linux-kernel - Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise

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Date: Thu, 11 Apr 2024 16:42:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, 21cnbao@...il.com, mhocko@...e.com,
 fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com,
 xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
 peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in
 madvise_free

On 11.04.24 16:39, Ryan Roberts wrote:
> On 11/04/2024 15:07, Lance Yang wrote:
>> On Thu, Apr 11, 2024 at 9:48 PM Ryan Roberts <ryan.roberts@....com> wrote:
>>>
>>> [...]
>>>
>>>>>> +
>>>>>> +             if (!folio_trylock(folio))
>>>>>> +                     continue;
>>>>>
>>>>> This is still wrong. This should all be protected by the "if
>>>>> (folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was previously
>>>>> so that you only call folio_trylock() if that condition is true. You are
>>>>> unconditionally locking here, then unlocking, then relocking below if the
>>>>> condition is met. Just put everything inside the condition and lock once.
>>>>
>>>> I'm not sure if it's safe to call folio_mapcount() without holding the
>>>> folio lock.
>>>>
>>>> As mentioned earlier by David in the v2[1]
>>>>> What could work for large folios is making sure that #ptes that map the
>>>>> folio here correspond to the folio_mapcount(). And folio_mapcount()
>>>>> should be called under folio lock, to avoid racing with swapout/migration.
>>>>
>>>> [1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@redhat.com/
>>>
>>> But I'm not suggesting that you should call folio_mapcount() without the lock.
>>> I'm proposing this:
>>>
>>>                  if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
>>>                          if (!folio_trylock(folio))
>>>                                  continue;
>>>                          /*
>>> -                        * If folio is shared with others, we mustn't clear
>>> -                        * the folio's dirty flag.
>>> +                        * If we have a large folio at this point, we know it is
>>> +                        * fully mapped so if its mapcount is the same as its
>>> +                        * number of pages, it must be exclusive.
>>>                           */
>>> -                       if (folio_mapcount(folio) != 1) {
>>> +                       if (folio_mapcount(folio) != folio_nr_pages(folio)) {
>>>                                  folio_unlock(folio);
>>>                                  continue;
>>>                          }
>>
>> IIUC, if the folio is clean and not in the swapcache, we still need to
>> compare the number of batched PTEs against folio_mapcount().
> 
> Why? That's not how the old code worked. In fact the comment says that the
> reason for the exclusive check is to avoid marking a dirty *folio* as clean if
> shared; that would be bad because we could throw away data that others relied
> upon. It's perfectly safe to clear the dirty flag from the *pte* even if it is
> shared; the ptes are private to the process so that won't affect sharers.
> 
> You should just follow the pattern already estabilished by the original code.
> The only difference is that because the folio is now (potentially) large, you
> have to change the way to detect exclusivity.

+1

-- 
Cheers,

David / dhildenb