Re: [PATCH RFC] xfs: add FALLOC_FL_WRITE_ZEROES to XFS code base

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Oct 14, 2025 at 10:10:51AM +0200, lukas@herbolt.com wrote:
> On 2025-10-04 06:16, Christoph Hellwig wrote:
> > On Fri, Oct 03, 2025 at 09:00:20PM -0700, Darrick J. Wong wrote:
> > > > > -	error = xfs_alloc_file_space(XFS_I(inode), offset, len);
> > > > > +	if (mode & FALLOC_FL_WRITE_ZEROES) {
> > > > > +		if (!bdev_write_zeroes_unmap_sectors(inode->i_sb->s_bdev))
> > > 
> > >      		                         not correct ^^^^^^^^^^^^^^^^^^^
> > > 
> > > You need to find the block device associated with the file because XFS
> > > is a multi-device filesystem.
> > 
> > Indeed. xfs_inode_buftarg will do the work, but we'll need to ensure
> 
> Thanks for xfs_inode_buftarg pointer.
> 
> > the RT bit doesn't get flipped, i.e. it needs to hold a lock between
> > that check and allocation the blocks if there were none yet.
> 
> I am having bit of trouble with that. If I get it right we should hold
> the XFS_ILOCK_EXCL but this lock is then grabbed in the
> xfs_trans_alloc_inode.
> 
> So I would need to release before and there would be again a small window
> where the RT flag can be flipped.
> 
> Looking at the xfs_alloc_file_space, there is also check for the RT bit
> without
> lock, so this also need an attention.
>     rt = XFS_IS_REALTIME_INODE(ip);
>     extsz = xfs_get_extsz_hint(ip);
> 
> Or the xfs_trans_alloc_inode would need to check if we are a;ready holding
> the
> lock Is there a way how to check the current thread is the owner of the
> xfs_ilock rw_sem?

Hm?  The VFS takes inode_lock in vfs_fileattr_set prior to changing the
rt status bit, and and __xfs_file_fallocate takes the same lock (aka the
IOLOCK) before calling xfs_falloc_*.

IOWs, a call to xfs_inode_buftarg would see a stable realtime flag and
return the correct results.

--D

> 
> Something like?
> ---
> static inline bool is_current_writer(struct rw_semaphore *sem)
> {
>     /* The rwsem_owner() helper can be used to get the owner */
>     return sem->owner == current;
> }
> ---
>