On 01/09/2015 02:24 AM, Christoffer Dall wrote: > On Thu, Jan 08, 2015 at 08:28:46AM -0800, Mario Smarduch wrote: >> On 01/08/2015 02:45 AM, Christoffer Dall wrote: >>> On Wed, Jan 07, 2015 at 05:43:18PM -0800, Mario Smarduch wrote: >>>> Hi Christoffer, >>>> before going through your comments, I discovered that >>>> in 3.18.0-rc2 - a generic __get_user_pages_fast() >>>> was implemented, now ARM picks this up. This causes >>>> gfn_to_pfn_prot() to return meaningful 'writable' >>>> value for a read fault, provided the region is writable. >>>> >>>> Prior to that the weak version returned 0 and 'writable' >>>> had no optimization effect to set pte/pmd - RW on >>>> a read fault. >>>> >>>> As a consequence dirty logging broke in 3.18, I was seeing >> Correction on this, proper __get_user_pages_fast() >> behavior exposed a bug in page logging code. >> >>>> weird but very intermittent issues. I just put in the >>>> additional few lines to fix it, prevent pte RW (only R) on >>>> read faults while logging writable region. >>>> >>>> On 01/07/2015 04:38 AM, Christoffer Dall wrote: >>>>> On Wed, Dec 17, 2014 at 06:07:29PM -0800, Mario Smarduch wrote: >>>>>> This patch is a followup to v15 patch series, with following changes: >>>>>> - When clearing/dissolving a huge, PMD mark huge page range dirty, since >>>>>> the state of whole range is unknown. After the huge page is dissolved >>>>>> dirty page logging is at page granularity. >>>>> >>>>> What is the sequence of events where you could have dirtied another page >>>>> within the PMD range after the user initially requested dirty page >>>>> logging? >>>> >>>> No there is none. My issue was the start point for tracking dirty pages >>>> and that would be second call to dirty log read. Not first >>>> call after initial write protect where any page in range can >>>> be assumed dirty. I'll remove this, not sure if there would be any >>>> use case to call dirty log only once. >>>> >>> >>> Calling dirty log once can not give you anything meaningful, right? You >>> must assume all memory is 'dirty' at this point, no? >> >> There is the interval between KVM_MEM_LOG_DIRTY_PAGES and first >> call to KVM_GET_DIRTY_LOG. Not sure of any use case, maybe enable >> logging, wait a while do a dirty log read, disable logging. >> Get an accumulated snapshot of dirty page activity. >> > ok, so from the time the user calls KVM_MEM_LOG_DIRTY_PAGES, then any > fault on any huge page will dissolve that huge page into pages, and each > dirty page will be logged accordingly for the first call to > KVM_GET_DIRTY_LOG, right? What am I missing here? Yes that's correct, this may or may not be meaningful in itself. The original point was first time access to a huge page (on first or some later call) and do we consider whole range dirty. Keeping track at page granularity + original image provides everything needed to reconstruct the source so it should not matter. I think I convoluted this issue a bit. - Mario > > -Christoffer > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
