Only WB pages should be swappable, but even so, the cacheability should be in the vma.
Jan Beulich <jbeul...@suse.com> wrote: >>>> On 21.08.13 at 16:12, Cyrill Gorcunov <gorcu...@gmail.com> wrote: >> On Wed, Aug 21, 2013 at 02:48:20PM +0100, David Vrabel 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. >>> >> >> David, could you please explain, Xen keeps and analyze _PTE_PAT bit >> for ptes which are not present? > >No, the problem isn't with not-present PTEs (i.e. swap entries), >but with present ones - the same bit (7) is being used for both, >according to this comment: > >/* > * Tracking soft dirty bit when a page goes to a swap is tricky. > * We need a bit which can be stored in pte _and_ not conflict > * with swap entry format. On x86 bits 6 and 7 are *not* involved > * into swap entry computation, but bit 6 is used for nonlinear > * file mapping, so we borrow bit 7 for soft dirty tracking. > */ > >Or are you telling me that the comment is misleading (at least me), >and this applies only to not-present PTEs? And even then - where >would the value of the original PAT bit be stored while swapped >out (or is it impossible - now and forever - for WC pages to get >swapped)? > >Jan -- 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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
