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Subject: Re: [PATCH] mm: Introduce page_size()
From: William Kucharski <william.kucharski@oracle.com>
In-Reply-To: <20190102031414.GG6310@bombadil.infradead.org>
Date: Wed, 2 Jan 2019 04:46:27 -0700
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References: <20181231134223.20765-1-willy@infradead.org>
 <87y385awg6.fsf@linux.ibm.com> <20190101063031.GD6310@bombadil.infradead.org>
 <87lg447knf.fsf@linux.ibm.com> <20190102031414.GG6310@bombadil.infradead.org>
Sender: owner-linux-mm@kvack.org
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To: Matthew Wilcox <willy@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org

It's tricky, simply because if someone doesn't know the size of their
current page, they would generally want to know what size the current
page is mapped as, based upon what is currently extant within that address
space.

So for example, assuming read-only pages, if an as has a PMD-sized THP
mapped, it seems as if page_size() for any address within that PMD
address range should return the PMD size as compound page head/tail is
an implementation issue, not a VM one per se.

On the other hand, if another as has a portion of the physical space
the THP occupies mapped as a PAGESIZE page, a page_size() for and address
within that range should return PAGESIZE.

Forgive me if I'm being impossibly naive here.