On Fri, Feb 28, 2020 at 10:50:48AM +0100, Michal Hocko wrote: > 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. And MADV_FREE > pages are a kind of cache as well. If the target node is short on memory > then those will be reclaimed as a cache so a pro-active freeing sounds > counter productive as you do not have any idea whether that cache is > going to be used in future. In other words you are not going to free a > clean page cache if you want to use that memory as a migration target > right? So you should make a clear case about why MADV_FREE cache is less > important than the clean page cache and ideally have a good > justification backed by real workloads. Agreed. MADV_FREE says that the *data* in the pages is no longer needed, and so the pages are cheaper to reclaim than regular anon pages (swap). But people use MADV_FREE in the hope that they can reuse the pages at a later point - otherwise, they'd unmap them. We should retain them until the memory is actively needed for other allocations.
