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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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