Re: Problems with VM_MIXEDMAP removal from /proc//smaps

Jan Kara
2018-10-18T14:55:55Z

On Thu 18-10-18 11:25:10, Dave Chinner wrote:
> On Wed, Oct 17, 2018 at 04:23:50PM -0400, Jeff Moyer wrote:
> > MAP_SYNC
> > - file system guarantees that metadata required to reach faulted-in file
> > data is consistent on media before a write fault is completed. A
> > side-effect is that the page cache will not be used for
> > writably-mapped pages.
>
> I think you are conflating current implementation with API
> requirements - MAP_SYNC doesn't guarantee anything about page cache
> use. The man page definition simply says "supported only for files
> supporting DAX" and that it provides certain data integrity
> guarantees. It does not define the implementation.
>
> We've /implemented MAP_SYNC/ as O_DSYNC page fault behaviour,
> because that's the only way we can currently provide the required
> behaviour to userspace. However, if a filesystem can use the page
> cache to provide the required functionality, then it's free to do
> so.
>
> i.e. if someone implements a pmem-based page cache, MAP_SYNC data
> integrity could be provided /without DAX/ by any filesystem using
> that persistent page cache. i.e. MAP_SYNC really only requires
> mmap() of CPU addressable persistent memory - it does not require
> DAX. Right now, however, the only way to get this functionality is
> through a DAX capable filesystem on dax capable storage.
>
> And, FWIW, this is pretty much how NOVA maintains DAX w/ COW - it
> COWs new pages in pmem and attaches them a special per-inode cache
> on clean->dirty transition. Then on data sync, background writeback
> or crash recovery, it migrates them from the cache into the file map
> proper via atomic metadata pointer swaps.
>
> IOWs, NOVA provides the correct MAP_SYNC semantics by using a
> separate persistent per-inode write cache to provide the correct
> crash recovery semantics for MAP_SYNC.

Corect. NOVA would be able to provide MAP_SYNC semantics without DAX. But
effectively it will be also able to provide MAP_DIRECT semantics, right?
Because there won't be DRAM between app and persistent storage and I don't
think COW tricks or other data integrity methods are that interesting for
the application. Most users of O_DIRECT are concerned about getting close
to media speed performance and low DRAM usage...

> > and what I think Dan had proposed:
> >
> > mmap flag, MAP_DIRECT
> > - file system guarantees that page cache will not be used to front storage.
> > storage MUST be directly addressable. This *almost* implies MAP_SYNC.
> > The subtle difference is that a write fault /may/ not result in metadata
> > being written back to media.
>
> SIimilar to O_DIRECT, these semantics do not allow userspace apps to
> replace msync/fsync with CPU cache flush operations. So any
> application that uses this mode still needs to use either MAP_SYNC
> or issue msync/fsync for data integrity.
>
> If the app is using MAP_DIRECT, the what do we do if the filesystem
> can't provide the required semantics for that specific operation? In
> the case of O_DIRECT, we fall back to buffered IO because it has the
> same data integrity semantics as O_DIRECT and will always work. It's
> just slower and consumes more memory, but the app continues to work
> just fine.
>
> Sending SIGBUS to apps when we can't perform MAP_DIRECT operations
> without using the pagecache seems extremely problematic to me. e.g.
> an app already has an open MAP_DIRECT file, and a third party
> reflinks it or dedupes it and the fs has to fall back to buffered IO
> to do COW operations. This isn't the app's fault - the kernel should
> just fall back transparently to using the page cache for the
> MAP_DIRECT app and just keep working, just like it would if it was
> using O_DIRECT read/write.

There's another option of failing reflink / dedupe with EBUSY if the file
is mapped with MAP_DIRECT and the filesystem cannot support relink &
MAP_DIRECT together. But there are downsides to that as well.

> The point I'm trying to make here is that O_DIRECT is a /hint/, not
> a guarantee, and it's done that way to prevent applications from
> being presented with transient, potentially fatal error cases
> because a filesystem implementation can't do a specific operation
> through the direct IO path.
>
> IMO, MAP_DIRECT needs to be a hint like O_DIRECT and not a
> guarantee. Over time we'll end up with filesystems that can
> guarantee that MAP_DIRECT is always going to use DAX, in the same
> way we have filesystems that guarantee O_DIRECT will always be
> O_DIRECT (e.g. XFS). But if we decide that MAP_DIRECT must guarantee
> no page cache will ever be used, then we are basically saying
> "filesystems won't provide MAP_DIRECT even in common, useful cases
> because they can't provide MAP_DIRECT in all cases." And that
> doesn't seem like a very good solution to me.

These are good points. I'm just somewhat wary of the situation where users
will map files with MAP_DIRECT and then the machine starts thrashing
because the file got reflinked and thus pagecache gets used suddently.
With O_DIRECT the fallback to buffered IO is quite rare (at least for major
filesystems) so usually people just won't notice. If fallback for
MAP_DIRECT will be easy to hit, I'm not sure it would be very useful.

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