David Hildenbrand <david@xxxxxxxxxx> writes: > On 02.03.20 15:12, Huang, Ying wrote: >> Michal Hocko <mhocko@xxxxxxxxxx> writes: >> >>> On Fri 28-02-20 16:55:40, Huang, Ying wrote: >>>> David Hildenbrand <david@xxxxxxxxxx> writes: >>> [...] >>>>> E.g., free page reporting in QEMU wants to use MADV_FREE. The guest will >>>>> report currently free pages to the hypervisor, which will MADV_FREE the >>>>> reported memory. As long as there is no memory pressure, there is no >>>>> need to actually free the pages. Once the guest reuses such a page, it >>>>> could happen that there is still the old page and pulling in in a fresh >>>>> (zeroed) page can be avoided. >>>>> >>>>> AFAIKs, after your change, we would get more pages discarded from our >>>>> guest, resulting in more fresh (zeroed) pages having to be pulled in >>>>> when a guest touches a reported free page again. But OTOH, page >>>>> migration is speed up (avoiding to migrate these pages). >>>> >>>> Let's look at this problem in another perspective. To migrate the >>>> MADV_FREE pages of the QEMU process from the node A to the node B, we >>>> need to free the original pages in the node A, and (maybe) allocate the >>>> same number of pages in the node B. So the question becomes >>>> >>>> - we may need to allocate some pages in the node B >>>> - these pages may be accessed by the application or not >>>> - we should allocate all these pages in advance or allocate them lazily >>>> when they are accessed. >>>> >>>> We thought the common philosophy in Linux kernel is to allocate lazily. >>> >>> The common philosophy is to cache as much as possible. >> >> Yes. This is another common philosophy. And MADV_FREE pages is >> different from caches such as the page caches because it has no valid >> contents. > > Side note: It might contain valid content until discarded/zeroed out. > E.g., an application could use a marker bit (e.g., first bit) to detect > if the page still contains valid data or not. If the data is still > marked valid, the content could be reuse immediately. Not sure if there > is any such application, though :) I don't think this is the typical use case. But I admit that this is possible. Best Regards, Huang, Ying
