Date:   Thu, 02 Dec 2021 10:11:39 +0800
From:   Chengguang Xu <cgxu519@mykernel.net>
Reply-To: cgxu519@mykernel.net
To:     "Amir Goldstein" <amir73il@gmail.com>
Cc:     "Jan Kara" <jack@suse.cz>, "Miklos Szeredi" <miklos@szeredi.hu>,
        "linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
        "overlayfs" <linux-unionfs@vger.kernel.org>,
        "linux-kernel" <linux-kernel@vger.kernel.org>,
        "ronyjin" <ronyjin@tencent.com>,
        "charliecgxu" <charliecgxu@tencent.com>,
        "Vivek Goyal" <vgoyal@redhat.com>
Message-ID: <17d78e95c35.ceeffaaf22655.2727336036618811041@mykernel.net>
In-Reply-To: <CAOQ4uxg6FATciQhzRifOft4gMZj15G=UA6MUiPX2n9-NR5+1Pg@mail.gmail.com>
References: <20211118112315.GD13047@quack2.suse.cz> <17d32ecf46e.124314f8f672.8832559275193368959@mykernel.net>
 <20211118164349.GB8267@quack2.suse.cz> <17d36d37022.1227b6f102736.1047689367927335302@mykernel.net>
 <20211130112206.GE7174@quack2.suse.cz> <17d719b79f9.d89bf95117881.5882353172682156775@mykernel.net>
 <CAOQ4uxidK-yDMZoZtoRwTZLgSTr1o2Mu2L55vJRNJDLV0-Sb1w@mail.gmail.com>
 <17d73da701b.e571c37220081.6904057835107693340@mykernel.net>
 <17d74b08dcd.c0e94e6320632.9167792887632811518@mykernel.net>
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 <20211201134610.GA1815@quack2.suse.cz> <17d76cf59ee.12f4517f122167.2687299278423224602@mykernel.net>
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Subject: Re: ovl_flush() behavior
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 ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E5=9B=9B, 2021-12-02 07:23:17 Amir Golds=
tein <amir73il@gmail.com> =E6=92=B0=E5=86=99 ----
 > > >
 > > > To be honest I even don't fully understand what's the ->flush() logi=
c 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 co=
py-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 u=
pper
 > > 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.
 > >
 >=20
 > 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().
 >=20
 > 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?
 >=20

So is that mean simply redirect the ->flush request to original underlying =
realfile?


Thanks,
Chengguang