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SubjectRe: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Fri 30-03-18 21:03:30, 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: Jan Kara <jack@suse.cz>
> 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>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> drivers/dax/super.c | 2 +
> fs/dax.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/dax.h | 25 ++++++++++++++
> mm/gup.c | 5 +++
> 4 files changed, 123 insertions(+), 1 deletion(-)

...

> +/**
> + * 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. It expects that get_user_pages() pte
> + * walks are performed under rcu_read_lock().
> + */
> +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;
> + /*
> + * Flush dax_layout_lock() sections to ensure all possible page
> + * references have been taken, or otherwise arrange for faults
> + * to block on the filesystem lock that is taken for
> + * establishing new mappings.
> + */
> + unmap_mapping_range(mapping, 0, 0, 1);
> + synchronize_rcu();

So I still don't like the use of RCU for this. It just seems as an abuse to
use RCU like that. Furthermore it has a hefty latency cost for the truncate
path. A trivial test to truncate 100 times the last page of a 16k file that
is mmaped (only the first page):

DAX+your patches 3.899s
non-DAX 0.015s

So you can see synchronize_rcu() increased time to run truncate(2) more
than 200 times (the process is indeed sitting in __wait_rcu_gp all the
time). IMHO that's just too costly.

> + 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;

This would be a bug - so WARN_ON_ONCE() here?

> +
> + spin_lock_irq(&mapping->tree_lock);
> + entry = get_unlocked_mapping_entry(mapping, index, NULL);
> + if (entry)
> + page = dax_busy_page(entry);
> + put_unlocked_mapping_entry(mapping, index, entry);
> + spin_unlock_irq(&mapping->tree_lock);
> + if (page)
> + break;
> + }
> + pagevec_remove_exceptionals(&pvec);
> + pagevec_release(&pvec);
> + index++;
> +
> + if (page)
> + break;
> + }
> + return page;
> +}
> +EXPORT_SYMBOL_GPL(dax_layout_busy_page);
> +
> static int __dax_invalidate_mapping_entry(struct address_space *mapping,
> pgoff_t index, bool trunc)
> {

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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