linux-kernel - Re: [PATCH] mempolicy: Optimize queue_folios_pte

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Message-ID: <1d6d7842-1700-40d2-9d5b-e044fbc242de@redhat.com>
Date: Tue, 15 Apr 2025 13:59:00 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, hughd@...gle.com, vishal.moola@...il.com,
 yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH] mempolicy: Optimize queue_folios_pte_range by PTE
 batching

On 15.04.25 13:47, Dev Jain wrote:
> 
> 
> On 15/04/25 3:47 pm, David Hildenbrand wrote:
>> On 11.04.25 10:13, Dev Jain wrote:
>>> After the check for queue_folio_required(), the code only cares about the
>>> folio in the for loop, i.e the PTEs are redundant. Therefore, optimize
>>> this
>>> loop by skipping over a PTE batch mapping the same folio.
>>>
>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>> ---
>>> Unfortunately I have only build tested this since my test environment is
>>> broken.
>>>
>>>    mm/mempolicy.c | 12 +++++++++++-
>>>    1 file changed, 11 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>>> index b28a1e6ae096..b019524da8a2 100644
>>> --- a/mm/mempolicy.c
>>> +++ b/mm/mempolicy.c
>>> @@ -573,6 +573,9 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>> unsigned long addr,
>>>        pte_t *pte, *mapped_pte;
>>>        pte_t ptent;
>>>        spinlock_t *ptl;
>>> +    int max_nr;
>>> +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>> +    int nr = 1;
>>
>> Try sticking to reverse xmas tree, please. (not completely the case
>> here, but fpb_flags can easily be moved all he way to the top)
> 
> I thought that the initializations were to be kept at the bottom.

Not that I am aware of.

> Asking for future patches, should I put all declarations in reverse-xmas
> fashion (even those which I don't intend to touch w.r.t the patch
> logic), or do I do that for only my additions?

We try to stay as close to reverse-xmas tree as possible. It's not 
always possible (e.g., dependent assignments), but fpb_flags in this 
case here can easily go all the way to the top.

...

> 
>>
>>   >       ptl = pmd_trans_huge_lock(pmd, vma);>       if (ptl) {
>>> @@ -586,7 +589,8 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>> unsigned long addr,
>>>            walk->action = ACTION_AGAIN;
>>>            return 0;
>>>        }
>>   > -    for (; addr != end; pte++, addr += PAGE_SIZE) {> +    for (;
>> addr != end; pte += nr, addr += nr * PAGE_SIZE) {
>>> +        nr = 1;
>>>            ptent = ptep_get(pte);
>>>            if (pte_none(ptent))
>>>                continue;
>>> @@ -607,6 +611,11 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>> unsigned long addr,
>>>            if (!queue_folio_required(folio, qp))
>>>                continue;
>>>            if (folio_test_large(folio)) {
>>> +            max_nr = (end - addr) >> PAGE_SHIFT;
>>> +            if (max_nr != 1)
>>> +                nr = folio_pte_batch(folio, addr, pte, ptent,
>>> +                             max_nr, fpb_flags,
>>> +                             NULL, NULL, NULL);
>>
>> We should probably do that immediately after we verified that
>> vm_normal_folio() have us something reasonable.
> 
> But shouldn't we keep the small folio case separate to avoid the
> overhead of folio_pte_batch()?

Yes, just do something like

if (folio_test_large(folio) && end - addr > 1)
	nr = folio_pte_batch(folio, addr, pte, ptent, end - addr,
			     max_nr, fpb_flags, ...);

before the folio_test_reserved().

Then you'd also skip the all ptes if !queue_folio_required.

> 
>>
>>>                /*
>>>                 * A large folio can only be isolated from LRU once,
>>>                 * but may be mapped by many PTEs (and Copy-On-Write may
>>> @@ -633,6 +642,7 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>> unsigned long addr,
>>>                qp->nr_failed++;
>>>                if (strictly_unmovable(flags))
>>>                    break;
>>> +            qp->nr_failed += nr - 1;
>>
>> Can't we do qp->nr_failed += nr; above?
> 
> I did not dive deep into the significance of nr_failed, but I did that
> to keep the code, before and after the change, equivalent:

And I question exactly that.

If we hit strictly_unmovable(flags), we end up returning "-EIO" from
queue_folios_pte_range().

And staring at queue_pages_range(), we ignore nr_failed if 
walk_page_range() returned an error.

So looks like we can just add everything in one shot, independent of 
strictly_unmovable()?

-- 
Cheers,

David / dhildenb