Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables

[Christoph Hellwig]

On Thu, Nov 22, 2018 at 07:16:32AM -0800, Matthew Wilcox wrote:
> Yes, your allocations from the page_frag allocator have to have similar
> lifetimes.  I thought that would be ideal for XFS though; as I understood
> the problem, these were per-IO allocations, and IOs to the same filesystem
> tend to take roughly the same amount of time.  Sure, in an error case,
> some IOs will take a long time before timing out, but it should be OK
> to have pages unavailable during that time in these rare situations.
> What am I missing?

No, thee are allocations for meatada buffers, which can stay around
for a long time.  Worse still that depends on usage, so one buffer
allocated from ma page might basically stay around forever, while
another one might get reclaimed very quickly.