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Message-ID: <20250514115211.GC9943@mit.edu> Date: Wed, 14 May 2025 07:52:11 -0400 From: "Theodore Ts'o" <tytso@....edu> To: 陈涛涛 Taotao Chen <chentaotao@...iglobal.com> Cc: "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "willy@...radead.org" <willy@...radead.org>, "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org> Subject: Re: [PATCH 2/3] ext4: implement IOCB_DONTCACHE handling in write operations On Mon, Apr 21, 2025 at 10:50:30AM +0000, 陈涛涛 Taotao Chen wrote: > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 94c7d2d828a6..787dd152a47e 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -1147,16 +1147,22 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping, > { > struct inode *inode = mapping->host; > int ret, needed_blocks; > + int iocb_flag; > handle_t *handle; > int retries = 0; > struct folio *folio; > pgoff_t index; > + fgf_t fgp = FGP_WRITEBEGIN; > unsigned from, to; > > ret = ext4_emergency_state(inode->i_sb); > if (unlikely(ret)) > return ret; > > + iocb_flag = (int)(uintptr_t)(*fsdata); > + if (iocb_flag & IOCB_DONTCACHE) > + fgp |= FGP_DONTCACHE; > + See my comment against the first patch in this series. It *should* be possible to solve the problem just for ext4 by adding this line here: *fsdata = (void *)0; The problem is that it's super-fragile, since how *fsdata gets used changes at different points in time, so it makes code review and maintenance more difficult. (As evidenced by the fact that you missed this; this is not a criticism on your programming ability, but rather for the design choise of overloading the use of *fsdata. This is a trap that someone else might fall into when doing future code changes.) And of course, the question is whether PATCH 1/3 could potentially break other file systems. We would need audit all of the other *_write_begin() functions, and then document this for the sake of future file system developers that might want to change their write_begin() function. This is why my preference would be to add an extra flags paramter to write_begin(), but that is going to be a lot more work. Cheers, - Ted
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