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Date: Mon, 21 Oct 2002 14:49:35 -0700
From: Andrew Morton <akpm@digeo.com>
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To: "Martin J. Bligh" <mbligh@aracnet.com>
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Subject: Re: ZONE_NORMAL exhaustion (dcache slab)
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"Martin J. Bligh" wrote:
> 
> >> Nope, kept OOMing and killing everything .
> >
> > Something broke.
> 
> Even I worked that out ;-)

Well I'm feeling especially helpful today.

> > Blockdevices only use ZONE_NORMAL for their pagecache.  That cat will
> > selectively put pressure on the normal zone (and DMA zone, of course).
> 
> Ah, I recall that now. That's fundamentally screwed.

When filesystems want to access metadata, they will typically read
a block into a buffer_head and access the memory directly.

 mnm:/usr/src/25> grep -rI b_data fs | wc -l
    844

That's a lot of kmaps need adding.

So we constrain blockdev->bd_inode->i_mapping->gfp_mask so that
the blockdev's pagecache memory is always in the direct-addressed
region.

It would be possible to fix on a per-fs basis - teach a filesystem
to kmap bh->b_page appropriately and then set __GFP_HIGHMEM in the
blockdev's gfp_mask.

But it doesn't seem to cause a lot of trouble in practice.
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