On 22/05/23 11:08PM, Jan Kara wrote:
> On Tue 24-05-22 01:38:44, Ritesh Harjani wrote:
> > On 22/05/21 09:42PM, Baokun Li wrote:
> > > When either of the "start + size <= ac->ac_o_ex.fe_logical" or
> > > "start > ac->ac_o_ex.fe_logical" conditions is met, it indicates
> > > that the fe_logical is not in the allocated range.
> >
> > Sounds about right to me based on the logic in ext4_mb_use_inode_pa().
> > We try to allocate/preallocate such that ac->ac_o_ex.fe_logical should fall
> > within the preallocated range. So if our start or start + size doesn't include
> > fe_logical then it is a bug in the ext4_mb_normalize_request() logic.
>
> I agree ac->ac_o_ex.fe_logical is a goal block. But AFAIK it never was a
> hard guarantee that we would allocate extent that includes that block. It
Agree that the guarantee is not about the extent which finally gets allocated.
It is only within ext4_mb_normalize_request() that the "start" and "size"
variable calculations is done in such a way that the ac->ac_o_ex.fe_logical
block will always fall within the "start" & "end" boundaries after
normalization.
That is how it also updates the goal block at the end too. ac->ac_g_ex.
> was always treated as a hint only. In particular if you look at the logic
> in ext4_mb_normalize_request() it does shift the start of the allocation to
> avoid preallocated ranges etc.
Yes, I checked the logic of ext4_mb_normalize_request() again.
As I see it (I can be wrong, so please correct me), there is always an attempt
to make "start" & "start + size" such that it covers ac->ac_o_ex.fe_logical
except just one change where we are trimming the size of the request to only
EXT4_BLOCKS_PER_GROUP.
For e.g. when it compares against preallocated ranges. It has a BUG() which
checks if the ac->ac_o_ex.fe_logical already lies in the preallocated range.
Because then we should never have come here to do allocation of a new block.
4143 /* PA must not overlap original request */
4144 BUG_ON(!(ac->ac_o_ex.fe_logical >= pa_end ||
4145 ac->ac_o_ex.fe_logical < pa->pa_lstart));
<...>
4152 BUG_ON(pa->pa_lstart <= start && pa_end >= end);
Then after skipping the preallocated regions which doesn't fall in between
"start" and "end"...
4147 /* skip PAs this normalized request doesn't overlap with */
4148 if (pa->pa_lstart >= end || pa_end <= start) {
4149 spin_unlock(&pa->pa_lock);
4150 continue;
4151 }
...it adjusts "start" and "end" boundary according to allocated PAs boundaries
such that fe_logical is always in between "start" and "end".
4154 /* adjust start or end to be adjacent to this pa */
4155 if (pa_end <= ac->ac_o_ex.fe_logical) {
4156 BUG_ON(pa_end < start);
4157 start = pa_end;
4158 } else if (pa->pa_lstart > ac->ac_o_ex.fe_logical) {
4159 BUG_ON(pa->pa_lstart > end);
4160 end = pa->pa_lstart;
4161 }
> so I don't see how we are guaranteed that
> ext4_mb_normalize_request() will result in an allocation request that
> includes ac->ac_o_ex.fe_logical.
It could be I am wrong, but looks like ext4_mb_normalize_request() keeps
ac->ac_o_ex.fe_logical within "start" and "end" of allocation request.
And then updates the goal block.
4196 ac->ac_g_ex.fe_logical = start;
4197 ac->ac_g_ex.fe_len = EXT4_NUM_B2C(sbi, size);
Thoughts?
-ritesh
