linux-kernel - Re: ovl_flush() behavior

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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