Re: pte_none versus pte_present

[Hugh Dickins <hugh@veritas.com>]

On Fri, 24 Aug 2007, Francis Moreau wrote:
> 
> Sorry for being blind but I cannot see any differences between
> these 2 helpers.  When should we prefer using one rather the other ?

pte_present says if there's a real page table entry there (including
the exceptional case of a pte which is not-present to the MMU, but
otherwise a good pte: sometimes required when handling PROT_NONE).

pte_none says if the slot is empty: when a pte is not present, we may
use its slot to note where to find the page when it's to be faulted
in; or if that's not needed leave it empty as pte_none.

The common case of !pte_present && !pte_none is when an anonymous page
is swapped out: the slot notes where the required page can be found
on swap.  Oddly we don't have a macro for that case, but for the less
common case of pte_file: used in a VM_NONLINEAR vma, to note what
offset of the file to pull the page from when faulting in.  (And
page migration uses a swap-like value, without actually using swap.)

Hope that helps you to decide which one you need.

Hugh

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