linux-kernel - Re: [PATCH] xfs: fix the judgment of whether the file already has extents

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Message-ID: <20241028104242.1114200-1-alexjlzheng@tencent.com>
Date: Mon, 28 Oct 2024 18:42:42 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: alexjlzheng@...il.com
Cc: alexjlzheng@...cent.com,
	cem@...nel.org,
	chandanbabu@...nel.org,
	dchinner@...hat.com,
	djwong@...nel.org,
	linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org,
	zhangjiachen.jaycee@...edance.com
Subject: Re: [PATCH] xfs: fix the judgment of whether the file already has extents

On Mon, 28 Oct 2024 18:33:32 +0800, alexjlzheng@...cent.com wrote:
> On Mon, 28 Oct 2024 02:41:01 -0700, hch@...radead.org wrote:
> > On Sun, Oct 27, 2024 at 02:01:16AM +0800, alexjlzheng@...il.com wrote:
> > > From: Jinliang Zheng <alexjlzheng@...cent.com>
> > > 
> > > When we call create(), lseek() and write() sequentially, offset != 0
> > > cannot be used as a judgment condition for whether the file already
> > > has extents.
> > > 
> > > This patch uses prev.br_startoff instead of offset != 0.
> >
> > This changed the predicate from "are we at offset 0" to "are there
> > any allocations before that".  That's a pretty big semantic change.
> > Maybe a good one, maybe not.  Can you explain what workload it helps
> > you with?
> 
> 
> Thanks for your reply.
> 
> I noticed this because I was confused when reading the code here. The code
> comment here says:
> 
> /*
>  * If there are already extents in the file, try an exact EOF block
>  * allocation to extend the file as a contiguous extent. If that fails,
>  * or it's the first allocation in a file, just try for a stripe aligned
>  * allocation.
>  */
> 
> But as you said, the semantics of the current code is "are we at offset 0",
> not "are there any allocations before that".

By the way, we only get here if got is or after EOF, so "are there any allocations
before that" means "are there already extents in the file".

Thank you, again. :)
Jinliang Zheng

> 
> Therefore, I think it is better to use "prev.br_startoff != NULLFILEOFF"
> instead of the current "offset != 0", at least its semantics are more
> consistent with the intention in the code comment and reduce confusion.
> 
> But if the semantics here have indeed changed to the point where it is
> inconsistent with the code comment, my suggestion is to update the code
> comment here.
> 
> Thank you. :)
> Jinliang Zheng