linux-ext4 - Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use iomap

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Date: Thu, 30 Nov 2023 10:57:28 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use iomap

Christoph Hellwig <hch@...radead.org> writes:

> On Thu, Nov 30, 2023 at 08:54:31AM +0530, Ritesh Harjani wrote:
>> So I took a look at this. Here is what I think -
>> So this is useful of-course when we have a large folio. Because
>> otherwise it's just one block at a time for each folio. This is not a
>> problem once FS buffered-io handling code moves to iomap (because we
>> can then enable large folio support to it).
>
> Yes.
>
>> However, this would still require us to pass a folio to ->map_blocks
>> call to determine the size of the folio (which I am not saying can't be
>> done but just stating my observations here).
>
> XFS currently maps based on the underlyig reservation (delalloc extent)
> and not the actual map size.   This works because on-disk extents are
> allocated as unwritten extents, and only the actual written part is
> the converted.  But if you only want to allocate blocks for the part
> actually written you actually need to pass in the dirty range and not
> just use the whole folio.  This would be the incremental patch to do
> that:

yes. dirty range indeed.

>
> http://git.infradead.org/users/hch/xfs.git/commitdiff/0007893015796ef2ba16bb8b98c4c9fb6e9e6752
>
> But unless your block allocator is very cheap doing what XFS does is
> probably going to work much better.
>
>> ...ok so here is the PoC for seq counter check for ext2. Next let me
>> try to see if we can lift this up from the FS side to iomap - 
>
> This looks good to me from a very superficial view.  Dave is the expert
> on this, though.

Sure.