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Date:   Thu, 16 Apr 2020 15:05:07 -0400
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Liu Bo <bo.liu@linux.alibaba.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-nvdimm@lists.01.org, virtio-fs@redhat.com, miklos@szeredi.hu,
        stefanha@redhat.com, dgilbert@redhat.com, mst@redhat.com
Subject: Re: [PATCH 20/20] fuse,virtiofs: Add logic to free up a memory range
Message-ID: <20200416190507.GC276932@redhat.com>
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On Thu, Apr 16, 2020 at 01:22:29AM +0800, Liu Bo wrote:
> On Tue, Apr 14, 2020 at 03:30:45PM -0400, Vivek Goyal wrote:
> > On Sat, Mar 28, 2020 at 06:06:06AM +0800, Liu Bo wrote:
> > > On Fri, Mar 27, 2020 at 10:01:14AM -0400, Vivek Goyal wrote:
> > > > On Thu, Mar 26, 2020 at 08:09:05AM +0800, Liu Bo wrote:
> > > > 
> > > > [..]
> > > > > > +/*
> > > > > > + * Find first mapping in the tree and free it and return it. Do not add
> > > > > > + * it back to free pool. If fault == true, this function should be called
> > > > > > + * with fi->i_mmap_sem held.
> > > > > > + */
> > > > > > +static struct fuse_dax_mapping *inode_reclaim_one_dmap(struct fuse_conn *fc,
> > > > > > +							 struct inode *inode,
> > > > > > +							 bool fault)
> > > > > > +{
> > > > > > +	struct fuse_inode *fi = get_fuse_inode(inode);
> > > > > > +	struct fuse_dax_mapping *dmap;
> > > > > > +	int ret;
> > > > > > +
> > > > > > +	if (!fault)
> > > > > > +		down_write(&fi->i_mmap_sem);
> > > > > > +
> > > > > > +	/*
> > > > > > +	 * Make sure there are no references to inode pages using
> > > > > > +	 * get_user_pages()
> > > > > > +	 */
> > > > > > +	ret = fuse_break_dax_layouts(inode, 0, 0);
> > > > > 
> > > > > Hi Vivek,
> > > > > 
> > > > > This patch is enabling inline reclaim for fault path, but fault path
> > > > > has already holds a locked exceptional entry which I believe the above
> > > > > fuse_break_dax_layouts() needs to wait for, can you please elaborate
> > > > > on how this can be avoided?
> > > > > 
> > > > 
> > > > Hi Liubo,
> > > > 
> > > > Can you please point to the exact lock you are referring to. I will
> > > > check it out. Once we got rid of needing to take inode lock in
> > > > reclaim path, that opended the door to do inline reclaim in fault
> > > > path as well. But I was not aware of this exceptional entry lock.
> > > 
> > > Hi Vivek,
> > > 
> > > dax_iomap_{pte,pmd}_fault has called grab_mapping_entry to get a
> > > locked entry, when this fault gets into inline reclaim, would
> > > fuse_break_dax_layouts wait for the locked exceptional entry which is
> > > locked in dax_iomap_{pte,pmd}_fault?
> > 
> > Hi Liu Bo,
> > 
> > This is a good point. Indeed it can deadlock the way code is written
> > currently.
> >
> 
> It's 100% reproducible on 4.19, but not on 5.x which has xarray for
> dax_layout_busy_page.
> 
> It was weird that on 5.x kernel the deadlock is gone, it turned out
> that xarray search in dax_layout_busy_page simply skips the empty
> locked exceptional entry, I didn't get deeper to find out whether it's
> reasonable, but with that 5.x doesn't run to deadlock.

I found more problems with enabling inline reclaim in fault path. I
am holding fi->i_mmap_sem, shared and fuse_break_dax_layouts() can
drop fi->i_mmap_sem if page is busy. I don't think we can drop and
reacquire fi->i_mmap_sem while in fault path.

Also fuse_break_dax_layouts() does not know if we are holding it
shared or exclusive.

So I will probably have to go back to disable inline reclaim in
fault path. If memory range is not available go back up in
fuse_dax_fault(), drop fi->i_mmap_sem lock and wait on wait queue for
a range to become free and retry.

I can retain the changes I did to break layout for a 2MB range only
and not the whole file. I think that's a good optimization to retain
anyway.

Vivek