linux-kernel - Re: [PATCH v2 4/6] iomap: correct the dirty length in page mkwrite

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Message-ID: <7824fcb7-1de9-7435-e9f7-03dd7da6ec0a@huaweicloud.com>
Date: Wed, 14 Aug 2024 15:49:41 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, djwong@...nel.org, brauner@...nel.org,
 david@...morbit.com, jack@...e.cz, willy@...radead.org, yi.zhang@...wei.com,
 chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH v2 4/6] iomap: correct the dirty length in page mkwrite

On 2024/8/14 13:36, Christoph Hellwig wrote:
> On Mon, Aug 12, 2024 at 08:11:57PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@...wei.com>
>>
>> When doing page mkwrite, iomap_folio_mkwrite_iter() dirty the entire
>> folio by folio_mark_dirty() even the map length is shorter than one
>> folio. However, on the filesystem with more than one blocks per folio,
>> we'd better to only set counterpart block's dirty bit according to
>> iomap_length(), so open code folio_mark_dirty() and pass the correct
>> length.
> 
> What about moving the folio_mark_dirty out of the loop and directly
> into iomap_page_mkwrite so that it is exactly called once?  The
> iterator then does nothing for the !buffer_head case (but we still
> need to call it to allocate the blocks).
> 

Sorry, this makes me confused. How does this could prevent setting
redundant dirty bits?

Suppose we have a 3K regular file on a filesystem with 1K block size.
In iomap_page_mkwrite(), the iter.len is 3K, if the folio size is 4K,
folio_mark_dirty() will also mark all 4 bits of ifs dirty. And then,
if we expand this file size to 4K, and this will still lead to a hole
with dirty bit set but without any block allocated/reserved. Am I
missing something?

Thanks,
Yi.