[Devel] [RFC PATCH vz9 v6 19/62] dm-ploop: simplify llseek

[Alexander Atanasov]

On 20.01.25 6:06, Pavel Tikhomirov wrote:
> 
> 
> On 12/6/24 05:55, Alexander Atanasov wrote:
>> From: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
>>
>> Do not bother with locked clusters. For llseek we can ignore all
>> concurrent operations.
>> Do not call ploop_bat_entries() on every cluster as this is slow.
>> Just remember md and read it directly.
>>
>> https://virtuozzo.atlassian.net/browse/VSTOR-91817
>> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
>> ---
>>   drivers/md/dm-ploop-map.c | 27 ++++++++-------------------
>>   1 file changed, 8 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/md/dm-ploop-map.c b/drivers/md/dm-ploop-map.c
>> index 7ecf79fcd2cc..ad7ca7d43dfc 100644
>> --- a/drivers/md/dm-ploop-map.c
>> +++ b/drivers/md/dm-ploop-map.c
>> @@ -2088,32 +2088,21 @@ 
>> ALLOW_ERROR_INJECTION(ploop_prepare_reloc_index_wb, ERRNO);
>>   loff_t ploop_llseek_hole(struct dm_target *ti, loff_t offset, int 
>> whence)
>>   {
>>       struct ploop *ploop = ti->private;
>> -    u32 clu, dst_clu;
>> +    u32 clu, dst_clu, id, *bat;
>> +    struct md_page *md;
>>       loff_t result;
>>       clu = SEC_TO_CLU(ploop, to_sector(offset) + ploop->skip_off);
>> +    id = U32_MAX;
>>       while (clu < ploop->nr_bat_entries) {
>> -
>> -        /*
>> -         * Assume a locked cluster to have a data.
>> -         * In worst case someone will read zeroes
>> -         */
>> -        if (ploop_postpone_if_cluster_locked(ploop, NULL, clu)) {
>> -            if (whence & SEEK_DATA)
>> -                break;
>> -            if (whence & SEEK_HOLE) {
>> -                clu++;
>> -                continue;
>> -            }
>> +        if (id != ploop_bat_clu_to_page_nr(clu)) {
>> +            id = ploop_bat_clu_to_page_nr(clu);
>> +            md = ploop_md_page_find(ploop, id);
> 
> We can probably put BUG_ON(!md); here same as in ploop_bat_entries.

I think it is better to have :

			if (!md)
				return -EINVAL;


this is a system call so we can return error instead of crashing - 
BUG_ON use is discouraged.

EINVAL is listed in llseek man page.


> 
>>           }
>> -        /*
>> -         * It *may* be possible to speed this up by iterating over 
>> clusters
>> -         * in mapped metadata page, but this is a bit problematic
>> -         * since we want to check if cluster is locked
>> -         */
>> -        dst_clu = ploop_bat_entries(ploop, clu, NULL, NULL);
>> +        bat = md->kmpage;
>> +        dst_clu = READ_ONCE(bat[ploop_bat_clu_idx_in_page(clu)]);
>>           if (whence & SEEK_DATA && dst_clu != BAT_ENTRY_NONE)
>>               break;
> 

-- 
Regards,
Alexander Atanasov