On Thu, Sep 08, 2022 at 09:48:35PM -0700, Andy Lutomirski wrote: > On 8/19/22 17:27, Kirill A. Shutemov wrote: > > On Thu, Aug 18, 2022 at 08:00:41PM -0700, Hugh Dickins wrote: > > > On Thu, 18 Aug 2022, Kirill A . Shutemov wrote: > > > > On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote: > > > > > > > > > > If your memory could be swapped, that would be enough of a good reason > > > > > to make use of shmem.c: but it cannot be swapped; and although there > > > > > are some references in the mailthreads to it perhaps being swappable > > > > > in future, I get the impression that will not happen soon if ever. > > > > > > > > > > If your memory could be migrated, that would be some reason to use > > > > > filesystem page cache (because page migration happens to understand > > > > > that type of memory): but it cannot be migrated. > > > > > > > > Migration support is in pipeline. It is part of TDX 1.5 [1]. And swapping > > > > theoretically possible, but I'm not aware of any plans as of now. > > > > > > > > [1] https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html > > > > > > I always forget, migration means different things to different audiences. > > > As an mm person, I was meaning page migration, whereas a virtualization > > > person thinks VM live migration (which that reference appears to be about), > > > a scheduler person task migration, an ornithologist bird migration, etc. > > > > > > But you're an mm person too: you may have cited that reference in the > > > knowledge that TDX 1.5 Live Migration will entail page migration of the > > > kind I'm thinking of. (Anyway, it's not important to clarify that here.) > > > > TDX 1.5 brings both. > > > > In TDX speak, mm migration called relocation. See TDH.MEM.PAGE.RELOCATE. > > > > This seems to be a pretty bad fit for the way that the core mm migrates > pages. The core mm unmaps the page, then moves (in software) the contents > to a new address, then faults it in. TDH.MEM.PAGE.RELOCATE doesn't fit into > that workflow very well. I'm not saying it can't be done, but it won't just > work. Hm. From what I see we have all necessary infrastructure in place. Unmaping is NOP for inaccessible pages as it is never mapped and we have mapping->a_ops->migrate_folio() callback that allows to replace software copying with whatever is needed, like TDH.MEM.PAGE.RELOCATE. What do I miss? -- Kiryl Shutsemau / Kirill A. Shutemov
