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

[Matthew Wilcox]

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?