Re: [PATCH v11 4/7] mm, fs, dax: handle layout changes to pinned dax mappings

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Fri, May 18, 2018 at 06:35:13PM -0700, Dan Williams wrote:
> Background:
> 
> get_user_pages() in the filesystem pins file backed memory pages for
> access by devices performing dma. However, it only pins the memory pages
> not the page-to-file offset association. If a file is truncated the
> pages are mapped out of the file and dma may continue indefinitely into
> a page that is owned by a device driver. This breaks coherency of the
> file vs dma, but the assumption is that if userspace wants the
> file-space truncated it does not matter what data is inbound from the
> device, it is not relevant anymore. The only expectation is that dma can
> safely continue while the filesystem reallocates the block(s).
> 
> Problem:
> 
> This expectation that dma can safely continue while the filesystem
> changes the block map is broken by dax. With dax the target dma page
> *is* the filesystem block. The model of leaving the page pinned for dma,
> but truncating the file block out of the file, means that the filesytem
> is free to reallocate a block under active dma to another file and now
> the expected data-incoherency situation has turned into active
> data-corruption.
> 
> Solution:
> 
> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> file while any page/block in the file is under active dma. This solution
> assumes that dma is transient. Cases where dma operations are known to
> not be transient, like RDMA, have been explicitly disabled via
> commits like 5f1d43de5416 "IB/core: disable memory registration of
> filesystem-dax vmas".
> 
> The dax_layout_busy_page() routine is called by filesystems with a lock
> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> The process of looking up a busy page invalidates all mappings
> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> The filesystem continues to call dax_layout_busy_page() until it finally
> returns no more active pages. This approach assumes that the page
> pinning is transient, if that assumption is violated the system would
> have likely hung from the uncompleted I/O.
> 
> Cc: Jeff Moyer <jmoyer@redhat.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Matthew Wilcox <mawilcox@microsoft.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Reported-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
<>
> @@ -492,6 +505,90 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
>  	return entry;
>  }
>  
> +/**
> + * 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)
> +{
> +	pgoff_t	indices[PAGEVEC_SIZE];
> +	struct page *page = NULL;
> +	struct pagevec pvec;
> +	pgoff_t	index, end;
> +	unsigned i;
> +
> +	/*
> +	 * In the 'limited' case get_user_pages() for dax is disabled.
> +	 */
> +	if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> +		return NULL;
> +
> +	if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> +		return NULL;
> +
> +	pagevec_init(&pvec);
> +	index = 0;
> +	end = -1;
> +
> +	/*
> +	 * If we race get_user_pages_fast() here either we'll see the
> +	 * elevated page count in the pagevec_lookup and wait, or
> +	 * get_user_pages_fast() will see that the page it took a reference
> +	 * against is no longer mapped in the page tables and bail to the
> +	 * get_user_pages() slow path.  The slow path is protected by
> +	 * pte_lock() and pmd_lock(). New references are not taken without
> +	 * holding those locks, and unmap_mapping_range() will not zero the
> +	 * pte or pmd without holding the respective lock, so we are
> +	 * guaranteed to either see new references or prevent new
> +	 * references from being established.
> +	 */
> +	unmap_mapping_range(mapping, 0, 0, 1);
> +
> +	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
> +				min(end - index, (pgoff_t)PAGEVEC_SIZE),
> +				indices)) {
> +		for (i = 0; i < pagevec_count(&pvec); i++) {
> +			struct page *pvec_ent = pvec.pages[i];
> +			void *entry;
> +
> +			index = indices[i];
> +			if (index >= end)
> +				break;
> +
> +			if (!radix_tree_exceptional_entry(pvec_ent))
> +				continue;
> +
> +			xa_lock_irq(&mapping->i_pages);
> +			entry = get_unlocked_mapping_entry(mapping, index, NULL);
> +			if (entry)
> +				page = dax_busy_page(entry);
> +			put_unlocked_mapping_entry(mapping, index, entry);
> +			xa_unlock_irq(&mapping->i_pages);
> +			if (page)
> +				break;
> +		}
> +		pagevec_remove_exceptionals(&pvec);
> +		pagevec_release(&pvec);

I must be missing something - now that we're using the common 4k zero page, we
should only ever have exceptional entries in the DAX radix tree, right?

If so, it seems like these two pagevec_* calls could/should go away, and the
!radix_tree_exceptional_entry() check in the for loop above should be
surrounded by a WARN_ON_ONCE()?

Or has something changed that I'm overlooking?