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

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Message-ID: <20181122082602.GB2049@infradead.org>
Date:   Thu, 22 Nov 2018 00:26:02 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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 Wed, Nov 21, 2018 at 06:35:58PM -0800, Matthew Wilcox wrote:
> I think you should look at using the page_frag allocator here.  You can
> use whatever GFP_DMA flags you like.

So I actually tries to use page_frag to solve the XFS unaligned kmalloc
allocations problem, and I don't think it is the right hammer for this
nail (or any other nail outside of networking).

The problem with the page_frag allocator is that it never reuses
fragments returned to the page, but only only frees the page once all
fragments are freed.  This means that if you have some long(er) term
allocations you are effectively creating memory leaks.