Re: [PATCH 00/16] Sanitize usage of ->flags and ->mapping for tail pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Jul 15, 2015 at 03:20:01PM -0500, Christoph Lameter wrote:
> On Thu, 19 Mar 2015, Kirill A. Shutemov wrote:
> 
> > Currently we take naive approach to page flags on compound -- we set the
> > flag on the page without consideration if the flag makes sense for tail
> > page or for compound page in general. This patchset try to sort this out
> > by defining per-flag policy on what need to be done if page-flag helper
> > operate on compound page.
> 
> Well we hand pointers to head pages around if handling compound pages.
> References to tail pages are dicey and should only be used in a limited
> way. At least that is true in the slab allocators and that was my
> understanding in earlier years. Therefore it does not make sense
> then check for tail pages.

This is preparation patchset for THP refcounting rework. With new
refcounting sub-pages for THP can be mapped with PTEs, therefore we will
see tail pages returned from pte_page().

I've tried ad-hoc approach to page flags wrt tail pages on earlier (pre
LFS/MM) revisions of THP refcounting patchset. And IIRC, *you* pointed
that it would be nice to have more systematic approach.

And here's my attempt.

> > For now I catched one case of illigal usage of page flags or ->mapping:
> > sound subsystem allocates pages with __GFP_COMP and maps them with PTEs.
> > It leads to setting dirty bit on tail pages and access to tail_page's
> > ->mapping. I don't see any bad behaviour caused by this, but worth fixing
> > anyway.
> 
> Does this catch any errors?

It helped to catch BUG fixed by c761471b58e6 (mm: avoid tail page
refcounting on non-THP compound pages) and helped with work on
refcounting patchset.
 
> > This patchset makes more sense if you take my THP refcounting into
> > account: we will see more compound pages mapped with PTEs and we need to
> > define behaviour of flags on compound pages to avoid bugs.
> 
> Ok that introduces the risk of pointers to tail pages becoming more of an
> issue. But that does not affect non pagecache pages.

We don't have huge pages in pagecache yet. Refcounting patchset only
affects anon-THP. And makes compound pages suitable for pagecache.

We also have PTE-mapped compound pages -- in sound subsystem and some
drivers (framebuffer, etc.)

-- 
 Kirill A. Shutemov

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