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

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

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.