Later, when the last reference to the inode is being closed, XFS frees the
> > > > unused 'prealloc EOF blocks' in xfs_inactive().
> > > >
> > > > In step 3 above, When allocating space on disk for the delayed allocation
> > > > range, the space allocator might sometimes allocate less blocks than
> > > > required. If such an allocation ends right at the current EOF of the
> > > > file, We will not be able to clear the "delayed allocation" flag for the
> > > > 'prealloc EOF blocks', since we won't have dirty buffer heads associated
> > > > with that range of the file.
> > > >
> > > > In such a situation if a Direct I/O read operation is performed on file
> > > > range [X, Y] (where X < EOF and Y > EOF), we flush dirty data in the
> > > > range [X, Y] and invalidate page cache for that range (Refer to
> > > > iomap_dio_rw()). Later for performing the Direct I/O read, XFS obtains
> > > > the extent items (which are still cached in memory) for the file
> > > > range. When doing so we are not supposed to get an extent item with
> > > > IOMAP_DELALLOC flag set, since the previous "flush" operation should
> > > > have converted any delayed allocation data in the range [X, Y]. Hence we
> > > > end up hitting a WARN_ON_ONCE(1) statement in iomap_dio_actor().
> > > >
> > > > This commit fixes the bug by preventing the read operation from going
> > > > beyond iomap_dio->i_size.
> > > >
> > > > Reported-by: Santhosh G <santhog4@xxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Chandan Rajendra <chandan@xxxxxxxxxxxxxxxxxx>
> > > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > >
> > > Looks ok to me, but I'll wait for the test before making further decisions.
> > >
> >
> > The command 'fsstress -d /mnt -l 1000 -n 1000 -p 1000' takes a long time for
> > execution. It does not complete even after 30 mins of run time.
> >
> > Hence I tried the following command line sequence for the new xfstests' test,
> >
> > # mkfs.xfs -f -d size=256m /dev/loop0
> > # mount /dev/loop0 /mnt/
> > # xfs_io -f -c 'pwrite 0 64k' /mnt/test-file
> > # dd if=/dev/zero of=/mnt/filler bs=4k
> > # for i in $(seq 1 2 32); do offset=$(($i * 4096)); xfs_io -f -c "fpunch $offset 4k" -c sync /mnt/filler; done
> > # xfs_io -f -c 'pwrite 64k 4k' /mnt/test-file # Prealloc blocks are reserved beyond file offset (68k - 1).
> > # xfs_io -f -d -c 'pread 64k 64k' /mnt/test-file
> >
> > The last command which performs a direct read operation beyond file's EOF
> > should have triggered the call trace. But I noticed (via printk statements)
> > that XFS was able to successfully allocate (4k + prealloc blocks) worth of
> > contiguous space for /mnt/test-file.
> >
> > I am still trying to figure out how XFS was able to allocate the EOF prealloc
> > blocks when it shouldn't have had any contiguous space larger than 4k bytes.
>
> Hi Darrick,
>
> I am unable to recreate the issue using the script provided below,
>
> --------------------------------------------------------------------------------
> #!/usr/bin/zsh -f
>
> [[ $ARGC != 2 ]] && { print "Usage: $0 <device> <mount point>"; exit 1 }
>
> device=$1
> mntpnt=$2
>
> test_file=$mntpnt/test-file
> filler_file=$mntpnt/filler
>
> print "Device = $device"
> print "Mount point = $mntpnt"
>
> umount $device > /dev/null 2>&1
> mkfs.xfs -f -d size=256m $device || { print "mkfs.xfs failed"; exit 1 }
> mount $device $mntpnt || { print "mounting $device failed"; exit 1 }
>
> xfs_io -f -c 'pwrite 0 64k' $test_file
> sync
> dd if=/dev/zero of=$filler_file bs=4k
> sync
By default, XFS reserves a bunch of blocks so that it can cut off
enthusiastic writers before they fill the filesystem completely up.
This makes it difficult to completely fragment the free space such that
we can guarantee that sync'ing a delalloc reservation that extends
beyond EOF. Normally, we reserve 8192 blocks to prevent weird ENOSPC
errors; you can adjust this via:
# xfs_io -x -c 'resblks 1' /mnt
That at least gets us to more severe fragmentation:
xfs_db> freesp
from to extents blocks pct
1 1 110452 110452 99.95
16 31 2 59 0.05
Though I'm not sure what one would have to do to knock out those last
two ~30 block free extents.
--D
> for i in $(seq 1 2 31); do
> offset=$(($i * 4096));
> printf "Fpunching hole at range: %d - %d\n" $offset $(($offset + 4096 - 1))
> xfs_io -f -c "fpunch $offset 4k" -c sync $filler_file;
> done
>
> xfs_io -f -c 'pwrite 64k 4k' $test_file
> xfs_io -f -d -c 'pread 64k 64k' $test_file
>
> exit 0
> --------------------------------------------------------------------------------
>
> When the last but one command (i.e. "xfs_io -f -c 'pwrite 64k 4k' $test_file")
> is executed, XFS is able to somehow reserve 16 blocks (1 required 4k block +
> 15 prealloc 4k blocks). Ideally this shouldn't have happened because we would
> have punched only 15 blocks in $filler_file.
>
> Also, During space allocation (i.e. xfs_map_blocks()), I see that the space
> allocation code was able to allocate an extent of 16 blocks (1 required 4k
> block + 15 prealloc blocks). Again, this shouldn't have happened since
> $filler_file would have occupied the rest of the free space and fpunch was
> performed on alternating blocks.
>
> Just after executing the series of "fpunch" commands in the above script, I
> observe the following,
>
> [root@localhost xfstests-dev]# xfs_db /dev/loop0
> xfs_db> freesp
> from to extents blocks pct
> 1 1 32 32 0.94
> 2 3 1 3 0.09
> 32 63 1 45 1.32
> 64 127 2 144 4.24
> 2048 4095 1 3173 93.41
>
> Basically, I feel that it is quite impossible to control the free space usage
> of an XFS filesystem in a granularity as required to recreate this bug.
>
> As I has informed earlier, "fsstress -d /mnt -l 1000 -n 1000 -p 1000" does not
> complete even after 30 mins of execution on a loop device which has a file on
> tmpfs as its backing device.
>
> Please let me know if you have any hints/thoughts about how I could proceed.
>
> --
> chandan
>
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