Re: [PATCH v2] fat: preallocate space before expanding in fat_cont_expand()
From: Matheus Alves de Almeida
Date: Fri Oct 23 2026 - 17:22:32 EST
On Fri, Oct 23, 2026 at 04:33:23PM +0900, OGAWA Hirofumi wrote:
>Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx> writes:
>
>> 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).
>>
>> 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.
>>
>> Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx>
>> ---
>>
>> Changes in v2:
>> - fat_cont_expand(): Hold truncate_lock around fat_truncate_blocks().
>>
>> This patch depends on:
>> https://lore.kernel.org/linux-fsdevel/20261012171106.146243-1-matheus.aalmeida@xxxxxxxxxxxx/T/#t
>>
>> fs/fat/file.c | 56 ++++++++++++++++++++++++++++++++++-----------------
>> 1 file changed, 37 insertions(+), 19 deletions(-)
>>
>> diff --git a/fs/fat/file.c b/fs/fat/file.c
>> index 3aa541c6b127..dd0ae795dc14 100644
>> --- a/fs/fat/file.c
>> +++ b/fs/fat/file.c
>> @@ -217,15 +217,50 @@ const struct file_operations fat_file_operations = {
>> .setlease = generic_setlease,
>> };
>>
>> +static int fat_preallocate(struct inode *inode, loff_t new_size)
>> +{
>> + int nr_cluster; /* Number of clusters to be allocated */
>> + loff_t mm_bytes; /* Number of bytes to be allocated for file */
>> + loff_t ondisksize; /* block aligned on-disk size in bytes*/
>> + struct super_block *sb = inode->i_sb;
>> + struct msdos_sb_info *sbi = MSDOS_SB(sb);
>> +
>> + ondisksize = inode->i_blocks << 9;
>> + if (new_size <= ondisksize)
>> + return 0;
>> +
>> + /* First compute the number of clusters to be allocated */
>> + mm_bytes = new_size - ondisksize;
>> + nr_cluster = (mm_bytes + (sbi->cluster_size - 1)) >>
>> + sbi->cluster_bits;
>> +
>> + /* Allocate the clusters. We are not zeroing them out */
>> + return fat_add_clusters(inode, nr_cluster);
>> +}
>> +
>> static int fat_cont_expand(struct inode *inode, loff_t size)
>> {
>> struct address_space *mapping = inode->i_mapping;
>> loff_t start = inode->i_size, count = size - inode->i_size;
>> int err;
>>
>> - err = generic_cont_expand_simple(inode, size);
>> + /*
>> + * Preallocating space here is more efficient than during
>> + * generic_cont_expand_simple() because of batching.
>> + */
>> + err = inode_newsize_ok(inode, size);
>> if (err)
>> goto out;
>> + err = fat_preallocate(inode, size);
>> + if (err)
>> + goto out;
>> + err = generic_cont_expand_simple(inode, size);
>> + if (err) {
>> + down_write(&MSDOS_I(inode)->truncate_lock);
>> + fat_truncate_blocks(inode, inode->i_size);
>> + up_write(&MSDOS_I(inode)->truncate_lock);
>
>Is this fat_truncate_blocks() really required?
>
Yes. generic_cont_expand_simple() calls inode_newsize_ok() as well,
which happens before fat_write_failed() can run and clean up. Another
thread could lower RLIMIT_FSIZE between the FAT and generic checks and
return without cleaning up.
Thanks,
Matheus
>> + goto out;
>> + }
>>
>> fat_truncate_time(inode, NULL, FAT_UPDATE_CMTIME);
>> mark_inode_dirty(inode);
>> @@ -263,12 +298,7 @@ static int fat_cont_expand(struct inode *inode, loff_t size)
>> static long fat_fallocate(struct file *file, int mode,
>> loff_t offset, loff_t len)
>> {
>> - int nr_cluster; /* Number of clusters to be allocated */
>> - loff_t mm_bytes; /* Number of bytes to be allocated for file */
>> - loff_t ondisksize; /* block aligned on-disk size in bytes*/
>> struct inode *inode = file->f_mapping->host;
>> - struct super_block *sb = inode->i_sb;
>> - struct msdos_sb_info *sbi = MSDOS_SB(sb);
>> int err = 0;
>>
>> /* No support for hole punch or other fallocate flags. */
>> @@ -281,19 +311,7 @@ static long fat_fallocate(struct file *file, int mode,
>>
>> inode_lock(inode);
>> if (mode & FALLOC_FL_KEEP_SIZE) {
>> - ondisksize = inode->i_blocks << 9;
>> - if ((offset + len) <= ondisksize)
>> - goto error;
>> -
>> - /* First compute the number of clusters to be allocated */
>> - mm_bytes = offset + len - ondisksize;
>> - nr_cluster = (mm_bytes + (sbi->cluster_size - 1)) >>
>> - sbi->cluster_bits;
>> -
>> - /* Allocate the clusters. We are not zeroing them out */
>> - err = fat_add_clusters(inode, nr_cluster);
>> - if (err)
>> - goto error;
>> + err = fat_preallocate(inode, offset + len);
>> } else {
>> if ((offset + len) <= i_size_read(inode))
>> goto error;
>
>--
>OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>