Linux-Fsdevel Archive on lore.kernel.org
 help / color / Atom feed
From: Jan Kara <jack@suse.cz>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Chengguang Xu <cgxu519@mykernel.net>,
	miklos <miklos@szeredi.hu>,
	linux-unionfs <linux-unionfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	charliecgxu <charliecgxu@tencent.com>
Subject: Re: [RFC PATCH v2 5/8] ovl: mark overlayfs' inode dirty on shared writable mmap
Date: Thu, 5 Nov 2020 16:54:34 +0100
Message-ID: <20201105155434.GI32718@quack2.suse.cz> (raw)
In-Reply-To: <CAOQ4uxiH+1rV9_hkjed2jt7YF0CMJJVa6Fc+kbzeTuMXYAQ8MQ@mail.gmail.com>

On Thu 05-11-20 16:21:27, Amir Goldstein wrote:
> On Thu, Nov 5, 2020 at 4:03 PM Jan Kara <jack@suse.cz> wrote:
> >
> > On Wed 04-11-20 19:54:03, Chengguang Xu wrote:
> > >  ---- 在 星期二, 2020-11-03 01:30:52 Jan Kara <jack@suse.cz> 撰写 ----
> > >  > On Sun 25-10-20 11:41:14, Chengguang Xu wrote:
> > >  > > Overlayfs cannot be notified when mmapped area gets dirty,
> > >  > > so we need to proactively mark inode dirty in ->mmap operation.
> > >  > >
> > >  > > Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
> > >  > > ---
> > >  > >  fs/overlayfs/file.c | 4 ++++
> > >  > >  1 file changed, 4 insertions(+)
> > >  > >
> > >  > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > >  > > index efccb7c1f9bc..cd6fcdfd81a9 100644
> > >  > > --- a/fs/overlayfs/file.c
> > >  > > +++ b/fs/overlayfs/file.c
> > >  > > @@ -486,6 +486,10 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
> > >  > >          /* Drop reference count from new vm_file value */
> > >  > >          fput(realfile);
> > >  > >      } else {
> > >  > > +        if (vma->vm_flags & (VM_SHARED|VM_MAYSHARE) &&
> > >  > > +            vma->vm_flags & (VM_WRITE|VM_MAYWRITE))
> > >  > > +            ovl_mark_inode_dirty(file_inode(file));
> > >  > > +
> > >  >
> > >  > But does this work reliably? I mean once writeback runs, your inode (as
> > >  > well as upper inode) is cleaned. Then a page fault comes so file has dirty
> > >  > pages again and would need flushing but overlayfs inode stays clean? Am I
> > >  > missing something?
> > >  >
> > >
> > > Yeah, this is key point of this approach, in order to  fix the issue I
> > > explicitly set I_DIRTY_SYNC flag in ovl_mark_inode_dirty(), so what i
> > > mean is during writeback we will call into ->write_inode() by this
> > > flag(I_DIRTY_SYNC) and at that place we get chance to check mapping and
> > > re-dirty overlay's inode. The code logic like below in ovl_write_inode().
> > >
> > >     if (mapping_writably_mapped(upper->i_mapping) ||
> > >          mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK))
> > >                  iflag |= I_DIRTY_PAGES;
> >
> > OK, but suppose the upper mapping is clean at this moment (upper inode has
> > been fully written out for whatever reason, but it is still mapped) so your
> > overlayfs inode becomes clean as well. Then I don't see a mechanism which
> > would make your overlayfs inode dirty again when a write to mmap happens,
> > set_page_dirty() will end up marking upper inode with I_DIRTY_PAGES flag.
> >
> > Note that ovl_mmap() gets called only at mmap(2) syscall time but then
> > pages get faulted in, dirtied, cleaned fully at discretion of the mm
> > / writeback subsystem.
> >
> 
> Perhaps I will add some background.
> 
> What I suggested was to maintain a "suspect list" in addition to
> the dirty ovl inodes.
> 
> ovl inode is added to the suspect list on mmap (writable) and removed
> from the suspect list on release() flush() or on sync_fs() if real inode is no
> longer writably mapped.
> 
> There was another variant where ovl inode is added to suspect list on open
> for write and removed from suspect list on release() flush() or sync_fs()
> if real inode is not inode_is_open_for_write().
> 
> In both cases the list will have inodes whose real is not dirty, but
> in both cases
> the list shouldn't be terribly large to traverse on sync_fs().
> 
> Chengguang tried to implement the idea without an actual list by
> re-dirtying the "suspect" inodes on every write_inode(), but I personally have
> no idea if his idea works.
> 
> I think we can resort to using an actual suspect list if you say that it
> cannot work like this?

Yeah, the suspect list (i.e., additional list of inodes to check on sync)
you describe should work fine. Also the "keep suspect inode dirty" idea
of Chengguang could work fine but we'd have to use something like
inode_is_open_for_write() or inode_is_writeably_mapped() (which would need
to be implemented but it should be easy vma_interval_tree_foreach() walk
checking each found VMA for vma->vm_flags & VM_WRITE) for checking whether
inode should be redirtied or not.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-25  3:41 [RFC PATCH v2 0/8] implement containerized syncfs for overlayfs Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 1/8] ovl: setup overlayfs' private bdi Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 2/8] ovl: implement ->writepages operation Chengguang Xu
2020-11-02 17:17   ` Jan Kara
2020-11-04 12:18     ` Chengguang Xu
2020-11-05 13:55       ` Jan Kara
2020-11-06  5:57         ` Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 3/8] ovl: implement overlayfs' ->evict_inode operation Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 4/8] ovl: mark overlayfs' inode dirty on modification Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 5/8] ovl: mark overlayfs' inode dirty on shared writable mmap Chengguang Xu
2020-11-02 17:30   ` Jan Kara
2020-11-04 11:54     ` Chengguang Xu
2020-11-05 14:03       ` Jan Kara
2020-11-05 14:21         ` Amir Goldstein
2020-11-05 15:54           ` Jan Kara [this message]
2020-11-06  2:41             ` Chengguang Xu
2020-11-06  8:50               ` Jan Kara
2020-11-06  9:47                 ` Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 6/8] ovl: implement overlayfs' ->write_inode operation Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 7/8] ovl: cache dirty overlayfs' inode Chengguang Xu
2020-10-25  3:41 ` [RFC PATCH v2 8/8] ovl: implement containerized syncfs for overlayfs Chengguang Xu
2020-10-30 15:46 ` [RFC PATCH v2 0/8] " Miklos Szeredi
2020-10-31 12:22   ` Chengguang Xu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201105155434.GI32718@quack2.suse.cz \
    --to=jack@suse.cz \
    --cc=amir73il@gmail.com \
    --cc=cgxu519@mykernel.net \
    --cc=charliecgxu@tencent.com \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-unionfs@vger.kernel.org \
    --cc=miklos@szeredi.hu \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fsdevel/0 linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ https://lore.kernel.org/linux-fsdevel \
		linux-fsdevel@vger.kernel.org
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fsdevel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git