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Date:   Thu, 07 Oct 2021 22:54:29 +0800
From:   Chengguang Xu <cgxu519@mykernel.net>
Reply-To: cgxu519@mykernel.net
To:     "Jan Kara" <jack@suse.cz>
Cc:     "miklos" <miklos@szeredi.hu>, "amir73il" <amir73il@gmail.com>,
        "linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
        "linux-unionfs" <linux-unionfs@vger.kernel.org>,
        "linux-kernel" <linux-kernel@vger.kernel.org>
Message-ID: <17c5b3fa20a.ff02131b26074.9058176981832458952@mykernel.net>
In-Reply-To: <20211007144156.GK12712@quack2.suse.cz>
References: <20210923130814.140814-1-cgxu519@mykernel.net>
 <20210923130814.140814-7-cgxu519@mykernel.net>
 <20211007090157.GB12712@quack2.suse.cz>
 <17c5ab83d6d.10cdb35ab25883.3563739472838823734@mykernel.net> <20211007144156.GK12712@quack2.suse.cz>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
 operation
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 ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E5=9B=9B, 2021-10-07 22:41:56 Jan Kara <=
jack@suse.cz> =E6=92=B0=E5=86=99 ----
 > On Thu 07-10-21 20:26:36, Chengguang Xu wrote:
 > >  ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E5=9B=9B, 2021-10-07 17:01:57 Jan K=
ara <jack@suse.cz> =E6=92=B0=E5=86=99 ----
 > >  >=20
 > >  > > +    if (mapping_writably_mapped(upper->i_mapping) ||
 > >  > > +        mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK=
))
 > >  > > +        iflag |=3D I_DIRTY_PAGES;
 > >  > > +
 > >  > > +    iflag |=3D upper->i_state & I_DIRTY_ALL;
 > >  >=20
 > >  > Also since you call ->write_inode directly upper->i_state won't be =
updated
 > >  > to reflect that inode has been written out (I_DIRTY flags get clear=
ed in
 > >  > __writeback_single_inode()). So it seems to me overlayfs will keep =
writing
 > >  > out upper inode until flush worker on upper filesystem also writes =
the
 > >  > inode and clears the dirty flags? So you rather need to call someth=
ing like
 > >  > write_inode_now() that will handle the flag clearing and do writeba=
ck list
 > >  > handling for you?
 > >  >=20
 > >=20
 > > Calling ->write_inode directly upper->i_state won't be updated, howeve=
r,
 > > I don't think overlayfs will keep writing out upper inode since
 > > ->write_inode will be called when only overlay inode itself marked dir=
ty.
 > > Am I missing something?
 >=20
 > Well, if upper->i_state is not updated, you are more or less guaranteed
 > upper->i_state & I_DIRTY_ALL !=3D 0 and thus even overlay inode stays di=
rty.
 > And thus next time writeback runs you will see dirty overlay inode and
 > writeback the upper inode again although it is not necessary.
 >=20

Hi Jan,

Yes, I get the point now. Thanks for the explanation.


Thanks,
Chengguang