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

[Yang Shi <yang.shi@linux.alibaba.com>]

On 10/22/19 6:31 PM, Hugh Dickins wrote:
> On Tue, 22 Oct 2019, Yang Shi wrote:
>> On 10/22/19 3:27 PM, Hugh Dickins wrote:
>>> I completely agree that the current PageTransCompoundMap() is wrong.
>>>
>>> A fix for that is one of many patches I've not yet got to upstreaming.
>>> Comparing yours and mine, I'm worried by your use of PageDoubleMap(),
>>> because really that's a flag for anon THP, and not properly supported
>>> on shmem (or now I suppose file) THP - I forget the details, is it
>>> that it sometimes gets set, but never cleared?  Generally, we just
>>> don't refer to PageDoubleMap() on shmem THPs (but there may be
>>> exceptions: sorting out the THP mapcount maze, and eliminating
>>> PageDoubleMap(), is one of my long-held ambitions, not yet reached).
>>>
>>> Here's the patch I've been carrying, but it's from earlier, so I
>>> should warn that I've done no more than build-testing it on 5.4,
>>> and I'm too far away from these issues at the moment to be able to
>>> make a good judgement or argue for it - I hope you and others can
>>> decide which patch is the better.  I should also add that we're
>>> barely using PageTransCompoundMap() at all: at best it can only
>>> give a heuristic guess as to whether the page is pmd-mapped in
>>> any particular case, and we preferred to take forward the KVM
>>> patches we posted back in April 2016, plumbing hva down to where
>>> it's needed - though of course those are somewhat different now.
>> Thanks for catching this. I was definitely thinking about using
>> compount_mapcount instead of DoubleMap flag when I was working the patch. I
>> just simply thought it would change less file by using DoubleMap flag but I
>> didn't notice it was kind of unbalanced for file THP.
>>
>> With the unbalanced DoubleMap flag, it sounds better to use
>> compound_mapcount.
> Yes: no doubt PageDoubleMap could be fixed on shmem+file, but I have no
> interest in doing that, because it's just unnecessary overhead for them.
> (They have their own overhead, of subpage mapcounting for pmd: which is
> something to eliminate and unify with anon when I get around to it.)

It might be worth fixing the unbalance since mlock depends on this flag 
too. There should be a little bit overhead when handling PTE rmap remove 
since we have to iterate every subpage to check if _mapcount is same 
with compound_mapcount or not in order to clear DoubleMap flag. It is 
easy to handle this when the last PMD map is gone.

>
>> Thanks for sharing your patch, I'm going to rework v2 by using
>> compound_mapcount. Do you mind I might steal your patch?
> Please do! One less for me to worry about, thanks.
>
>> I'm supposed we'd better fix this bug regardless of whether you would like to
>> move forward your KVM patches.
> Absolutely. There remain a few other uses of PageTransCompoundMap
> anyway, and I really wanted this outright mm fix to go in before
> re-submitting AndresLC's KVM patch (I'll ask a KVM-savvy colleague
> to take that over, Cc'ing you, once the mm end is correct).

Thanks.

>
> Hugh