fat: preallocate space before expanding in fat_cont_expand()

The fat_cont_expand() implementation leaves allocation of new clusters
to generic_cont_expand_simple(). This path does many small cluster
allocations, harming performance.

Preallocate cluster space in fat_cont_expand(). Move preallocation to
a new fat_preallocate() helper used by both fat_cont_expand() and
fat_fallocate(FALLOC_FL_KEEP_SIZE). Use a FAT-specific
fat_cont_expand_simple() instead of generic_cont_expand_simple() to
avoid rechecking the new size after clusters have been allocated.

Benchmarking was done under QEMU/KVM with a virtio-blk device using
cache=none. Each result is the arithmetic mean of the listed samples.

start = now_ns();
for (unsigned i = 0; i < count; i++) {
    if (truncate_mode)
        ftruncate(fds[i], bytes);
    else
        fallocate(fds[i], 0, 0, bytes);
}
elapsed = now_ns() - start;

16 files, 512-byte clusters:
length    mode      sync?    before    after    improvement    samples
256MiB    ftruncate no       9.576s    5.345s   1.79x          10
256MiB    fallocate no       9.634s    5.640s   1.71x          10
1MiB      ftruncate yes      58.66s    0.362s   162.15x        5
1MiB      fallocate yes      61.36s    0.390s   157.15x        5

128 files, 512-byte clusters:
length    mode      sync?    before    after    improvement    samples
512KiB    ftruncate no       0.433s    0.352s   1.23x          10
512KiB    fallocate no       0.446s    0.393s   1.13x          10
64KiB     ftruncate yes      31.32s    0.429s   73.00x         10
64KiB     fallocate yes      29.59s    0.410s   72.42x         10

16 files, 4096-byte clusters:
length    mode      sync?    before    after    improvement    samples
256MiB    ftruncate no       5.181s    4.417s   1.17x          10
256MiB    fallocate no       5.166s    4.442s   1.16x          10
1MiB      ftruncate yes      9.294s    0.111s   84.06x         10
1MiB      fallocate yes      8.710s    0.110s   79.36x         10

The biggest improvement is on filesystems with a small cluster size and
on synchronous mounts.

Link: https://lore.kernel.org/20261024013021.251898-1-matheus.aalmeida@inf.ufrgs.br
Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

0 files changed, 0 insertions, 0 deletions