>>> 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 -- 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/
