Re: [PATCH v2 06/10] ext4: update delalloc data reserve spcae in ext4_es_insert_extent()

[Jan Kara]

On Thu 08-08-24 19:18:30, Zhang Yi wrote:
> On 2024/8/8 1:41, Jan Kara wrote:
> > On Fri 02-08-24 19:51:16, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> >>
> >> Now that we update data reserved space for delalloc after allocating
> >> new blocks in ext4_{ind|ext}_map_blocks(), and if bigalloc feature is
> >> enabled, we also need to query the extents_status tree to calculate the
> >> exact reserved clusters. This is complicated now and it appears that
> >> it's better to do this job in ext4_es_insert_extent(), because
> >> __es_remove_extent() have already count delalloc blocks when removing
> >> delalloc extents and __revise_pending() return new adding pending count,
> >> we could update the reserved blocks easily in ext4_es_insert_extent().
> >>
> >> Thers is one special case needs to concern is the quota claiming, when
> >> bigalloc is enabled, if the delayed cluster allocation has been raced
> >> by another no-delayed allocation(e.g. from fallocate) which doesn't
> >> cover the delayed blocks:
> >>
> >>   |<       one cluster       >|
> >>   hhhhhhhhhhhhhhhhhhhdddddddddd
> >>   ^            ^
> >>   |<          >| < fallocate this range, don't claim quota again
> >>
> >> We can't claim quota as usual because the fallocate has already claimed
> >> it in ext4_mb_new_blocks(), we could notice this case through the
> >> removed delalloc blocks count.
> >>
> >> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> > ...
> >> @@ -926,9 +928,27 @@ void ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
> >>  			__free_pending(pr);
> >>  			pr = NULL;
> >>  		}
> >> +		pending = err3;
> >>  	}
> >>  error:
> >>  	write_unlock(&EXT4_I(inode)->i_es_lock);
> >> +	/*
> >> +	 * Reduce the reserved cluster count to reflect successful deferred
> >> +	 * allocation of delayed allocated clusters or direct allocation of
> >> +	 * clusters discovered to be delayed allocated.  Once allocated, a
> >> +	 * cluster is not included in the reserved count.
> >> +	 *
> >> +	 * When bigalloc is enabled, allocating non-delayed allocated blocks
> >> +	 * which belong to delayed allocated clusters (from fallocate, filemap,
> >> +	 * DIO, or clusters allocated when delalloc has been disabled by
> >> +	 * ext4_nonda_switch()). Quota has been claimed by ext4_mb_new_blocks(),
> >> +	 * so release the quota reservations made for any previously delayed
> >> +	 * allocated clusters.
> >> +	 */
> >> +	resv_used = rinfo.delonly_cluster + pending;
> >> +	if (resv_used)
> >> +		ext4_da_update_reserve_space(inode, resv_used,
> >> +					     rinfo.delonly_block);
> > 
> > I'm not sure I understand here. We are inserting extent into extent status
> > tree. We are replacing resv_used clusters worth of space with delayed
> > allocation reservation with normally allocated clusters so we need to
> > release the reservation (mballoc already reduced freeclusters counter).
> > That I understand. In normal case we should also claim quota because we are
> > converting from reserved into allocated state. Now if we allocated blocks
> > under this range (e.g. from fallocate()) without
> > EXT4_GET_BLOCKS_DELALLOC_RESERVE, we need to release quota reservation here
> > instead of claiming it. But I fail to see how rinfo.delonly_block > 0 is
> > related to whether EXT4_GET_BLOCKS_DELALLOC_RESERVE was set when allocating
> > blocks for this extent or not.
> 
> Oh, this is really complicated due to the bigalloc feature, please let me
> explain it more clearly by listing all related situations.
> 
> There are 2 types of paths of allocating delayed/reserved cluster:
> 1. Normal case, normally allocate delayed clusters from the write back path.
> 2. Special case, allocate blocks under this delayed range, e.g. from
>    fallocate().
> 
> There are 4 situations below:
> 
> A. bigalloc is disabled. This case is simple, after path 2, we don't need
>    to distinguish path 1 and 2, when calling ext4_es_insert_extent(), we
>    set EXT4_GET_BLOCKS_DELALLOC_RESERVE after EXT4_MAP_DELAYED bit is
>    detected. If the flag is set, we must be replacing a delayed extent and
>    rinfo.delonly_block must be > 0. So rinfo.delonly_block > 0 is equal
>    to set EXT4_GET_BLOCKS_DELALLOC_RESERVE.

Right. So fallocate() will call ext4_map_blocks() and
ext4_es_lookup_extent() will find delayed extent and set EXT4_MAP_DELAYED
which you (due to patch 2 of this series) transform into
EXT4_GET_BLOCKS_DELALLOC_RESERVE. We used to update the delalloc
accounting through in ext4_ext_map_blocks() but this patch moved the update
to ext4_es_insert_extent(). But there is one cornercase even here AFAICT:

Suppose fallocate is called for range 0..16k, we have delalloc extent at
8k..16k. In this case ext4_map_blocks() at block 0 will not find the
delalloc extent but ext4_ext_map_blocks() will allocate 16k from mballoc
without using delalloc reservation but then ext4_es_insert_extent() will
still have rinfo.delonly_block > 0 so we claim the quota reservation
instead of releasing it?

> B. bigalloc is enabled, there a 3 sub-cases of allocating a delayed
>    cluster:
> B0.Allocating a whole delayed cluster, this case is the same to A.
> 
>      |<         one cluster       >|
>      ddddddd+ddddddd+ddddddd+ddddddd
>      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ allocating the whole range

I agree. In this case there's no difference.

 
> B1.Allocating delayed blocks in a reserved cluster, this case is the same
>    to A, too.
> 
>      |<         one cluster       >|
>      hhhhhhh+hhhhhhh+ddddddd+ddddddd
>                              ^^^^^^^
>                              allocating this range

Yes, if the allocation starts within delalloc range, we will have
EXT4_GET_BLOCKS_DELALLOC_RESERVE set and ndelonly_blocks will always be >
0.

> B2.Allocating blocks which doesn't cover the delayed blocks in one reserved
>    cluster,
> 
>      |<         one cluster       >|
>      hhhhhhh+hhhhhhh+hhhhhhh+ddddddd
>      ^^^^^^^
>      fallocating this range
> 
>   This case must from path 2, which means allocating blocks without
>   EXT4_GET_BLOCKS_DELALLOC_RESERVE. In this case, rinfo.delonly_block must
>   be 0 since we are not replacing any delayed extents, so
>   rinfo.delonly_block == 0 means allocate blocks without EXT4_MAP_DELAYED
>   detected, which further means that EXT4_GET_BLOCKS_DELALLOC_RESERVE is
>   not set. So I think we could use rinfo.delonly_block to identify this
>   case.

Well, this is similar to the non-bigalloc case I was asking about above.
Why the allocated unwritten extent cannot extend past the start of delalloc
extent? I didn't find anything that would disallow that...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR