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

[Yang Shi]

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.

I think the commit 127393fbe597 ("mm: thp: kvm: fix memory corruption in 
KVM with THP enabled") elaborates the trade-off clearly (not full commit 
log, just paste the most related part):

ÂÂ Ideally instead of the page->_mapcount < 1 check, get_user_pages()
ÂÂÂ should return the granularity of the "page" mapping in the "mm" passed
 to get_user_pages(). However it's non trivial change to pass the "pmd"
ÂÂÂ status belonging to the "mm" walked by get_user_pages up the stack (up
 to the caller of get_user_pages). So the fix just checks if there is
ÂÂÂ not a single pte mapping on the page returned by get_user_pages, and in
ÂÂÂ turn if the caller can assume that the whole compound page is mapped in
 the current "mm" (in a pmd_trans_huge()). In such case the entire
ÂÂÂ compound page is safe to map into the secondary MMU without additional
 get_user_pages() calls on the surrounding tail/head pages. In addition
ÂÂÂ of being faster, not having to run other get_user_pages() calls also
ÂÂÂ reduces the memory footprint of the secondary MMU fault in case the pmd
ÂÂÂ split happened as result of memory pressure.