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SubjectRe: Pagecache: find_or_create_page does not call a proper page allocator function
On Tue, 24 Apr 2007, Hugh Dickins wrote:

> I've not yet looked at the patch under discussion, but this remark
> prompts me... a couple of days ago I got very worried by the various
> hard-wired GFP_HIGHUSER allocations in mm/migrate.c and mm/mempolicy.c,
> and wondered how those would work out if someone has a blockdev mmap'ed.

Hmmm.... These not that critical given that 32 bit NUMA systems are a bit
rare. And if a page is in the wrong area then it can be bounced before I/O
is performed on it.

> (If vma->vm_file is non-NULL, we can be sure vma->vm_file->f_mapping
> is non-NULL, can't we? Some common code assumes that, some does not:
> I've avoided cargo-cult safety below, but don't let me make it unsafe.)
> Is there a problem with page migration to HIGHMEM, if pages were
> mapped from a GFP_USER block device? I failed to demonstrate any
> problem, but here's a quick fix if needed.

If a page is migrated into a different zone and then a write attempt is
made on it then the page will be bounced into a zone from which the device
can write from.
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