linux-kernel - Re: ovl_flush() behavior

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Message-ID: <CAOQ4uxg6FATciQhzRifOft4gMZj15G=UA6MUiPX2n9-NR5+1Pg@mail.gmail.com>
Date:   Thu, 2 Dec 2021 01:23:17 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Chengguang Xu <cgxu519@...ernel.net>
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

> >
> > 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?

Thanks,
Amir.