Recent kernels (>3.x) fail when writing to ext4 filesystems with the
message "file too large". The critical size is 28672 byte (7*2^12). This
has been traced down to ext4_max_size() by Tobias Ulmer, which looks like
this:
fs/ext4/super.c, comments stripped:
loff_t ext4_max_size(int blkbits, int has_huge_files)
{
loff_t res;
loff_t upper_limit = MAX_LFS_FILESIZE;
if (!has_huge_files || sizeof(blkcnt_t) < sizeof(u64)) {
upper_limit = (1LL << 32) - 1;
upper_limit >>= (blkbits - 9);
upper_limit <<= blkbits;
}
res = (1LL << 32) - 1;
res <<= blkbits;
if (res > upper_limit)
res = upper_limit;
return res;
}
The 64 assembler output using gcc for 64 bit looks like this:
0000000000000000 <ext4_max_size>:
0: 37 de 00 80 ldo 40(sp),sp
4: 8f 20 20 80 cmpib,<> 0,r25,4c <ext4_max_size+0x4c>
8: db 5c 0f e0 extrd,s,* r26,63,32,ret0
c: 97 9c 00 7e subi 3f,ret0,ret0
10: 01 7c 18 40 mtsar ret0
14: 73 dc 3f 91 std ret0,-38(sp)
18: 08 1c 02 53 copy ret0,r19
1c: 37 5a 3f ef ldo -9(r26),r26
20: d7 9f 12 1d depdi,z,* -1,sar,32,ret0
24: db 5a 0f e0 extrd,s,* r26,63,32,r26
28: 01 7a 18 c0 mtsarcm r26
2c: f7 ff 00 00 depdi,z,* -1,63,32,r31
30: d3 ff 17 00 extrd,s,* r31,sar,64,r31
34: 01 73 18 40 mtsar r19
38: d7 ff 03 00 depd,z,* r31,sar,64,r31
3c: 0b 9f 78 a0 cmpclr,*> r31,ret0,r0
40: 08 1f 02 5c copy r31,ret0
44: e8 40 d0 00 bve (rp)
48: 37 de 3f 81 ldo -40(sp),sp
4c: 01 7c 18 c0 mtsarcm ret0
50: d7 9f 12 1d depdi,z,* -1,sar,3,ret0
54: e8 40 d0 00 bve (rp)
58: 37 de 3f 81 ldo -40(sp),sp
5c: 00 00 00 00 break 0,0
I think that blkbits is 12, so the depdi instructions (0x2c and 0x50) do:
shift a -1 of length 3 by 12 bits, which is 28672. Reading the C code they
should do "of length 32", so I fear gcc is somehow damaging it's length
output here.
A part from the 32 bit code looks like this:
9c: 01 7c 18 40 mtsar ret0
a0: d7 9f 18 01 depwi,z -1,31,31,ret0
a4: d6 bf 10 00 depwi,z -1,sar,32,r21
a8: 08 14 02 5d copy r20,ret1
ac: 01 7a 18 40 mtsar r26
b0: d3 80 00 1a shrpw r0,ret0,sar,r26
So here a 32 bit -1 is used if I'm reading this correctly.
My current 2.6.39.2 kernel is built with the same compiler (4.5.3) and
runs fine, while 3.x doesn't work. That likely comes because the later of
those shift things has been fixed in
f17722f917b2f21497deb6edc62fb1683daa08e6, and the former is never hit.
I've also compiled this file with 4.6.2, but the ASM code looks the same.
Dave, any idea?
Eike
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