>On Fri, Mar 24, 2023 at 02:55:02PM +0000, Matthew Wilcox wrote: >> No, that's not true. You can allocate kernel memory from ZONE_MOVABLE. >> You have to be careful when you do that, but eg filesystems put symlinks >> and directories in ZONE_MOVABLE, and zswap allocates memory from >> ZONE_MOVABLE. Of course, then you have to be careful that the kernel >> doesn't try to move it while you're accessing it. That's the tradeoff. > >I want to talk a little bit about what it would take to use MOVABLE >allocations for slab. > >Initially, one might presume that it is impossible to have slab use a >movable allocation. Usually, we need a relatively complex mechanism of >reference counting where one takes a reference on the page, uses it, >then puts the reference. Then migration can check the page reference >and if it's unused, it knows it's safe to migrate (much handwaving here, >of course it's more complex). > >The general case of kmalloc slabs cannot use MOVABLE allocations. >The API has no concept of "this pointer is temporarily not in use", >so we can never migrate any slab which has allocated objects. > >But for slab caches, individual objects may have access rules which allow >them to be moved. For example, we might be able to migrate every dentry >in a slab, then RCU-free the slab. Similarly for radix_tree_nodes. > >There was some work along these lines a few years ago: >https://lore.kernel.org/all/20190603042637.2018-16-tobin@xxxxxxxxxx/ > >There are various practical problems with that patchset, but they can >be overcome with sufficient work. The question is: Why do we need to do >this work? What is the high-level motivation to make slab caches movable? Thank you for sharing us the case. We studied your sentences and the patchset. Let me summarize our understanding. A kernel context is migratable at certain in case the reference count of a page is traceable and it is 0. As I answered previously, our intention is the attribute of CXL DRAM page can be movable as well as unmovable. A memory allocator context should be able to determine it.
