Re: [PATCH v2 3/6] iomap: advance the ifs allocation if we have more than one blocks per folio

[Zhang Yi <yi.zhang@huaweicloud.com>]

On 2024/8/14 13:32, Christoph Hellwig wrote:
> On Mon, Aug 12, 2024 at 08:11:56PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> Now we allocate ifs if i_blocks_per_folio is larger than one when
>> writing back dirty folios in iomap_writepage_map(), so we don't attach
>> an ifs after buffer write to an entire folio until it starts writing
>> back, if we partial truncate that folio, iomap_invalidate_folio() can't
>> clear counterpart block's dirty bit as expected. Fix this by advance the
>> ifs allocation to __iomap_write_begin().
> 
> Wouldn't it make more sense to only allocate the ifѕ in
> iomap_invalidate_folio when it actually is needed?
> 

Therefore, you mean current strategy of allocating ifs is to try to delay
the allocation time as much as possible? The advantage is that it could
avoid some unnecessary allocation operations if the whole folio are
invalidated before write back. right?

> Also do you have a reproducer for this?
> 

This mistake doesn't case any real problem now, because once the folio
has been partial truncated, the counterpart range becomes a hole, although
the ifs dirty bit is not cleared, iomap_writepage_map_blocks() can deal
with it and won't cause any problem. Hence I don't have reproducer for
this.

Thanks,
Yi.