Re: [f2fs-dev] [RFC] f2fs: fix a race condition between evict & gc

[Hou Pengyang]

On 2016/5/18 1:23, Jaegeuk Kim wrote:
> On Tue, May 17, 2016 at 11:00:53AM +0800, Hou Pengyang wrote:
> > On 2016/5/16 23:10, Chao Yu wrote:
> > Hi chao,
> > > Hi Pengyang,
> > >
> > > On 2016/5/16 18:40, Hou Pengyang wrote:
> > > > When collecting data segment(gc_data_segment), there is a race condition
> > > > between evict and phases of gc:
> > > > 0) ra_node_page(dnode)
> > > > 1) ra_node_page(inode)
> > > > 		<--- evict the inode
> > > > 2) f2fs_iget get the inode and add it to gc_list
> > > > 3) move_data_page
> > > >
> > > > In step 2), f2fs_iget does NOT find the inode and allocs a new inode as result,
> > >
> > > If inode was unlinked and then be evicted, f2fs_iget should fail when reading
> > > inode's page as blkaddr of this node is null.
> > agree, after do_read_inode fail, the newly created inode would be
> > freed as a bad inode and f2fs_iget fails. But this may lead to create
> > file fail:
> > gc:iget_locked
> >          <---- touch/mkdir(reuse the evicted ino)
> > gc:free the bad inode
>
> Seems there is no problem.
>
> After f2fs_evict_inode(ino),
>
> f2fs_iget(ino)
>   - iget_failed()                          f2fs_create()
>                                             - f2fs_new_inode()
>                                             - ino = alloc_nid()
>                                             - insert_inode_locked()
>                                              *** spin_lock(&inode_hash_lock)
>                                              - spin_lock(&old->i_lock)
>                                              - __iget(old)
>    - make_bad_inode()                        - spin_unlock(&old->i_lock)
>     - remove_inode_hash()                    - spin_unlock(&inode_hash_lock)
>      - spin_lock(&inode_hash_lock)           - wait_on_inode(old)

oh.. wait_on_inode. OK, Kim, get it, thanks for your answer.

>      - spin_lock(&inode->i_lock)
>      - list_del
>      - spin_unlock(&inode->i_lock)
>      - spin_unlock(&inode_hash_lock)
>     - unlock_new_inode()
>      - wake_up_bit(&inode->i_state, __I_NEW) --> wake up!
>                                              - iput(old) whish was unhashed.
>                                              - goto to ***
>     - iput()
>
> > during the bad inode allocated and freed in gc, the inode is reserved
> > in the global inode_hash, while the ino is a free nid in free_nid tree.
> >
> > touch/mkdir may reuse the ino, during the touch/mkdir path, the global
> > inode_hash would be checked if the ino file exists. Under this
> > scenario, because of the gc bad inode in inode_hash, touch/mkdir would
> > fail.
> >
> > ilookup seems better, as no need to alloc and free a bad inode.
> >
> > if ilookup fails, that exactly means inode has been evicted and no need
> > to gc;
>
> No, we should do gc for data blocks owned by *evicted* inodes as well.
>
> Thanks,
>
> > if ilookup success, before phase 3, is_alive to deal with the ino reuse
> > scenario;
> >
> > Do I miss anything else?
> > thanks
> > > If inode still have non-zero nlink value and then be evicted, we should allow gc
> > > thread to reference this inode for moving its data pages.
> > >
> > > Thanks,
> > >
> > > > which is not resonable.
> > > >
> > > > This patch changes f2fs_iget to ilookup. when no inode is found, no new inode is
> > > > created.
> > > >
> > > > Signed-off-by: Hou Pengyang <houpengyang@xxxxxxxxxx>
> > > > ---
> > > >    fs/f2fs/gc.c | 4 ++--
> > > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > > > index 38d56f6..6e73193 100644
> > > > --- a/fs/f2fs/gc.c
> > > > +++ b/fs/f2fs/gc.c
> > > > @@ -717,8 +717,8 @@ next_step:
> > > >    		ofs_in_node = le16_to_cpu(entry->ofs_in_node);
> > > >
> > > >    		if (phase == 2) {
> > > > -			inode = f2fs_iget(sb, dni.ino);
> > > > -			if (IS_ERR(inode) || is_bad_inode(inode))
> > > > +			inode = ilookup(sb, dni.ino);
> > > > +			if (!inode || IS_ERR(inode) || is_bad_inode(inode))
> > > >    				continue;
> > > >
> > > >    			/* if encrypted inode, let's go phase 3 */
> > >
> > > .
> >
> >
> >
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