Re: [PATCH 3/4] xfs: fix incorrect use of gfp flags in xfs_buf_alloc_backing_mem

[Christoph Hellwig <hch@lst.de>]

On Mon, Jun 29, 2026 at 06:49:35PM -0700, Darrick J. Wong wrote:
> > +	return xfs_buf_alloc_folio(bp, size, gfp_mask);
> 
> Hrmm, ok.  So first we special-case buffers > PAGE_SIZE -- if they're a
> power of two, we try (not very hard) to allocate a single large folio.
> If that fails or it's not a power-of-two, then we just do vmalloc, which
> allows direct reclaim and retries.

Yes.

> For smaller than PAGE_SIZE buffers that are powers of two, I guess we
> use the slab allocator, otherwise a full folio.  That part strikes me as
> a little strange (efficiency, I guess?), but that's what the code did
> before so I guess it's ok.

That's because slab doesn't guarantee alignent for non-power-of two
allocations, and we need that for the block layer.  There is a comment
in the code explaining that.