On Wed, Sep 05, 2018 at 08:49:20AM -0400, Zi Yan wrote: > On 5 Sep 2018, at 3:30, Peter Xu wrote: > > > On Tue, Sep 04, 2018 at 10:00:28AM -0400, Zi Yan wrote: > >> On 4 Sep 2018, at 4:01, Kirill A. Shutemov wrote: > >> > >>> On Tue, Sep 04, 2018 at 03:55:10PM +0800, Peter Xu wrote: > >>>> When splitting a huge page, we should set all small pages as dirty if > >>>> the original huge page has the dirty bit set before. Otherwise we'll > >>>> lose the original dirty bit. > >>> > >>> We don't lose it. It got transfered to struct page flag: > >>> > >>> if (pmd_dirty(old_pmd)) > >>> SetPageDirty(page); > >>> > >> > >> Plus, when split_huge_page_to_list() splits a THP, its subroutine __split_huge_page() > >> propagates the dirty bit in the head page flag to all subpages in __split_huge_page_tail(). > > > > Hi, Kirill, Zi, > > > > Thanks for your responses! > > > > Though in my test the huge page seems to be splitted not by > > split_huge_page_to_list() but by explicit calls to > > change_protection(). The stack looks like this (again, this is a > > customized kernel, and I added an explicit dump_stack() there): > > > > kernel: dump_stack+0x5c/0x7b > > kernel: __split_huge_pmd+0x192/0xdc0 > > kernel: ? update_load_avg+0x8b/0x550 > > kernel: ? update_load_avg+0x8b/0x550 > > kernel: ? account_entity_enqueue+0xc5/0xf0 > > kernel: ? enqueue_entity+0x112/0x650 > > kernel: change_protection+0x3a2/0xab0 > > kernel: mwriteprotect_range+0xdd/0x110 > > kernel: userfaultfd_ioctl+0x50b/0x1210 > > kernel: ? do_futex+0x2cf/0xb20 > > kernel: ? tty_write+0x1d2/0x2f0 > > kernel: ? do_vfs_ioctl+0x9f/0x610 > > kernel: do_vfs_ioctl+0x9f/0x610 > > kernel: ? __x64_sys_futex+0x88/0x180 > > kernel: ksys_ioctl+0x70/0x80 > > kernel: __x64_sys_ioctl+0x16/0x20 > > kernel: do_syscall_64+0x55/0x150 > > kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > At the very time the userspace is sending an UFFDIO_WRITEPROTECT ioctl > > to kernel space, which is handled by mwriteprotect_range(). In case > > you'd like to refer to the kernel, it's basically this one from > > Andrea's (with very trivial changes): > > > > https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git userfault > > > > So... do we have two paths to split the huge pages separately? > > > > Another (possibly very naive) question is: could any of you hint me > > how the page dirty bit is finally applied to the PTEs? These two > > dirty flags confused me for a few days already (the SetPageDirty() one > > which sets the page dirty flag, and the pte_mkdirty() which sets that > > onto the real PTEs). > > change_protection() only causes splitting a PMD entry into multiple PTEs > but not the physical compound page, so my answer does not apply to your case. > It is unclear how the dirty bit makes your QEMU get a SIGBUS. I think you > need to describe your problem with more details. Hi, Zi, I explained with some more details on my problem in my other reply to Kirill. Please have a look. > > AFAIK, the PageDirty bit will not apply back to any PTEs. So for your case, > when reporting a page’s dirty bit information, some function in the kernel only checks > the PTE’s dirty bit but not the dirty bit in the struct page flags, which > might provide a wrong answer. Are you suggesting that we should always check both places (the PTE dirty bit) and also the page flag to know whether a page is dirty (hence, either of the bit set should mean the page is dirty)? Thanks, -- Peter Xu
