linux-kernel - Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

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Message-ID: <611dd616-a626-4968-b88a-4db82dccfb90@redhat.com>
Date: Mon, 8 Jul 2024 11:49:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <baohua@...nel.org>
Cc: Zhiguo Jiang <justinjiang@...o.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

On 08.07.24 11:46, Barry Song wrote:
> On Mon, Jul 8, 2024 at 9:36 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 08.07.24 11:04, Zhiguo Jiang wrote:
>>> The releasing process of the non-shared anonymous folio mapped solely by
>>> an exiting process may go through two flows: 1) the anonymous folio is
>>> firstly is swaped-out into swapspace and transformed into a swp_entry
>>> in shrink_folio_list; 2) then the swp_entry is released in the process
>>> exiting flow. This will increase the cpu load of releasing a non-shared
>>> anonymous folio mapped solely by an exiting process, because the folio
>>> go through swap-out and the releasing the swapspace and swp_entry.
>>>
>>> When system is low memory, it is more likely to occur, because more
>>> backend applidatuions will be killed.
>>>
>>> The modification is that shrink skips the non-shared anonymous folio
>>> solely mapped by an exting process and the folio is only released
>>> directly in the process exiting flow, which will save swap-out time
>>> and alleviate the load of the process exiting.
>>>
>>> Signed-off-by: Zhiguo Jiang <justinjiang@...o.com>
>>> ---
>>>
>>> Change log:
>>> v4->v5:
>>> 1.Modify to skip non-shared anonymous folio only.
>>> 2.Update comments for pra->referenced = -1.
>>> v3->v4:
>>> 1.Modify that the unshared folios mapped only in exiting task are skip.
>>> v2->v3:
>>> Nothing.
>>> v1->v2:
>>> 1.The VM_EXITING added in v1 patch is removed, because it will fail
>>> to compile in 32-bit system.
>>>
>>>    mm/rmap.c   | 13 +++++++++++++
>>>    mm/vmscan.c |  7 ++++++-
>>>    2 files changed, 19 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 26806b49a86f..5b5281d71dbb
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -843,6 +843,19 @@ static bool folio_referenced_one(struct folio *folio,
>>>        int referenced = 0;
>>>        unsigned long start = address, ptes = 0;
>>>
>>> +     /*
>>> +      * Skip the non-shared anonymous folio mapped solely by
>>> +      * the single exiting process, and release it directly
>>> +      * in the process exiting.
>>> +      */
>>> +     if ((!atomic_read(&vma->vm_mm->mm_users) ||
>>> +             test_bit(MMF_OOM_SKIP, &vma->vm_mm->flags)) &&
>>> +             folio_test_anon(folio) && folio_test_swapbacked(folio) &&
>>> +             !folio_likely_mapped_shared(folio)) {
>>
>> I'm currently working on moving all folio_likely_mapped_shared() under
>> the PTL, where we are then sure that the folio is actually mapped by
>> this process (e.g., no concurrent unmapping poisslbe).
>>
>> Can we do the same here directly?
> 
> Implementing this is challenging because page_vma_mapped_walk() is
> responsible for
> traversing the page table to acquire and release the PTL. This becomes
> particularly
> complex with mTHP, as we may need to interrupt the page_vma_mapped_walk
> loop at the first PTE.

Why can't we perform the check under the PTL and bail out? I'm missing 
something important.

-- 
Cheers,

David / dhildenb