Re: [PATCH v2 07/13] xfs: Introduce FORCEALIGN inode flag

[John Garry]

On 24/07/2024 01:04, Darrick J. Wong wrote:
> > So why not just enable the per-inode flag with RT right from the
> > start given that this functionality is supposed to work and be
> > globally supported by the rtdev right now? It seems like a whole lot
> > less work to just enable it for RT now than it is to disable it...
> What needs to be done to the rt allocator, anyway?
>
> I think it's mostly turning off the fallback to unaligned allocation,
> just like what was done for the data device allocator, right?  And
> possibly tweaking whatever this does:
>
> 	/*
> 	 * Only bother calculating a real prod factor if offset & length are
> 	 * perfectly aligned, otherwise it will just get us in trouble.
> 	 */
> 	div_u64_rem(ap->offset, align, &mod);
> 	if (mod || ap->length % align) {
> 		prod = 1;
> 	} else {
> 		prod = xfs_extlen_to_rtxlen(mp, align);
> 		if (prod > 1)
> 			xfs_rtalloc_align_minmax(&raminlen, &ralen, &prod);
> 	}

My initial impression is that calling xfs_bmap_rtalloc() -> 
xfs_rtpick_extent() for XFS_ALLOC_INITIAL_USER_DATA won't always give an 
aligned extent. However the rest of the allocator paths are giving 
extents aligned as requested - that is from limited testing.

And we would need to not take the xfs_bmap_rtalloc() retry fallback for 
-ENOSPC when align > rtextsize, but I have not hit that yet - maybe 
because xfs_trans_reserve() stops us getting to this point due to lack 
of free rtextents.