On Wed, 22 Dec 2021, Peng Liang wrote: > Hi all, > > Recently we are working on implementing CRIU [1] for QEMU based on > Steven's work [2]. It will use memfd to allocate guest memory in order > to restore (inherit) it in the new QEMU process. However, memfd will > allocate a new page for reading while anonymous memory will map to zero > page for reading. For QEMU, memfd may cause that all memory are > allocated during the migration because QEMU will read all pages in > migration. It may lead to OOM if over-committed memory is enabled, > which is usually enabled in public cloud. > > In this patch I try to add support mapping to zero pages on reading > memfd. On reading, memfd will map to zero page instead of allocating a > new page. Then COW it when a write occurs. > > For now it's just a demo for discussion. There are lots of work to do, > e.g.: > 1. don't support THP; > 2. don't support shared reading and writing, only for inherit. For > example: > task1 | task2 > 1) read from addr | > | 2) write to addr > 3) read from addr again | > then 3) will read 0 instead of the data task2 writed in 2). > > Would something similar be welcome in the Linux? David has made good suggestions on better avoiding the need for such a change, for the use case you have in mind. And I don't care for the particular RFC patch that you posted. But I have to say that use of ZERO_PAGE for shmem/memfd/tmpfs read-fault might (potentially) be very welcome. Not as some MFD_ZEROPAGE special case, but as how it would always work. Deleting the shmem_recalc_inode() cruft, which is there to correct accounting for the unmodified read-only pages, after page reclaim has got around to freeing them later. It does require more work than you gave it in 1/1: mainly, as you call out above, there's a need to note in the mapping's XArray when ZERO_PAGE has been used at an offset, and do an rmap walk to unmap those ptes when a writable page is substituted - see __xip_unmap() in Linux 3.19's mm/filemap_xip.c for such an rmap walk. Though when this came up before (in the "no-fault mmap" MAP_NOSIGBUS thread last year - which then got forgotten), Linus was wary of that unmapping, and it was dropped for a simple MAP_PRIVATE implementation. And I've never scoped out what is needed to protect the page from writing in all circumstances: in principle, it ought to be easy by giving shmem_vm_ops a page_mkwrite; but that's likely to come with a performance penalty, which may not be justified for this case. I didn't check what you did for write protection: maybe what you did was enough, but one has to be very careful about that. Making this change to ZERO_PAGE has never quite justified the effort so far: temporarily allocated pages have worked well enough in most circumstances. Hugh > > Thanks, > Peng > > [1] https://criu.org/Checkpoint/Restore > [2] https://patchwork.kernel.org/project/qemu-devel/cover/1628286241-217457-1-git-send-email-steven.sistare@xxxxxxxxxx/ > > Peng Liang (1): > memfd: Support mapping to zero page on reading memfd > > include/linux/fs.h | 2 ++ > include/uapi/linux/memfd.h | 1 + > mm/memfd.c | 8 ++++++-- > mm/memory.c | 37 ++++++++++++++++++++++++++++++++++--- > mm/shmem.c | 10 ++++++++-- > 5 files changed, 51 insertions(+), 7 deletions(-) > > -- > 2.33.1
