From: Chengguang Xu <cgxu519@mykernel.net>
To: "Amir Goldstein" <amir73il@gmail.com>
Cc: "Jan Kara" <jack@suse.cz>, "miklos" <miklos@szeredi.hu>,
"linux-unionfs" <linux-unionfs@vger.kernel.org>,
"linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"cgxu519" <cgxu519@mykernel.net>
Subject: Re: [RFC PATCH 1/5] fs: introduce notifier list for vfs inode
Date: Thu, 15 Oct 2020 21:13:48 +0800 [thread overview]
Message-ID: <1752c652963.113ee3fbc44343.6282793280578516240@mykernel.net> (raw)
In-Reply-To: <CAOQ4uxhEA=ggONsJrUzGfHOGHob+81-UHk1Wo9ejj=CziAjtTQ@mail.gmail.com>
---- 在 星期四, 2020-10-15 20:32:24 Amir Goldstein <amir73il@gmail.com> 撰写 ----
> > > Perhaps you can combine that with the shadow overlay sbi approach.
> > > Instead of dirtying overlay inode when underlying is dirtied, you can
> > > "pre-dirty" overlayfs inode in higher level file ops and add them to the
> > > "maybe-dirty" list (e.g. after write).
> > >
> >
> > Main problem is we can't be notified by set_page_dirty from writable mmap.
> > Meanwhile, if we dirty overlay inode then writeback will pick up dirty overlay
> > inode and clear it after syncing, then overlay inode could be release at any time,
> > so in the end, maybe overlay inode is released but upper inode is still dirty and
> > there is no any pointer to find upper dirty inode out.
> >
>
> But we can control whether overlay inode is release with ovl_drop_inode()
> right? Can we prevent dropping overlay inode if upper inode is
> inode_is_open_for_write()?
>
> About re-dirty, see below...
>
> >
> > > ovl_sync_fs() can iterate the maybe-dirty list and re-dirty overlay inodes
> > > if the underlying inode is still dirty on the (!wait) pass.
> > >
> > > As for memory mapped inodes via overlayfs (which can be dirtied without
> > > notifying overlayfs) I am not sure that is a big problem in practice.
> > >
> >
> > Yes, it's key problem here.
> >
> > > When an inode is writably mapped via ovarlayfs, you can flag the
> > > overlay inode with "maybe-writably-mapped" and then remove
> > > it from the maybe dirty list when the underlying inode is not dirty
> > > AND its i_writecount is 0 (checked on write_inode() and release()).
> > >
> > > Actually, there is no reason to treat writably mapped inodes and
> > > other dirty inodes differently - insert to suspect list on open for
> > > write, remove from suspect list on last release() or write_inode()
> > > when inode is no longer dirty and writable.
I have to say inserting to suspect list cannot prevent dropping,
thinking of the problem of previous approach that we write dirty upper
inode with current->flags & PF_MEMALLOC while evicting clean overlay inode.
> > >
> > > Did I miss anything?
> > >
> >
> > If we dirty overlay inode that means we have launched writeback mechanism,
> > so in this case, re-dirty overlay inode in time becomes important.
> >
>
> My idea was to use the first call to ovl_sync_fs() with 'wait' false
> to iterate the
> maybe-dirty list and re-dirty overlay inodes whose upper is dirty.
>
I'm curious how we prevent dropping of clean overlay inode with dirty upper?
Hold another reference during iput_final operation? in the drop_inode() or something
else?
> Then in the second call to __sync_filesystem, sync_inodes_sb() will take
> care of calling ovl_write_inode() for all the re-dirty inodes.
>
> In current code we sync ALL dirty upper fs inodes and we do not overlay
> inodes with no reference in cache.
>
> The best code would sync only upper fs inodes dirtied by this overlay
> and will be able to evict overlay inodes whose upper inodes are clean.
>
> The compromise code would sync only upper fs inodes dirtied by this overlay,
> and would not evict overlay inodes as long as upper inodes are "open for write".
> I think its a fine compromise considering the alternatives.
>
> Is this workable?
>
In your approach, the key point is how to prevent dropping overlay inode that has
dirty upper and no reference but I don't understand well how to achieve it from
your descriptions.
Thanks,
Chengguang
next prev parent reply other threads:[~2020-10-15 13:30 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-10 14:23 [RFC PATCH 0/5] implement containerized syncfs for overlayfs Chengguang Xu
2020-10-10 14:23 ` [RFC PATCH 1/5] fs: introduce notifier list for vfs inode Chengguang Xu
2020-10-14 16:15 ` Jan Kara
2020-10-15 3:03 ` Chengguang Xu
2020-10-15 6:11 ` Amir Goldstein
2020-10-15 11:29 ` Chengguang Xu
2020-10-15 12:32 ` Amir Goldstein
2020-10-15 13:13 ` Chengguang Xu [this message]
2020-10-15 16:02 ` Amir Goldstein
2020-10-15 16:06 ` Amir Goldstein
2020-10-16 1:56 ` Chengguang Xu
2020-10-16 4:39 ` Amir Goldstein
2020-10-16 7:43 ` Chengguang Xu
2020-10-15 3:25 ` Al Viro
2020-10-15 3:42 ` Chengguang Xu
2020-10-15 4:57 ` Al Viro
2020-10-15 10:56 ` Chengguang Xu
2020-10-16 7:09 ` Chengguang Xu
2020-10-16 9:22 ` Jan Kara
2020-10-10 14:23 ` [RFC PATCH 2/5] fs: export symbol of writeback_single_inode() Chengguang Xu
2020-10-10 14:23 ` [RFC PATCH 3/5] ovl: setup overlayfs' private bdi Chengguang Xu
2020-10-10 14:23 ` [RFC PATCH 4/5] ovl: monitor marking dirty activity of underlying upper inode Chengguang Xu
2020-10-10 14:23 ` [RFC PATCH 5/5] ovl: impement containerized syncfs for overlayfs Chengguang Xu
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