Re: [PATCH 05/16] page-flags: define behavior of FS/IO-related flags on compound pages

From: Kirill A. Shutemov
Date: Thu Mar 19 2015 - 16:03:28 EST

On Thu, Mar 19, 2015 at 11:29:52AM -0700, Dave Hansen wrote:
> On 03/19/2015 10:08 AM, Kirill A. Shutemov wrote:
> > The odd exception is PG_dirty: sound uses compound pages and maps them
> > with PTEs. NO_COMPOUND triggers VM_BUG_ON() in set_page_dirty() on
> > handling shared fault. Let's use HEAD for PG_dirty.
> Can we get the sound guys to look at this, btw? It seems like an odd
> thing that we probably don't want to keep around, right?

CC: +sound guys

I'm not sure what is right fix here. At the time adding __GFP_COMP was a
fix: see f3d48f0373c1.

Other odd part about __GFP_COMP here is that we have ->_mapcount in tail
pages to be used for both: mapcount of the individual page and for gup
pins. __compound_tail_refcounted() doesn't recognize that we don't need
tail page accounting for these pages.

Hugh, I tried to ask you about the situation several times (last time on
the summit). Any comments?

Kirill A. Shutemov
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