linux-kernel - Re: [PATCH 20/20] fuse,virtiofs: Add logic to free up a memory range

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Date:   Sat, 28 Mar 2020 06:06:06 +0800
From:   Liu Bo <bo.liu@...ux.alibaba.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, virtio-fs@...hat.com, miklos@...redi.hu,
        stefanha@...hat.com, dgilbert@...hat.com, mst@...hat.com
Subject: Re: [PATCH 20/20] fuse,virtiofs: Add logic to free up a memory range

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?

thanks,
liubo