Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Buffered atomic writes

[Christoph Hellwig <hch@lst.de>]

On Tue, Feb 17, 2026 at 10:47:07AM -0500, Andres Freund wrote:
> Most prominently: With DIO concurrently extending multiple files leads to
> quite terrible fragmentation, at least with XFS. Forcing us to
> over-aggressively use fallocate(), truncating later if it turns out we need
> less space. The fallocate in turn triggers slowness in the write paths, as
> writing to uninitialized extents is a metadata operation.  It'd be great if
> the allocation behaviour with concurrent file extension could be improved and
> if we could have a fallocate mode that forces extents to be initialized.

As Dave already mentioned, if you do concurrent allocations (extension
or hole filling), setting an extent size hint is probably a good idea.
We could try to look into heuristics, but chances are that they would
degrade other use caes.  Details would be useful as a report on the
XFS list.

> 
> A secondary issue is that with the buffer pool sizes necessary for DIO use on
> bigger systems, creating the anonymous memory mapping becomes painfully slow
> if we use MAP_POPULATE - which we kinda need to do, as otherwise performance
> is very inconsistent initially (often iomap -> gup -> handle_mm_fault ->
> folio_zero_user uses the majority of the CPU). We've been experimenting with
> not using MAP_POPULATE and using multiple threads to populate the mapping in
> parallel, but that feels not like something that userspace ought to have to
> do.  It's easier to work around for us that the uninitialized extent
> conversion issue, but it still is something we IMO shouldn't have to do.

Please report this to linux-mm.