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SubjectRe: [PATCH] mm: hugepage: mark splitted page dirty when needed
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

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