[Ocfs2-devel] [PATCH 1/3] ocfs2: Add ocfs2_trim_fs for SSD trim support.

[Tao Ma]

On 03/08/2011 02:23 PM, Tristan Ye wrote:
> Tao Ma wrote:
>> On 03/08/2011 12:55 PM, Tristan Ye wrote:
>>> Hi Tao,
>>>
>>>    Most of codes looks pretty neat to me, few comments inlined below:
>> Thanks for the review.
>>> Tao Ma wrote:
>>>> From: Tao Ma <boyu.mt at taobao.com>
>>>>
>>>> Add ocfs2_trim_fs to support trimming freed clusters in the
>>>> volume. A range will be given and all the freed clusters greater
>>>> than minlen will be discarded to the block layer.
>>>>
>>>> Signed-off-by: Tao Ma <boyu.mt at taobao.com>
>>>> ---
>>>>  fs/ocfs2/alloc.c |  154
>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  fs/ocfs2/alloc.h |    1 +
>>>>  2 files changed, 155 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/fs/ocfs2/alloc.c b/fs/ocfs2/alloc.c
>>>> index b27a0d8..6e1b3b5 100644
>>>> --- a/fs/ocfs2/alloc.c
>>>> +++ b/fs/ocfs2/alloc.c
>>>> @@ -29,6 +29,7 @@
>>>>  #include <linux/highmem.h>
>>>>  #include <linux/swap.h>
>>>>  #include <linux/quotaops.h>
>>>> +#include <linux/blkdev.h>
>>>>  
>>>>  #include <cluster/masklog.h>
>>>>  
>>>> @@ -7184,3 +7185,156 @@ out_commit:
>>>>  out:
>>>>      return ret;
>>>>  }
>>>> +
>>>> +static int ocfs2_trim_extent(struct super_block *sb,
>>>> +                 struct ocfs2_group_desc *gd,
>>>> +                 int start, int count)
>>>> +{
>>>> +    u64 discard;
>>>> +
>>>> +    count = ocfs2_clusters_to_blocks(sb, count);
>>>> +    discard = le64_to_cpu(gd->bg_blkno) +
>>>> +            ocfs2_clusters_to_blocks(sb, start);
>>>> +
>>>> +    return sb_issue_discard(sb, discard, count, GFP_NOFS, 0);
>>>> +}
>>>> +
>>>> +static int ocfs2_trim_group(struct super_block *sb,
>>>> +                struct ocfs2_group_desc *gd,
>>>> +                int start, int max, int minbits)
>>>> +{
>>>> +    int ret = 0, count = 0, next;
>>>> +    void *bitmap = gd->bg_bitmap;
>>>> +
>>>> +    while (start < max) {
>>>> +        start = ocfs2_find_next_zero_bit(bitmap, max, start);
>>>> +        if (start >= max)
>>>> +            break;
>>>    /* What if the 'start' stands within a hole */
>>>
>>>    if (ocfs2_test_bit(...)) {
>>>       start = ocfs2_find_next_zero_bit(...);
>>>       if ((start == -1) || (start >= max))
>>>          break;
>>>    }
>>>
>>>> +        next = ocfs2_find_next_bit(bitmap, max, start);
>>>      next = ocfs2_find_next_bit(...);
>>>    if (next == -1)
>>>       break;
>> next will be set to "-1"? sorry, but where do you get it?
>>>    if (next > max)
>>>       next = max;
>> again, ocfs2_find_next_bit will return a value larger than 'max'? I am
>> afraid not. Otherwise, it will be nonsense to pass a 'max' to it.
> 
> 
> Say we're handling the last group, and the 'start + len' was within a
> hole, then the 'max'
> is 'first_bit + len', while the next none-zero bit we found may be
> larger than 'max', isn't
> that possible?
ocfs2_find_next_bit(and ext2_find_next_bit) won't parse, check and
return 'bit' after 'max'. otherwise there should be a problem of memory
overflow(you read and check some memory which isn't owned and handled by
you). So the same goes here. If it can return a value larger than 'max',
every caller will have to check the overflow. That would be too painful.
>>>> +int ocfs2_trim_fs(struct super_block *sb, struct fstrim_range *range)
>>>> +{
>>>> +    struct ocfs2_super *osb = OCFS2_SB(sb);
>>>> +    u64 start, len, minlen, trimmed, first_group, last_group, group;
>>>    why not using u32 start, len, minlen, trimmed;
>> we may use 64 bit clusters later I guess. And what's more, they will be
>> set by the user later. and it may overflow. Say the user pass a u64
>> range->len, it will overflow with range->len >> osb->s_clustersize_bits.
> 
> I just found we were using u32 for counting clusters all around ocfs2
> codes, e.g truncate/punching_hole
> codes, also passing an u64 byte_offset from userspace, so my original
> intention is to keep an unification;-)
> 
> Overflow can theoretically happen anyway, however, it's not very likely
> to pass a 16TB+ byte_offset from userspace.
I am afraid it is very likely. So say you want to trim all the clusters
within the volume, how could you set 'range->len'? Will you first fdisk
to get the volume size and then set it accordingly?
Most guys will set it to ULLONG_MAX and let the file system handles it.
This is not my personal view, please check this article:
http://lwn.net/Articles/417809/
Jonathan also suggests to set len to ULLONG_MAX so that you can trim the
whole volume.

Regards,
Tao