Re: [PATCH v2 02/20] dax: Create a range version of dax_layout_busy_page()

[Vivek Goyal]

On Mon, Aug 17, 2020 at 06:53:39PM +0200, Jan Kara wrote:
> On Fri 07-08-20 15:55:08, Vivek Goyal wrote:
> > virtiofs device has a range of memory which is mapped into file inodes
> > using dax. This memory is mapped in qemu on host and maps different
> > sections of real file on host. Size of this memory is limited
> > (determined by administrator) and depending on filesystem size, we will
> > soon reach a situation where all the memory is in use and we need to
> > reclaim some.
> > 
> > As part of reclaim process, we will need to make sure that there are
> > no active references to pages (taken by get_user_pages()) on the memory
> > range we are trying to reclaim. I am planning to use
> > dax_layout_busy_page() for this. But in current form this is per inode
> > and scans through all the pages of the inode.
> > 
> > We want to reclaim only a portion of memory (say 2MB page). So we want
> > to make sure that only that 2MB range of pages do not have any
> > references  (and don't want to unmap all the pages of inode).
> > 
> > Hence, create a range version of this function named
> > dax_layout_busy_page_range() which can be used to pass a range which
> > needs to be unmapped.
> > 
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: linux-nvdimm@xxxxxxxxxxxx
> > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> 
> The API looks OK. Some comments WRT the implementation below.
> 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 11b16729b86f..0d51b0fbb489 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -558,27 +558,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
> >  	return xa_mk_internal(VM_FAULT_FALLBACK);
> >  }
> >  
> > -/**
> > - * 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.
> > +/*
> > + * Partial pages are included. If end is LLONG_MAX, pages in the range from
> > + * start to end of the file are inluded.
> >   */
> 
> I think the big kerneldoc comment should stay with
> dax_layout_busy_page_range() since dax_layout_busy_page() will be just a
> trivial wrapper around it..

Hi Jan,

Thanks for the review.

Will move kerneldoc comment.


> 
> > -struct page *dax_layout_busy_page(struct address_space *mapping)
> > +struct page *dax_layout_busy_page_range(struct address_space *mapping,
> > +					loff_t start, loff_t end)
> >  {
> > -	XA_STATE(xas, &mapping->i_pages, 0);
> >  	void *entry;
> >  	unsigned int scanned = 0;
> >  	struct page *page = NULL;
> > +	pgoff_t start_idx = start >> PAGE_SHIFT;
> > +	pgoff_t end_idx = end >> PAGE_SHIFT;
> > +	XA_STATE(xas, &mapping->i_pages, start_idx);
> > +	loff_t len, lstart = round_down(start, PAGE_SIZE);
> >  
> >  	/*
> >  	 * In the 'limited' case get_user_pages() for dax is disabled.
> > @@ -589,6 +582,22 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> >  	if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> >  		return NULL;
> >  
> > +	/* If end == LLONG_MAX, all pages from start to till end of file */
> > +	if (end == LLONG_MAX) {
> > +		end_idx = ULONG_MAX;
> > +		len = 0;
> > +	} else {
> > +		/* length is being calculated from lstart and not start.
> > +		 * This is due to behavior of unmap_mapping_range(). If
> > +		 * start is say 4094 and end is on 4096 then we want to
> > +		 * unamp two pages, idx 0 and 1. But unmap_mapping_range()
> > +		 * will unmap only page at idx 0. If we calculate len
> > +		 * from the rounded down start, this problem should not
> > +		 * happen.
> > +		 */
> > +		len = end - lstart + 1;
> > +	}
> 
> Maybe it would be more understandable to use
> 	unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1);
> below and avoid all this rounding and special-casing.

Will do.

> 
> > +
> >  	/*
> >  	 * If we race get_user_pages_fast() here either we'll see the
> >  	 * elevated page count in the iteration and wait, or
> > @@ -601,10 +610,10 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> >  	 * guaranteed to either see new references or prevent new
> >  	 * references from being established.
> >  	 */
> > -	unmap_mapping_range(mapping, 0, 0, 0);
> > +	unmap_mapping_range(mapping, start, len, 0);
> >  
> >  	xas_lock_irq(&xas);
> > -	xas_for_each(&xas, entry, ULONG_MAX) {
> > +	xas_for_each(&xas, entry, end_idx) {
> >  		if (WARN_ON_ONCE(!xa_is_value(entry)))
> >  			continue;
> >  		if (unlikely(dax_is_locked(entry)))
> > @@ -625,6 +634,27 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> >  	xas_unlock_irq(&xas);
> >  	return page;
> >  }
> > +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);
> 
> Should the 'end' rather be LLONG_MAX?

My bad. I forgot to change this. Previous version of patches had the
semantic that 'end == 0' signifies till the end of file. Yes, 'end'
should be LLONG_MAX now. Will fix it.

Thanks
Vivek

> 
> Otherwise the patch looks good to me.
> 
> 								Honza
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
>