Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

["H. Peter Anvin" <hpa@zytor.com>]

Eep.  This should be reverted, indeed.  This isn't a manifest bug on !Xen but we have gotten requests for WT support which would mean adding in the PAT but again.

David Vrabel <david.vrabel@citrix.com> wrote:
>All,
>
>179ef71c (mm: save soft-dirty bits on swapped pages) introduces a new
>PTE bit on x86 _PTE_SWP_SOFT_DIRTY which has the same value as _PTE_PSE
>and _PTE_PAT.
>
>With a Xen PV guest, the use of the _PTE_PAT will result in the page
>having unexpected cachability which will introduce a range of subtle
>performance and correctness issues.  Xen programs the entry 4 in the
>PAT
>table with WC so a page that was previously WB will end up as WC.
>
>The use of this bit also appears to preclude the use of (transparent)
>huge pages by the application.  It is not clear if there is something
>else guaranteeing that that there will be no huge pages.
>
>To fix this regression I suggest one or more of:
>
>1. If no other changes are made, at a mimimum, MEM_SOFT_DIRTY must
>require !XEN and possibly !TRANSPARENT_HUGEPAGE and !HUGETLBFS.  This
>would prevent this option being enabled on the majority of standard
>Linux distributions.
>
>2. Find a different PTE bit to (re)use.
>
>3. Avoid clearing the soft dirty bit when repopulating a swapped out
>page.
>
>4. Redesign the soft dirty tracking to not require the use of
>architecture specific PTE bits.  e.g., by using a shadow set of
>structures for the soft dirty bit tracking.
>
>David

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