On Wed, Sep 05, 2018 at 03:30:37PM +0800, 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? We have two entiries that can be split: page table enties and underlying compound page. split_huge_pmd() (and variants of it) split the PMD entry into a PTE page table. It doens't touch underlying compound page. The page still can be mapped in other place as huge. split_huge_page() (and ivariants of it) split compound page into a number of 4k (or whatever PAGE_SIZE is). The operation requires splitting all PMD, but not other way around. > > 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). Dirty bit from page table entries transferes to sturct page flug and used for decision making in reclaim path. -- Kirill A. Shutemov
