On Thu, Sep 9, 2021 at 2:21 PM Jann Horn <jannh@xxxxxxxxxx> wrote: > [...] > > > > Option 1: as you suggest, pin pages holding struct umcg_task in sys_umcg_ctl; > > FWIW, there is a variant on this that might also be an option: > > You can create a new memory mapping from kernel code and stuff pages > into it that were originally allocated as normal kernel pages. This is > done in a bunch of places, e.g.: > > This has the advantage that it avoids pinning random pages that were > originally allocated from ZONE_MOVABLE blocks. (Or pinning hugepages, > or something like that.) > The downsides are that it reduces userspace's freedom to place the > UAPI structs wherever it wants (so userspace e.g. probably can't > directly put the struct in thread-local storage, instead it has to > store a pointer to the struct), and that you need to write a bunch of > code to create the mapping and allocate slots in these pages for > userspace threads. Thanks again, Jann! Why do you think using custom mapping like this is preferable to doing just kzalloc(size, GFP_USER), or maybe alloc_page(GFP_USER)? The documentation here https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html says: "GFP_USER means that the allocated memory is not movable and it must be directly accessible by the kernel", which sounds exactly what we need here. [...]