Re: [PATCH v3 2/6] fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP

[Dave Chinner]

On Fri, Aug 11, 2017 at 04:42:18PM -0700, Dan Williams wrote:
> On Fri, Aug 11, 2017 at 4:27 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > On Thu, Aug 10, 2017 at 11:39:28PM -0700, Dan Williams wrote:
> >> >From falloc.h:
> >>
> >>     FALLOC_FL_SEAL_BLOCK_MAP is used to seal (make immutable) all of the
> >>     file logical-to-physical extent offset mappings in the file. The
> >>     purpose is to allow an application to assume that there are no holes
> >>     or shared extents in the file and that the metadata needed to find
> >>     all the physical extents of the file is stable and can never be
> >>     dirtied.
> >>
> >> For now this patch only permits setting the in-memory state of
> >> S_IOMAP_IMMMUTABLE. Support for clearing and persisting the state is
> >> saved for later patches.
> >>
> >> The implementation is careful to not allow the immutable state to change
> >> while any process might have any established mappings. It reuses the
> >> existing xfs_reflink_unshare() and xfs_alloc_file_space() to unshare
> >> extents and fill all holes in the file. It then holds XFS_ILOCK_EXCL
> >> while it validates the file is in the proper state and sets
> >> S_IOMAP_IMMUTABLE.
> >
> > SO I've been thinking about this - I'm thinking that we need to
> > separate the changes to the extent map from the action of sealing
> > the extent map.
> >
> > That is, I have a need to freeze an extent map without any
> > modification to it at all and breaking all the sharing and filling
> > all the holes completely screws up the file layout I need to
> > preserve. i.e. I want to be able to freeze the maps of a pair of
> > reflinked files so I can use FIEMAP to query only the changed blocks
> > and then read out the data in the private/changed blocks in the
> > reflinked file. I only need a temporary seal (if we crash it goes
> > away), so maybe there's another command modifier flag needed here,
> > too.
> >
> > The DAX allocation requirements can be handled by a preallocation
> > call to filll holes with zeros, followed by an unshare call to break
> > all the COW mappings, and then the extent map can be sealed. If you
> > need to check for holes after sealing, SEEK_HOLE will tell you what
> > you need to know...
> >
> > My preference really is for each fallocate command to do just one
> > thing - having the seal operation also modify the extent map
> > means it's not useful for the use cases where we need the extent map
> > to remain unmodified....
> >
> > Thoughts?
> 
> That does seem to better follow the principle of least surprise and
> make the interface more composable. However, for the DAX case do we
> now end up needing a SEEK_SHARED, or something similar to check that
> we sealed the file without shared extents?

Don't we have an inode attribute flag for that? There's definitely a
flag in the on disk XFS inode to indicate that there are shared
extents on the file.

Hmmmm - for some reason it's not exposed in FS_IOC_FSGETXATTR.
Darrick? Any reason we don't expose the "this file has shared
extents" flag to userspace? How are we checking that on-disk state
in xfstests?

As it is, if we're adding an immutable extent flag, we've got to be
able to query the immutable extent state of a file from userspace so
I'm thinking that we'd need to expose it via the same interface that
exposes the immutable flag. i.e. we could add the "shared extents
present" flag to FS_IOC_FSGETXATTR at the same time...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx