Re: [git pull] vfs.git - including i_mutex wrappers

[Al Viro]

On Sun, Jan 24, 2016 at 11:26:58AM +1100, Dave Chinner wrote:

> That's fair enough. However, compare this to how core locking
> changes occur in the mm subsystem - they go through multiple patch
> postings and review so there's no surprise when the pull request
> comes.

... and the thread in question has grown from precisely that (and not the first
iteration, either) for earlier such change (RCU symlinks).  Subsequent one
(follow_link -> get_link, with RCU symlink traversal for non-embedded
symlinks) also went through fsdevel mailbombs (a couple of iterations, IIRC).

Seriously, when it comes to actual fs-visible behaviour changes (rather than
"please, try and use these helpers instead of open-coding ->i_mutex access"
done exactly to avoid the inter-tree dependencies from hell while that work
is being done) fsdevel will be hit by such mailbomb and probably more than
once.

For now it's really just a reduction of trivial conflicts for the next cycle;
eventually it's going to be a weaker VFS exclusion on ->lookup().  Which had
been loudly demanded quite a few times, and I don't recall any filesystem
developers _ever_ objecting to that.

Speaking of the earlier changes - IIRC, there had been plans to start
hashing (at least some of) XFS symlinks.  I think it was from hch, along
the lines of "stash a buffer with symlink body into ->i_link the first time
around, free it from inode eviction".  As long as that freeing is RCU-delayed,
doing so would work just fine and give you symlink traversal without dropping
from RCU mode...  OTOH, if that gets resurrected, it probably ought to go
through XFS tree - all VFS infrastructure is there (since 4.2), so it's
purely XFS business at this point...  One thing to watch out for is that
RCU delay - see shmem.c fix in the same pull request for related example.