Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock

[Dave Chinner <david@fromorbit.com>]

On Thu, May 13, 2021 at 11:52:52AM -0700, Darrick J. Wong wrote:
> On Thu, May 13, 2021 at 07:44:59PM +0200, Jan Kara wrote:
> > On Wed 12-05-21 08:23:45, Darrick J. Wong wrote:
> > > On Wed, May 12, 2021 at 03:46:11PM +0200, Jan Kara wrote:
> > > > +->fallocate implementation must be really careful to maintain page cache
> > > > +consistency when punching holes or performing other operations that invalidate
> > > > +page cache contents. Usually the filesystem needs to call
> > > > +truncate_inode_pages_range() to invalidate relevant range of the page cache.
> > > > +However the filesystem usually also needs to update its internal (and on disk)
> > > > +view of file offset -> disk block mapping. Until this update is finished, the
> > > > +filesystem needs to block page faults and reads from reloading now-stale page
> > > > +cache contents from the disk. VFS provides mapping->invalidate_lock for this
> > > > +and acquires it in shared mode in paths loading pages from disk
> > > > +(filemap_fault(), filemap_read(), readahead paths). The filesystem is
> > > > +responsible for taking this lock in its fallocate implementation and generally
> > > > +whenever the page cache contents needs to be invalidated because a block is
> > > > +moving from under a page.
> > > > +
> > > > +->copy_file_range and ->remap_file_range implementations need to serialize
> > > > +against modifications of file data while the operation is running. For blocking
> > > > +changes through write(2) and similar operations inode->i_rwsem can be used. For
> > > > +blocking changes through memory mapping, the filesystem can use
> > > > +mapping->invalidate_lock provided it also acquires it in its ->page_mkwrite
> > > > +implementation.
> > > 
> > > Question: What is the locking order when acquiring the invalidate_lock
> > > of two different files?  Is it the same as i_rwsem (increasing order of
> > > the struct inode pointer) or is it the same as the XFS MMAPLOCK that is
> > > being hoisted here (increasing order of i_ino)?
> > > 
> > > The reason I ask is that remap_file_range has to do that, but I don't
> > > see any conversions for the xfs_lock_two_inodes(..., MMAPLOCK_EXCL)
> > > calls in xfs_ilock2_io_mmap in this series.
> > 
> > Good question. Technically, I don't think there's real need to establish a
> > single ordering because locks among different filesystems are never going
> > to be acquired together (effectively each lock type is local per sb and we
> > are free to define an ordering for each lock type differently). But to
> > maintain some sanity I guess having the same locking order for doublelock
> > of i_rwsem and invalidate_lock makes sense. Is there a reason why XFS uses
> > by-ino ordering? So that we don't have to consider two different orders in
> > xfs_lock_two_inodes()...
> 
> I imagine Dave will chime in on this, but I suspect the reason is
> hysterical raisins^Wreasons.

It's the locking rules that XFS has used pretty much forever.
Locking by inode number always guarantees the same locking order of
two inodes in the same filesystem, regardless of the specific
in-memory instances of the two inodes.

e.g. if we lock based on the inode structure address, in one
instancex, we could get A -> B, then B gets recycled and
reallocated, then we get B -> A as the locking order for the same
two inodes.

That, IMNSHO, is utterly crazy because with non-deterministic inode
lock ordered like this you can't make consistent locking rules for
locking the physical inode cluster buffers underlying the inodes in
the situation where they also need to be locked.

We've been down this path before more than a decade ago when the
powers that be decreed that inode locking order is to be "by
structure address" rather than inode number, because "inode number
is not unique across multiple superblocks".

I'm not sure that there is anywhere that locks multiple inodes
across different superblocks, but here we are again....

> It might simply be time to convert all
> three XFS inode locks to use the same ordering rules.

Careful, there lie dragons along that path because of things like
how the inode cluster buffer operations work - they all assume
ascending inode number traversal within and across inode cluster
buffers and hence we do have locking order constraints based on
inode number...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com