Re: [v4 PATCH] mm: thp: handle page cache THP correctly in PageTransCompoundMap

[Matthew Wilcox <willy@infradead.org>]

On Thu, Oct 24, 2019 at 09:33:11AM -0700, Yang Shi wrote:
> On 10/24/19 6:55 AM, Matthew Wilcox wrote:
> > On Thu, Oct 24, 2019 at 05:19:35AM +0800, Yang Shi wrote:
> > > We have usecase to use tmpfs as QEMU memory backend and we would like to
> > > take the advantage of THP as well.  But, our test shows the EPT is not
> > > PMD mapped even though the underlying THP are PMD mapped on host.
> > > The number showed by /sys/kernel/debug/kvm/largepage is much less than
> > > the number of PMD mapped shmem pages as the below:
> > > 
> > > 7f2778200000-7f2878200000 rw-s 00000000 00:14 262232 /dev/shm/qemu_back_mem.mem.Hz2hSf (deleted)
> > > Size:            4194304 kB
> > > [snip]
> > > AnonHugePages:         0 kB
> > > ShmemPmdMapped:   579584 kB
> > > [snip]
> > > Locked:                0 kB
> > > 
> > > cat /sys/kernel/debug/kvm/largepages
> > > 12
> > > 
> > > And some benchmarks do worse than with anonymous THPs.
> > > 
> > > By digging into the code we figured out that commit 127393fbe597 ("mm:
> > > thp: kvm: fix memory corruption in KVM with THP enabled") checks if
> > > there is a single PTE mapping on the page for anonymous THP when
> > > setting up EPT map.  But, the _mapcount < 0 check doesn't fit to page
> > > cache THP since every subpage of page cache THP would get _mapcount
> > > inc'ed once it is PMD mapped, so PageTransCompoundMap() always returns
> > > false for page cache THP.  This would prevent KVM from setting up PMD
> > > mapped EPT entry.
> > > 
> > > So we need handle page cache THP correctly.  However, when page cache
> > > THP's PMD gets split, kernel just remove the map instead of setting up
> > > PTE map like what anonymous THP does.  Before KVM calls get_user_pages()
> > > the subpages may get PTE mapped even though it is still a THP since the
> > > page cache THP may be mapped by other processes at the mean time.
> > > 
> > > Checking its _mapcount and whether the THP has PTE mapped or not.
> > > Although this may report some false negative cases (PTE mapped by other
> > > processes), it looks not trivial to make this accurate.
> > I don't understand why you care how it's mapped into userspace.  If there
> > is a PMD-sized page in the page cache, then you can use a PMD mapping
> > in the EPT tables to map it.  Why would another process having a PTE
> > mapping on the page cause you to not use a PMD mapping?
> 
> We don't care if the THP is PTE mapped by other process, but either
> PageDoubleMap flag or _mapcount/compound_mapcount can't tell us if the PTE
> map comes from the current process or other process unless gup could return
> pmd's status.

But why do you care if the THP is PTE mapped by _this_ process?
This process has a reference to the page; the page is PMD sized and PMD
aligned, so you can use a PMD mapping in the guest, regardless of how
it's mapped by userspace.  Maybe this process doesn't even have the page
mapped at all?