Re: PFs on pages pinned with get_user_pages()

[Peter Zijlstra]

On Thu, 2009-01-29 at 14:08 +0100, Frank Mehnert wrote:
> Peter,

(please retain CC's)

> On Thursday 29 January 2009, Peter Zijlstra wrote:
> > On Thu, 2009-01-29 at 09:05 +0100, Frank Mehnert wrote:
> > > please could someone explain me under which circumstances a pagefault,
> > > either generated from kernel code or from userland code, can occur on
> > > pages which are pinned with get_user_pages()?
> > >
> > > So far my understanding was that this can _never_ happen but I seems to
> > > be wrong. Under high memory pressure I get PFs on such pages raised from
> > > kernel code and the PFs are handled by do_swap_page(). When this happens,
> > > page_count is 3 but page_mapped() returns false.
> >
> > Under memory pressure the page reclaim will first unmap the physical
> > page from the virtual address range, and then try to free it.
> 
> Which means the page table entry is removed but the physical page
> is not swapped out, right?

Correct.

> > Obviously the freeing bit fails if you hold a reference to it, but the
> > unmap will work.
> 
> Right.
> 
> > After that, userspace will have to (minor) fault the stuff back in.
> 
> So do_swap_page does only 'restore' the page table entry, no further
> reading from the swapfile is necessary?

Indeed.

> > Also, that same page-reclaim, or pdflush might decide to write out dirty
> > data, which will also result in (minor) faults when userspace will
> > re-dirty the pages.
> >
> > Having a page reference will only avoid the physical page from getting
> > removed from its current mapping (and thereby also pins the mapping).
> 
> Question: Is it possible to prevent these minor page faults at all?

Not without some serious tinkering to the VM -- and in the case of the
dirty fault, not at all.

Why are you asking?

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