linux-kernel - Re: [External] Re: [PATCH v2 05/19] mm/hugetlb: Introduce pgtable allocation/freeing helpers

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Date:   Wed, 28 Oct 2020 16:42:13 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v2 05/19] mm/hugetlb: Introduce pgtable
 allocation/freeing helpers

On 10/28/20 12:26 AM, Muchun Song wrote:
> On Wed, Oct 28, 2020 at 8:33 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>> On 10/26/20 7:51 AM, Muchun Song wrote:
>>
>> I see the following routines follow the pattern for vmemmap manipulation
>> in dax.
> 
> Did you mean move those functions to mm/sparse-vmemmap.c?

No.  Sorry, that was mostly a not to myself.

>>> +static void vmemmap_pgtable_deposit(struct page *page, pte_t *pte_p)
>>> +{
>>> +     pgtable_t pgtable = virt_to_page(pte_p);
>>> +
>>> +     /* FIFO */
>>> +     if (!page_huge_pte(page))
>>> +             INIT_LIST_HEAD(&pgtable->lru);
>>> +     else
>>> +             list_add(&pgtable->lru, &page_huge_pte(page)->lru);
>>> +     page_huge_pte(page) = pgtable;
>>> +}
>>> +
>>> +static pte_t *vmemmap_pgtable_withdraw(struct page *page)
>>> +{
>>> +     pgtable_t pgtable;
>>> +
>>> +     /* FIFO */
>>> +     pgtable = page_huge_pte(page);
>>> +     if (unlikely(!pgtable))
>>> +             return NULL;
>>> +     page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
>>> +                                                    struct page, lru);
>>> +     if (page_huge_pte(page))
>>> +             list_del(&pgtable->lru);
>>> +     return page_to_virt(pgtable);
>>> +}
>>> +
...
>>> @@ -1783,6 +1892,14 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
>>>       if (!page)
>>>               return NULL;
>>>
>>> +     if (vmemmap_pgtable_prealloc(h, page)) {
>>> +             if (hstate_is_gigantic(h))
>>> +                     free_gigantic_page(page, huge_page_order(h));
>>> +             else
>>> +                     put_page(page);
>>> +             return NULL;
>>> +     }
>>> +
>>
>> It seems a bit strange that we will fail a huge page allocation if
>> vmemmap_pgtable_prealloc fails.  Not sure, but it almost seems like we shold
>> allow the allocation and log a warning?  It is somewhat unfortunate that
>> we need to allocate a page to free pages.
> 
> Yeah, it seems unfortunate. But if we allocate success, we can free some
> vmemmap pages later. Like a compromise :) . If we can successfully allocate
> a huge page, I also prefer to be able to successfully allocate another one page.
> If we allow the allocation when vmemmap_pgtable_prealloc fails, we also
> need to mark this page that vmemmap has not been released. Seems
> increase complexity.

Yes, I think it is better to leave code as it is and avoid complexity.

-- 
Mike Kravetz