linux-kernel - Re: [PATCH 02/20] dax: Create a range version of dax_layout_busy

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Date:   Tue, 10 Mar 2020 16:29:46 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Ira Weiny <ira.weiny@...el.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 02/20] dax: Create a range version of
 dax_layout_busy_page()

On Tue, Mar 10, 2020 at 08:19:07AM -0700, Ira Weiny wrote:
> On Wed, Mar 04, 2020 at 11:58:27AM -0500, Vivek Goyal wrote:
> >  
> > +	/* If end == 0, all pages from start to till end of file */
> > +	if (!end) {
> > +		end_idx = ULONG_MAX;
> > +		len = 0;
> 
> I find this a bit odd to specify end == 0 for ULONG_MAX...
> 
> >  }
> > +EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);
> > +
> > +/**
> > + * dax_layout_busy_page - find first pinned page in @mapping
> > + * @mapping: address space to scan for a page with ref count > 1
> > + *
> > + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> > + * 'onlined' to the page allocator so they are considered idle when
> > + * page->count == 1. A filesystem uses this interface to determine if
> > + * any page in the mapping is busy, i.e. for DMA, or other
> > + * get_user_pages() usages.
> > + *
> > + * It is expected that the filesystem is holding locks to block the
> > + * establishment of new mappings in this address_space. I.e. it expects
> > + * to be able to run unmap_mapping_range() and subsequently not race
> > + * mapping_mapped() becoming true.
> > + */
> > +struct page *dax_layout_busy_page(struct address_space *mapping)
> > +{
> > +	return dax_layout_busy_page_range(mapping, 0, 0);
> 
> ... other functions I have seen specify ULONG_MAX here.  Which IMO makes this
> call site more clear.

I think I looked at unmap_mapping_range() where holelen=0 implies till the
end of file and followed same pattern.

But I agree that LLONG_MAX (end is of type loff_t) is probably more
intuitive. I will change it.

Vivek