linux-ext4 - Re: [PATCH v3 4/4] ext4: support uncached buffered I/O

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Message-ID: <aF7OzbVwXqbJaLQA@casper.infradead.org>
Date: Fri, 27 Jun 2025 18:03:09 +0100
From: Matthew Wilcox <willy@...radead.org>
To: 陈涛涛 Taotao Chen <chentaotao@...iglobal.com>
Cc: "tytso@....edu" <tytso@....edu>,
	"hch@...radead.org" <hch@...radead.org>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"brauner@...nel.org" <brauner@...nel.org>,
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	"tursulin@...ulin.net" <tursulin@...ulin.net>,
	"airlied@...il.com" <airlied@...il.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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	"frank.li@...o.com" <frank.li@...o.com>
Subject: Re: [PATCH v3 4/4] ext4: support uncached buffered I/O

On Fri, Jun 27, 2025 at 11:03:13AM +0000, 陈涛涛 Taotao Chen wrote:
> +++ b/fs/ext4/inode.c
> @@ -1270,6 +1270,9 @@ static int ext4_write_begin(const struct kiocb *iocb,
>  	if (unlikely(ret))
>  		return ret;
>  
> +	if (iocb->ki_flags & IOCB_DONTCACHE)
> +		fgp |= FGP_DONTCACHE;

I think this needs to be:

	if (iocb && iocb->ki_flags & IOCB_DONTCACHE)

because it's legit to call write_begin with a NULL argument.  The
'file' was always an optional argument, and we should preserve that
optionality with this transformation.

I wonder if it's worth abstracting some of this boilerplate.  Something
like:

struct folio *write_begin_get_folio(iocb, mapping, index, len)
{
	fgf_t fgflags = FGP_WRITEBEGIN;

	if (iocb && iocb->ki_flags & IOCB_DONTCACHE)
		fgflags |= FGP_DONTCACHE;
	fgflags |= fgf_set_order(len);

	return __filemap_get_folio(mapping, index, fgflags,
			mapping_gfp_mask(mapping));
}