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Date: Thu, 22 Nov 2018 07:19:41 -0800
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page
 tables
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To: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>, Robin Murphy <robin.murphy@arm.com>, Michal Hocko <mhocko@suse.com>, Will Deacon <will.deacon@arm.com>, Levin Alexander <Alexander.Levin@microsoft.com>, linux-mm@kvack.org, Christopher Lameter <cl@linux.com>, Nicolas Boichat <drinkcat@chromium.org>, Huaisheng Ye <yehs1@lenovo.com>, David Rientjes <rientjes@google.com>, yingjoe.chen@mediatek.com, Vlastimil Babka <vbabka@suse.cz>, Tomasz Figa <tfiga@google.com>, Mike Rapoport <rppt@linux.vnet.ibm.com>, Matthias Brugger <matthias.bgg@gmail.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>, iommu@lists.linux-foundation.org, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@techsingularity.net>

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.