Re: [PATCH 2/2] ext4: correct the judgment of BUG in ext4_mb_normalize_request

[Ritesh Harjani]

On 22/05/24 11:39AM, Jan Kara wrote:
> On Tue 24-05-22 11:56:55, Ritesh Harjani wrote:
> > 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?
>
> Right, after some more inspection the only thing I'm concerned about is:
>
>         /* don't cover already allocated blocks in selected range */
>         if (ar->pleft && start <= ar->lleft) {
>                 size -= ar->lleft + 1 - start;
>                 start = ar->lleft + 1;
>         }
>
> which can shift start beyond ac->ac_o_ex.fe_logical if the block would be
> already allocated. But I guess in that case we should not be calling
> ext4_mb_normalize_request()? ... some more code digging .. Yes, that is
> guaranteed in how lleft is initialized in ext4_ext_map_blocks().

Yes.

> So OK, I withdraw my objection to the stronger check but the changelog really needs

Thanks Jan for confirming it.


> to do a better job to explain why the stronger condition should be true.

Agreed.


diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 252c168454c7..9e7c145e9aa2 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4179,7 +4179,22 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
        }
        rcu_read_unlock();

-       if (start + size <= ac->ac_o_ex.fe_logical &&
+       /*
+        * In this function "start" and "size" are normalized for better
+        * alignment and length such that we could preallocate more blocks.
+        * This normalization is done such that original request of
+        * ac->ac_o_ex.fe_logical & fe_len should always lie within "start" and
+        * "size" boundaries.

Does above comment look good to you?


+        * (Note fe_len can be relaxed since FS block allocation API does not
+        * provide gurantee on number of contiguous blocks allocation since that
+        * depends upon free space left, etc).
+        * In case of inode pa, later we use the allocated blocks
+        * [pa_start + fe_logical - pa_lstart, fe_len/size] from the preallocated
+        * range of goal/best blocks [start, size] to put it at the
+        * ac_o_ex.fe_logical extent of this inode.
+        * (See ext4_mb_use_inode_pa() for more details)
+        */

^^^ We can even put more info if needed (maybe in commit message?)

+       if (start + size <= ac->ac_o_ex.fe_logical ||
                        start > ac->ac_o_ex.fe_logical) {
                ext4_msg(ac->ac_sb, KERN_ERR,
                         "start %lu, size %lu, fe_logical %lu",

FYI - I ran the fsstress test (with -g 256) shared by Baokun with only above
change (&& -> ||) and not the original fix. And I see that we can hit this
mentioned BUG() now.

<logs>
========
[  599.619875] EXT4-fs (loop2): start 692, size 196, fe_logical 982
	...I think we should also add (orig_size >> bsbits) in above print msg ^^

[  599.621043] ------------[ cut here ]------------
[  599.625099] kernel BUG at fs/ext4/mballoc.c:4188!

-ritesh