btrfs: handle aligned EOF truncation correctly for subpage cases

btrfs: handle aligned EOF truncation correctly for subpage cases

[BUG]
For the following fsx -e 1 run, the btrfs still fails the run on 64K
page size with 4K fs block size:

  READ BAD DATA: offset = 0x26b3a, size = 0xfafa, fname = /mnt/btrfs/junk
  OFFSET      GOOD    BAD     RANGE
  0x26b3a     0x0000  0x15b4  0x0
  operation# (mod 256) for the bad data may be 21
  [...]
  LOG DUMP (28 total operations):
  1(  1 mod 256): SKIPPED (no operation)
  2(  2 mod 256): SKIPPED (no operation)
  3(  3 mod 256): SKIPPED (no operation)
  4(  4 mod 256): SKIPPED (no operation)
  5(  5 mod 256): WRITE    0x1ea90 thru 0x285e0	(0x9b51 bytes) HOLE
  6(  6 mod 256): ZERO     0x1b1a8 thru 0x20bd4	(0x5a2d bytes)
  7(  7 mod 256): FALLOC   0x22b1a thru 0x272fa	(0x47e0 bytes) INTERIOR
  8(  8 mod 256): WRITE    0x741d thru 0x13522	(0xc106 bytes)
  9(  9 mod 256): MAPWRITE 0x73ee thru 0xdeeb	(0x6afe bytes)
  10( 10 mod 256): FALLOC   0xb719 thru 0xb994	(0x27b bytes) INTERIOR
  11( 11 mod 256): COPY 0x15ed8 thru 0x18be1	(0x2d0a bytes) to 0x25f6e thru 0x28c77
  12( 12 mod 256): ZERO     0x1615e thru 0x1770e	(0x15b1 bytes)
  13( 13 mod 256): SKIPPED (no operation)
  14( 14 mod 256): DEDUPE 0x20000 thru 0x27fff	(0x8000 bytes) to 0x1000 thru 0x8fff
  15( 15 mod 256): SKIPPED (no operation)
  16( 16 mod 256): CLONE 0xa000 thru 0xffff	(0x6000 bytes) to 0x36000 thru 0x3bfff
  17( 17 mod 256): ZERO     0x14adc thru 0x1b78a	(0x6caf bytes)
  18( 18 mod 256): TRUNCATE DOWN	from 0x3c000 to 0x1e2e3	******WWWW
  19( 19 mod 256): CLONE 0x4000 thru 0x11fff	(0xe000 bytes) to 0x16000 thru 0x23fff
  20( 20 mod 256): FALLOC   0x311e1 thru 0x3681b	(0x563a bytes) PAST_EOF
  21( 21 mod 256): FALLOC   0x351c5 thru 0x40000	(0xae3b bytes) EXTENDING
  22( 22 mod 256): WRITE    0x920 thru 0x7e51	(0x7532 bytes)
  23( 23 mod 256): COPY 0x2b58 thru 0xc508	(0x99b1 bytes) to 0x117b1 thru 0x1b161
  24( 24 mod 256): TRUNCATE DOWN	from 0x40000 to 0x3c9a5
  25( 25 mod 256): SKIPPED (no operation)
  26( 26 mod 256): MAPWRITE 0x25020 thru 0x26b06	(0x1ae7 bytes)
  27( 27 mod 256): SKIPPED (no operation)
  28( 28 mod 256): READ     0x26b3a thru 0x36633	(0xfafa bytes)	***RRRR***

[CAUSE]
The involved operations are:

  fallocating to largest ever: 0x40000
  21 pollute_eof	0x24000 thru	0x2ffff	(0xc000 bytes)
  21 falloc	from 0x351c5 to 0x40000 (0xae3b bytes)
  28 read	0x26b3a thru	0x36633	(0xfafa bytes)

At operation #21 a pollute_eof is done, by memory mapped write into
range [0x24000, 0x2ffff).
At this stage, the inode size is 0x24000, which is block aligned.

Then fallocate happens, and since it's expanding the inode, it will call
btrfs_truncate_block() to truncate any unaligned range.

But since the inode size is already block aligned,
btrfs_truncate_block() does nothing and exits.

However remember the folio at 0x20000 has some range polluted already,
although it will not be written back to disk, it still affects the
page cache, resulting the later operation #28 to read out the polluted
value.

[FIX]
Instead of early exit from btrfs_truncate_block() if the range is
already block aligned, do extra filio zeroing if the fs block size is
smaller than the page size and we're truncating beyond EOF.

This is to address exactly the above case where memory mapped write can
still leave some garbage beyond EOF.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>