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

[Kirill A. Shutemov]

On Tue, Mar 24, 2015 at 08:39:49PM +0300, Konstantin Khlebnikov wrote:
> On Thu, Mar 19, 2015 at 8:08 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> 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.
> >
> > The last patch in patchset also sanitize usege of page->mapping for tail
> > pages. We don't define meaning of page->mapping for tail pages. Currently
> > it's always NULL, which can be inconsistent with head page and potentially
> > lead to problems.
> >
> > 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.
> 
> Do you mean call of set_page_dirty() from zap_pte_range() ?

No. I trigger it earlier: set_page_dirty() from do_shared_fault().

> I think this should be replaced with vma operation:
> vma->vm_ops->set_page_dirty()

Does anybody know why would we want to dirtying pages with ->mapping ==
NULL?

I don't see a place where we can make any use of this. We probably could
avoid dirting such pages. Hm?

-- 
 Kirill A. Shutemov
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