[PATCH] fix corruption case for block size < page size

[Eric Sandeen]

Eric Sandeen wrote:
> On a 4k page system and 512-byte blocksize, this:
> 
> xfs_io \
> -c "pwrite -S 0x11 -b 4096 0 4096" \
> -c "mmap -r 0 512" -c "mread 0 512" -c "munmap" \
> -c "truncate 256" \
> -c "truncate 513" \
> -c "pwrite -S 0x22 -b 512 2048 512" \
> -t -d -f testfile

Not to keep belaboring the point, but if anyone reviews this here's a
bit more info.

If I blktrace the testcase it looks like this:

8,16  0   1   0.000000000  4222  C   W 166979666 + 8 [0] 4k wr
8,16  0   2   0.000367043  4222  C   R 166979666 + 8 [0] 4k map rd
8,16  0   3   0.002923548  4222  C   N (35 00 ..) [0]
8,16  0   4   0.003108924  4222  C   W 200708307 + 9 [0] Log?(trunc)
8,16  0   5   0.020357902  4222  C   N (35 00 ..) [0]
8,16  0   6   0.020361434  4222  C   W 200708307 + 9 [0] Log?(trunc)
8,16  0   7   0.020745509  4222  C   W 166979666 + 1 [0] 512 wr @0
8,16  0   8   0.020940005  4222  C   W 166979667 + 1 [0] 512 wr @1
8,16  0   9   0.021172749  4222  C   W 166979670 + 1 [0] 512 wr @4

and a detailed look at the data on disk is this:

00000000  11 11 11 11 11 11 11 11  11 11 11 11 11 11 11 11  Block 0(OK)
*
00000100  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  Block 0...
*
00000200  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  Block 1(OK)
*
00000400  11 11 11 11 11 11 11 11  11 11 11 11 11 11 11 11  Block 2(BAD)
*
00000600  11 11 11 11 11 11 11 11  11 11 11 11 11 11 11 11  Block 3(BAD)
*
00000800  22 22 22 22 22 22 22 22  22 22 22 22 22 22 22 22  Block 4(OK)
*
00000a00

And the bmap information is this:

 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL
   0: [0..4]:          56..60            0 (56..60)             5

So the bad data in blocks 2 and 3 were never rewritten; the buffer heads
probably were fine (containing 0's, but I should check) and we simply
re-mapped blocks 2 and 3 back into existence, along with their stale
data, it seems.

So I think this was just a bad mapping decision, and not a buffer head
state/zeroing problem...?

-Eric