Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings

[Dan Williams]

On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara <jack@xxxxxxx> wrote:
> 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@xxxxxxx>
>> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
>> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
>> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
>> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Reported-by: Christoph Hellwig <hch@xxxxxx>
>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  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.

Agree. I was quietly hoping that it wouldn't be that bad, but numbers
are numbers.

At this point I think we should just go with the
address_space_operations conversions and the sector-to-pfn conversion
for what's stored in the dax radix for 4.17-rc1, and circle back on a
better way to do this synchronization for 4.18.