On Tue, Jan 11, 2022 at 6:30 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > 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. I'm wondering if we could use ZERO_PAGE for shmem_getpage() too so that we have simpler return value? Currently shmem_getpage() returns: #1. errno and NULL *pagep #2. 0 and valid *pagep #3. 0 and NULL *pagep if SGP_READ Using ZERO_PAGE should be able to consolidate #2 and #3 so that we know there must be valid *pagep if 0 is returned. This should make read-fault use ZERO_PAGE automatically. > > 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 >
