On Wed, Jan 15, 2014 at 08:24:18PM -0500, Matthew Wilcox wrote:

This series of patches add support for XIP to ext4.  Unfortunately,
it turns out to be necessary to rewrite the existing XIP support code
first due to races that are unfixable in the current design.

Since v4 of this patchset, I've improved the documentation, fixed a
couple of warnings that a newer version of gcc emitted, and fixed a
bug where we would read/write the wrong address for I/Os that were not
aligned to PAGE_SIZE.

Looks like there's something fundamentally broken with the patch set
as it stands. I get this same data corruption on both ext4 and XFS
with XIP using fsx. It's as basic as it gets - the first read after
a mmapped write fails to see the data written by mmap:

$ sudo mkfs.xfs -f /dev/ram0
meta-data=/dev/ram0              isize=256    agcount=4, agsize=256000 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0
data     =                       bsize=4096   blocks=1024000, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal log           bsize=4096   blocks=12800, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
$ sudo mount -o xip /dev/ram0 /mnt/scr
$ sudo chmod 777 /mnt/scr
$ ltp/fsx -d -N 1000 -S 0 /mnt/scr/fsx
Seed set to 3774
1 mapwrite      0x3db39 thru    0x3ffff (0x24c7 bytes)
2 mapread       0x2e947 thru    0x33163 (0x481d bytes)
3 read  0x2e836 thru    0x3cba1 (0xe36c bytes)
4 punch from 0x2e7 to 0x5c43, (0x595c bytes)
5 mapwrite      0xcaea thru     0x13ba9 (0x70c0 bytes)
6 punch from 0x31645 to 0x38d1d, (0x76d8 bytes)
7 falloc        from 0x24f92 to 0x2f2b7 (0xa325 bytes)
fallocating to largest ever: 0x171ac
8 falloc        from 0xbcf1 to 0x171ac (0xb4bb bytes)
9 read  0x126f thru     0x11136 (0xfec8 bytes)
READ BAD DATA: offset = 0x126f, size = 0xfec8, fname = /mnt/scr/fsx
OFFSET  GOOD    BAD     RANGE
0x caea 0x05f9  0x0000  0x    0
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caeb 0xf905  0x0000  0x    1
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caec 0x0599  0x0000  0x    2
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caed 0x9905  0x0000  0x    3
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caee 0x05e8  0x0000  0x    4
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caef 0xe805  0x0000  0x    5
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf0 0x0580  0x0000  0x    6
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf1 0x8005  0x0000  0x    7
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf2 0x056c  0x0000  0x    8
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf3 0x6c05  0x0000  0x    9
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf4 0x05ad  0x0000  0x    a
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf5 0xad05  0x0000  0x    b
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf6 0x0539  0x0000  0x    c
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf7 0x3905  0x0000  0x    d
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf8 0x05db  0x0000  0x    e
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
0x caf9 0xdb05  0x0000  0x    f
operation# (mod 256) for the bad data unknown, check HOLE and EXTEND ops
LOG DUMP (9 total operations):
1(  1 mod 256): MAPWRITE 0x3db39 thru 0x3ffff   (0x24c7 bytes)
2(  2 mod 256): MAPREAD  0x2e947 thru 0x33163   (0x481d bytes)
3(  3 mod 256): READ     0x2e836 thru 0x3cba1   (0xe36c bytes)
4(  4 mod 256): PUNCH    0x2e7 thru 0x5c42      (0x595c bytes)
5(  5 mod 256): MAPWRITE 0xcaea thru 0x13ba9    (0x70c0 bytes)  ******WWWW
6(  6 mod 256): PUNCH    0x31645 thru 0x38d1c   (0x76d8 bytes)
7(  7 mod 256): FALLOC   0x24f92 thru 0x2f2b7   (0xa325 bytes) INTERIOR
8(  8 mod 256): FALLOC   0xbcf1 thru 0x171ac    (0xb4bb bytes) INTERIOR ******FFFF
9(  9 mod 256): READ     0x126f thru 0x11136    (0xfec8 bytes)  ***RRRR***
Correct content saved for comparison
(maybe hexdump "/mnt/scr/fsx" vs "/mnt/scr/fsx.fsxgood")

XFS gives a good indication that we aren't doing something correctly
w.r.t. mapped XIP writes, as trying to fiemap the file ASSERT fails
with a delayed allocation extent somewhere inside the file after a
sync. I shall keep digging.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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