[Ocfs2-devel] [PATCH 1/1] Ocfs2: Optimize punching-hole codes v3.

[tristan]

Tao Ma wrote:
> Hi Tristan,
>
> tristan wrote:
>> Thanks for your so quick review;)
>>
>>
>> Tao Ma wrote:
>>> Hi Tristan,
>>> here comes the review.
>>>
>>> Tristan Ye wrote:
>>>> Changes from v2 to v3:
>>>>
>>> <snip>
>>>> + /*
>>>> + * Simply jump to previous record if the pos is
>>>> + * the start of a record.
>>>> + */
>>>> + if (le32_to_cpu(rec->e_cpos) == *pos) {
>>>> + i--;
>>>> + if (i < 0) {
>>>> + *rec_got = NULL;
>>>> + return;
>>>> + }
>>>> +
>>>> + rec = &el->l_recs[i];
>>>> + range = le32_to_cpu(rec->e_cpos) +
>>>> + ocfs2_rec_clusters(el, rec);
>>>> + /*
>>>> + * Skip a hole.
>>>> + */
>>>> + if (range < *pos)
>>>> + *pos = range;
>>> do we still need to check range < *pos? As you have already check
>>> le32_to_cpu(rec->e_cpos) == *pos.
>>
>> I'm afraid we have to,
>>
>> See we decreased the 'i' in the if clause;) so the rec here is not 
>> the same as previous one.
> yes, we have decreased. so we should have range < 
> old_rec->e_cpos(range now is the previous rec's end). And *pos == 
> old_rec->e_cpos. That is of course range < *pos.

Oh, normally range(previous) should be equal to *pos, right?

when we say range < *pos, there must be a hole in it.

>>
>>>> +
>>>> + break;
>>>> + }
>>>> + }
>>>> +
>>>> + *rec_got = &el->l_recs[i];
>>>> +}
>>>> +
>>>> +/*
>>>> + * Helper to calculate the punching pos and length in one run, we 
>>>> handle the
>>>> + * following three cases in order:
>>>> + *
>>>> + * - remove the entire record
>>>> + * - remove a partial record
>>>> + * - no record needs to be removed (hole-punching completed)
>>>> +*/
>>>> +static void ocfs2_calc_trunc_pos(struct inode *inode,
>>>> + struct ocfs2_extent_list *el,
>>>> + struct ocfs2_extent_rec *rec,
>>>> + u32 trunc_start, u32 *trunc_cpos,
>>>> + u32 *trunc_len, u32 *trunc_end,
>>>> + u64 *blkno, int *done)
>>>> +{
>>>> + int ret = 0;
>>>> + u32 coff, range;
>>>> +
>>>> + range = le32_to_cpu(rec->e_cpos) + ocfs2_rec_clusters(el, rec);
>>>> +
>>>> + if (le32_to_cpu(rec->e_cpos) >= trunc_start) {
>>>> + *trunc_cpos = le32_to_cpu(rec->e_cpos);
>>>> + *trunc_len = *trunc_end - le32_to_cpu(rec->e_cpos);
>>>> + *blkno = le64_to_cpu(rec->e_blkno);
>>>> + *trunc_end = le32_to_cpu(rec->e_cpos);
>>>> + } else if (range > trunc_start) {
>>>> + *trunc_cpos = trunc_start;
>>>> + *trunc_len = range - trunc_start;
>>>> + coff = trunc_start - le32_to_cpu(rec->e_cpos);
>>>> + *blkno = le64_to_cpu(rec->e_blkno) +
>>>> + ocfs2_clusters_to_blocks(inode->i_sb, coff);
>>>> + *trunc_end = trunc_start;
>>>> + } else {
>>>> + /*
>>>> + * It may have two following possibilities:
>>>> + *
>>>> + * - last record has been removed
>>> what do you mean last record here? Sorry, I don't understand this case.
>>
>> Here we reached the 'trunc_start', means we've got nothing to truncate;)
>>
>> It's the case when 'range == trunc_start'
>>
>>>> + * - trunc_start was within a hole
>>
>> So here is the case when 'range < trunc_start'
> oh, so do please change 'last record has been removed' somehow, I 
> can't understand what it means actually.
>>>> @@ -1482,16 +1588,13 @@ static int ocfs2_remove_inode_range(struct 
>>>> inode *inode,
>>>> }
>>>>
>>>> trunc_start = ocfs2_clusters_for_bytes(osb->sb, byte_start);
>>>> - trunc_len = (byte_start + byte_len) >> osb->s_clustersize_bits;
>>>> - if (trunc_len >= trunc_start)
>>>> - trunc_len -= trunc_start;
>>>> - else
>>>> - trunc_len = 0;
>>>> + trunc_end = (byte_start + byte_len) >> osb->s_clustersize_bits;
>>>> + cluster_within_list = trunc_end;
>>> You said in the comments below that "we want to find a path which 
>>> contains (trunc_end - 1)". So we also need to set trunc_end -1 here? 
>>> Otherwise you may enter the wrong extent block in some case?
>>
>> No, I'm afraid we'd rather do like this, since 'trunc_end - 1' is 
>> used for finding the next extent block when crossing the blocks, 
>> trunc_end is for contiguous truncating..
> why? you use cluster_with_list to search the path. So consider 
> trunc_end is just the cpos for a extent block(not the leaf extent 
> rec). You go to the wrong extent block actually in the first 
> ocfs2_find_path. But you will find that the first rec->e_cpos == 
> trunc_end and go to the previous extent block. So an extra loop here.

cluster_with_list and trunc_end will be the same in first ocfs2_find_path.

>>>> + /*
>>>> + * Unlike truncate codes, here we want to find a path which
>>>> + * contains (trunc_end - 1) cpos, and then trunc_end will be
>>>> + * decreased after each removal of a record range.
>>>> + *
>>>> + * Why not using trunc_end to search the path?
>>>> + * The reason is simple, think about the situation of crossing
>>>> + * the extent block, we need to find the adjacent block by
>>>> + * decreasing one cluster, otherwise, it will run into a loop.
>>>> + */
>>>> + ret = ocfs2_find_path(INODE_CACHE(inode), path,
>>>> + cluster_within_list);
>>>> if (ret) {
>>>> mlog_errno(ret);
>>>> goto out;
>>>> }
>>>>
>>>> - if (alloc_size > trunc_len)
>>>> - alloc_size = trunc_len;
>>>> + el = path_leaf_el(path);
>>>>
>>>> - /* Only do work for non-holes */
>>>> - if (phys_cpos != 0) {
>>>> - ret = ocfs2_remove_btree_range(inode, &et, cpos,
>>>> - phys_cpos, alloc_size,
>>>> - &dealloc, refcount_loc,
>>>> - flags);
>>>> - if (ret) {
>>>> - mlog_errno(ret);
>>>> - goto out;
>>>> - }
>>>> + ocfs2_find_rec_with_pos(el, &rec, &trunc_end);
>>>> + /*
>>>> + * Go to next extent block directly when last rec was truncated.
>>>> + */
>>>> + if (!rec) {
>>>> + cluster_within_list = trunc_end - 1;
>>>> + ocfs2_reinit_path(path, 1);
>>>> + break;
>>> ocfs2_find_rec_with_pos will find a rec contains trunc_end and if 
>>> there is no such rec and trunc_end < el->l_recs[0].e_cpos, rec will 
>>> be set NULL, right?
>>
>> Correct.
>>
>>> So what if the el is in the left most extent block? Your function 
>>> will iterate again and again with every time trunc_end--?
>>
>> Cool, you've found a bug here:
>>
>> OCFS2: ERROR (device sda8): ocfs2_remove_extent: Owner 66050 has an 
>> extent at cpos 1245184 which can no longer be found.
>>
>> File system is now read-only due to the potential of on-disk 
>> corruption. Please run fsck.ocfs2 once the file system is unmounted.
>> (5975,0):ocfs2_remove_btree_range:5799 ERROR: status = -30
>> (5975,0):ocfs2_remove_inode_range:1654 ERROR: status = -30
>> (5975,0):__ocfs2_change_file_space:1778 ERROR: status = -30
>>
>>
>> Thanks for pointing this out.
>>
>> This happens when we've alreay have a hole from 0 to the first extent 
>> block.
>>
>>
>>> another problem is that your comments is "Go to next extent block 
>>> directly when last rec was truncated", I guess you mean 'previous'? 
>>> And you 'break'
>>
>> I hate being a moron to make such a mistake, it definitely should be 
>> 'previous' extent block.
> And we need to change 'break' to 'contiue' also, right?

I'll change the code somehow above, I'd rather still to break if no 
record was found, since no record found means we reached the leftmost 
extent block already.


>
> Regards,
> Tao