Re: [PATCH 17/17] xfs: support for synchronous DAX faults

On Tue, Oct 31, 2017 at 8:19 AM, Jan Kara <jack(a)suse.cz&gt; wrote: > On Fri 27-10-17 12:08:34, Jan Kara wrote: >> On Fri 27-10-17 08:16:11, Dave Chinner wrote: >> > On Thu, Oct 26, 2017 at 05:48:04PM +0200, Jan Kara wrote: >> > > > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c >> > > > > index f179bdf1644d..b43be199fbdf 100644 >> > > > > --- a/fs/xfs/xfs_iomap.c >> > > > > +++ b/fs/xfs/xfs_iomap.c >> > > > > @@ -33,6 +33,7 @@ >> > > > > #include "xfs_error.h" >> > > > > #include "xfs_trans.h" >> > > > > #include "xfs_trans_space.h" >> > > > > +#include "xfs_inode_item.h" >> > > > > #include "xfs_iomap.h" >> > > > > #include "xfs_trace.h" >> > > > > #include "xfs_icache.h" >> > > > > @@ -1086,6 +1087,10 @@ xfs_file_iomap_begin( >> > > > > trace_xfs_iomap_found(ip, offset, length, 0, &imap); >> > > > > } >> > > > > >> > > > > + if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) && >> > > > > + (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP)) >> > > > > + iomap->flags |= IOMAP_F_DIRTY; >> > > > >> > > > This is the very definition of an inode that is "fdatasync dirty". >> > > > >> > > > Hmmmm, shouldn't this also be set for read faults, too? >> > > >> > > No, read faults don't need to set IOMAP_F_DIRTY since user cannot write any >> > > data to the page which he'd then like to be persistent. The only reason why >> > > I thought it could be useful for a while was that it would be nice to make >> > > MAP_SYNC mapping provide the guarantee that data you see now is the data >> > > you'll see after a crash >> > >> > Isn't that the entire point of MAP_SYNC? i.e. That when we return >> > from a page fault, the app knows that the data and it's underlying >> > extent is on persistent storage? >> > >> > > but we cannot provide that guarantee for RO >> > > mapping anyway if someone else has the page mapped as well. So I just >> > > decided not to return IOMAP_F_DIRTY for read faults. >> > >> > If there are multiple MAP_SYNC mappings to the inode, I would have >> > expected that they all sync all of the data/metadata on every page >> > fault, regardless of who dirtied the inode. An RO mapping doesn't >> >> Well, they all do sync regardless of who dirtied the inode on every *write* >> fault. >> >> > mean the data/metadata on the inode can't change, it just means it >> > can't change through that mapping. Running fsync() to guarantee the >> > persistence of that data/metadata doesn't actually changing any >> > data.... >> > >> > IOWs, if read faults don't guarantee the mapped range has stable >> > extents on a MAP_SYNC mapping, then I think MAP_SYNC is broken >> > because it's not giving consistent guarantees to userspace. Yes, it >> > works fine when only one MAP_SYNC mapping is modifying the inode, >> > but the moment we have concurrent operations on the inode that >> > aren't MAP_SYNC or O_SYNC this goes out the window.... >> >> MAP_SYNC as I've implemented it provides guarantees only for data the >> process has actually written. I agree with that and it was a conscious >> decision. In my opinion that covers most usecases, provides reasonably >> simple semantics (i.e., if you write data through MAP_SYNC mapping, you can >> persist it just using CPU instructions), and reasonable performance. >> >> Now you seem to suggest the semantics should be: "Data you have read from or >> written to a MAP_SYNC mapping can be persisted using CPU instructions." And >> from implementation POV we can do that rather easily (just rip out the >> IOMAP_WRITE checks). But I'm unsure whether this additional guarantee would >> be useful enough to justify the slowdown of read faults? I was not able to >> come up with a good usecase and so I've decided for current semantics. What >> do other people think? > > Nobody commented on this for couple of days so how do we proceed? I would > prefer to go just with a guarantee for data written and we can always make > the guarantee stronger (i.e. apply it also for read data) when some user > comes with a good usecase? I think it is easier to strengthen the guarantee than loosen it later especially since it is not yet clear that we have a use case for the stronger semantic. At least the initial motivation for MAP_SYNC was for writers.