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 <[email protected]> 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 -- Sent from my mobile phone. Please excuse brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
