This patchset prepares the allocator infrastructure for extent size hint alignment to guarantee alignment of extents rather than just be a hint. Extent alignment is currently driven by multiple variables that come from different sources and are applied at different times. Stripe alignment currently defines the extent start alignment, whilst extent size hints only affect the extent length and not the start alignment. There are also assumptions about alignment of allocation parameters (such as the maximum length of the allocation) and if these aren't followed the extent size hints don't actually trim extents properly. This patch set aims to unify alignment entirely via args->alignment and args->prod/args->mod. Alignment at both ends of the extent should always occur when the free space selected allows for start alignment of an extent that is at least args->minlen in length. Hence we need to modify args->alignment setup to take into account extent size hints in addition to stripe alignment, and we need to ensure that extent length is always aligned to extent size hints, even if it means we cannot do a args->maxlen allocation. This means that only when we completely fail to find aligned free space to allocate from will extent size hints no longer be start aligned. They will continue to be tail aligned up until we run out of contiguous free space extents large enough to hold an extent the size of a hint. Hence there is no real change of behaviour for extent size hints; they will simply prefer aligned allocations first, then naturally fall back to the current unaligned hint sized allocation strategy. Unifying the allocation alignment stratgies simplifies the code, too. It means that the only time we don't align allocation is when trying to do contiguous allocation when extending the file (which should already be aligned if alignment is required!) or when there are no alignment constraints to begin with. As a result, we can simplify the allocation failure fallback strategies and make the code easier to understand and follow. Finally, we introduce a stub for force aligned allocation and add the logic fail the allocation if aligned allocation cannot be done. This has run through fstests for some time here without inducing new failures or causing any obvious near-ENOSPC performance regressions.
