On Tue 18-05-21 12:32:12, David Hildenbrand wrote: > On 18.05.21 12:07, Michal Hocko wrote: > > [sorry for a long silence on this] > > > > On Tue 11-05-21 10:15:31, David Hildenbrand wrote: > > [...] > > > > Thanks for the extensive usecase description. That is certainly useful > > background. I am sorry to bring this up again but I am still not > > convinced that READ/WRITE variant are the best interface. > > Thanks for having time to look into this. > > > > While the use case for MADV_POPULATE_WRITE is fairly obvious (i.e., > > > preallocate memory and prefault page tables for VMs), one issue is that > > > whenever we prefault pages writable, the pages have to be marked dirty, > > > because the CPU could dirty them any time. while not a real problem for > > > hugetlbfs or dax/pmem, it can be a problem for shared file mappings: each > > > page will be marked dirty and has to be written back later when evicting. > > > > > > MADV_POPULATE_READ allows for optimizing this scenario: Pre-read a whole > > > mapping from backend storage without marking it dirty, such that eviction > > > won't have to write it back. As discussed above, shared file mappings > > > might require an explciit fallocate() upfront to achieve > > > preallcoation+prepopulation. > > > > This means that you want to have two different uses depending on the > > underlying mapping type. MADV_POPULATE_READ seems rather weak for > > anonymous/private mappings. Memory backed by zero pages seems rather > > unhelpful as the PF would need to do all the heavy lifting anyway. > > Or is there any actual usecase when this is desirable? > > Currently, userfaultfd-wp, which requires "some mapping" to be able to arm > successfully. In QEMU, we currently have to prefault the shared zeropage for > userfaultfd-wp to work as expected. Just for clarification. The aim is to reduce the memory footprint at the same time, right? If that is really the case then this is worth adding. > I expect that use case might vanish over > time (eventually with new kernels and updated user space), but it might > stick for a bit. Could you elaborate some more please? > Apart from that, populating the shared zeropage might be relevant in some > corner cases: I remember there are sparse matrix algorithms that operate > heavily on the shared zeropage. I am not sure I see why this would be a useful interface for those? Zero page read fault is really low cost. Or are you worried about cummulative overhead by entering the kernel many times? > > So the split into these two modes seems more like gup interface > > shortcomings bubbling up to the interface. I do expect userspace only > > cares about pre-faulting the address range. No matter what the backing > > storage is. > > > > Or do I still misunderstand all the usecases? > > Let me give you an example where we really cannot tell what would be best > from a kernel perspective. > > a) Mapping a file into a VM to be used as RAM. We might expect the guest > writing all memory immediately (e.g., booting Windows). We would want > MADV_POPULATE_WRITE as we expect a write access immediately. > > b) Mapping a file into a VM to be used as fake-NVDIMM, for example, ROOTFS > or just data storage. We expect mostly reading from this memory, thus, we > would want MADV_POPULATE_READ. I am afraid I do not follow. Could you be more explicit about advantages of using those two modes for those example usecases? Is that to share resources (e.g. by not breaking CoW)? > Instead of trying to be smart in the kernel, I think for this case it makes > much more sense to provide user space the options. IMHO it doesn't really > hurt to let user space decide on what it thinks is best. I am mostly worried that this will turn out to be more confusing than helpful. People will need to grasp non trivial concepts and kernel internal implementation details about how read/write faults are handled. Thanks! -- Michal Hocko SUSE Labs
