Re: [PATCH] mm: get page_cache_get_speculative() work on tail pages

[Hugh Dickins <hughd@google.com>]

On Thu, 2 Apr 2015, Kirill A. Shutemov wrote:
> On Wed, Apr 01, 2015 at 04:21:30PM -0700, Hugh Dickins wrote:
> > On Thu, 2 Apr 2015, Kirill A. Shutemov wrote:
> > 
> > > Generic RCU fast GUP rely on page_cache_get_speculative() to obtain pin
> > > on pte-mapped page.  As pointed by Aneesh during review of my compound
> > > pages refcounting rework, page_cache_get_speculative() would fail on
> > > pte-mapped tail page, since tail pages always have page->_count == 0.
> > > 
> > > That means we would never be able to successfully obtain pin on
> > > pte-mapped tail page via generic RCU fast GUP.
> > > 
> > > But the problem is not exclusive to my patchset. In current kernel some
> > > drivers (sound, for instance) already map compound pages with PTEs.
> > 
> > Hah, you were sending this as I was replying to the original thread.
> > 
> > Do we care if fast gup fails on some hardware driver's compound pages?
> > I don't think we do, and it would be better not to complicate the
> > low-level page_cache_get_speculative for them.
> 
> Fair enough :-/
> 
> I'll check tomorrow if it will look more reasonable on gup_pte_range()
> level, rather than page_cache_get_speculative().

But we don't need it on the (fast) gup_pte_range() level either, do we?
Or do you have THP changes in mmotm which are now demanding this?

Hugh

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