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?
> 

I forget to mention that truncate_inode_partial_folio() call
folio_invalidate()->iomap_invalidate_folio() only when the folio has
private, if the folio doesn't has ifs, the iomap_invalidate_folio()
would nerver be called, hence allocate the ifs in
iomap_invalidate_folio() is useless.

In my opinion, one solution is change to always call folio_invalidate()
in truncate_inode_partial_folio(), all callbacks should handle the no
private case. Another solution is add a magic (a fake ifs) to
folio->private and then convert it to a real one in
iomap_invalidate_folio(), any thoughts?

Thanks,
Yi.