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

[Peter Xu]

On Wed, Sep 05, 2018 at 03:55:22PM +0300, Kirill A. Shutemov wrote:
> 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.

Thanks for explaining.  It's much clearer for me.

Though for the issue I have encountered, I am still confused on why
that dirty bit can be ignored for the splitted PTEs.  Indeed we have:

	if (pmd_dirty(old_pmd))
		SetPageDirty(page);

However to me this only transfers (as you explained above) the dirty
bit (AFAIU it's possibly set by the hardware when the page is written)
to the page struct of the compound page.  It did not really apply to
every small page of the splitted huge page.  As you also explained,
this __split_huge_pmd() only splits the PMD entry but it keeps the
compound huge page there, then IMHO it should also apply the dirty
bits from the huge page to all the small page entries, no?

These dirty bits are really important to my scenario since AFAIU the
change_protection() call is using these dirty bits to decide whether
it should append the WRITE bit - it finally corresponds to the lines
in change_pte_range():

        /* Avoid taking write faults for known dirty pages */
        if (dirty_accountable && pte_dirty(ptent) &&
                        (pte_soft_dirty(ptent) ||
                                !(vma->vm_flags & VM_SOFTDIRTY))) {
                ptent = pte_mkwrite(ptent);
        }

So when mprotect() with that range (my case is UFFDIO_WRITEPROTECT,
which is similar) although we pass in the new protocol with VM_WRITE
here it'll still mask it since the dirty bit is not set, then the
userspace program (in my case, the QEMU thread that handles write
protect failures) can never fixup the write-protected page fault.

Am I missing anything important here?

Regards,

-- 
Peter Xu