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Message-ID: <17d78e95c35.ceeffaaf22655.2727336036618811041@mykernel.net>
Date: Thu, 02 Dec 2021 10:11:39 +0800
From: Chengguang Xu <cgxu519@...ernel.net>
To: "Amir Goldstein" <amir73il@...il.com>
Cc: "Jan Kara" <jack@...e.cz>, "Miklos Szeredi" <miklos@...redi.hu>,
"linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
"overlayfs" <linux-unionfs@...r.kernel.org>,
"linux-kernel" <linux-kernel@...r.kernel.org>,
"ronyjin" <ronyjin@...cent.com>,
"charliecgxu" <charliecgxu@...cent.com>,
"Vivek Goyal" <vgoyal@...hat.com>
Subject: Re: ovl_flush() behavior
---- 在 星期四, 2021-12-02 07:23:17 Amir Goldstein <amir73il@...il.com> 撰写 ----
> > >
> > > To be honest I even don't fully understand what's the ->flush() logic in overlayfs.
> > > Why should we open new underlying file when calling ->flush()?
> > > Is it still correct in the case of opening lower layer first then copy-uped case?
> > >
> >
> > The semantics of flush() are far from being uniform across filesystems.
> > most local filesystems do nothing on close.
> > most network fs only flush dirty data when a writer closes a file
> > but not when a reader closes a file.
> > It is hard to imagine that applications rely on flush-on-close of
> > rdonly fd behavior and I agree that flushing only if original fd was upper
> > makes more sense, so I am not sure if it is really essential for
> > overlayfs to open an upper rdonly fd just to do whatever the upper fs
> > would have done on close of rdonly fd, but maybe there is no good
> > reason to change this behavior either.
> >
>
> On second thought, I think there may be a good reason to change
> ovl_flush() otherwise I wouldn't have submitted commit
> a390ccb316be ("fuse: add FOPEN_NOFLUSH") - I did observe
> applications that frequently open short lived rdonly fds and suffered
> undesired latencies on close().
>
> As for "changing existing behavior", I think that most fs used as
> upper do not implement flush at all.
> Using fuse/virtiofs as overlayfs upper is quite new, so maybe that
> is not a problem and maybe the new behavior would be preferred
> for those users?
>
So is that mean simply redirect the ->flush request to original underlying realfile?
Thanks,
Chengguang
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