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Message-ID: <5d910327-fbf0-46ed-9655-846236b555db@arm.com>
Date: Wed, 11 Sep 2024 17:32:11 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
willy@...radead.org, kirill.shutemov@...ux.intel.com
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
jack@...e.cz, mark.rutland@....com, hughd@...gle.com,
aneesh.kumar@...nel.org, yang@...amperecomputing.com, peterx@...hat.com,
ioworker0@...il.com, jglisse@...gle.com, wangkefeng.wang@...wei.com,
ziy@...dia.com, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 1/2] mm: Abstract THP allocation
On 9/11/24 14:59, David Hildenbrand wrote:
> On 11.09.24 11:27, David Hildenbrand wrote:
>> On 11.09.24 08:55, Dev Jain wrote:
>>> In preparation for the second patch, abstract away the THP allocation
>>> logic present in the create_huge_pmd() path, which corresponds to the
>>> faulting case when no page is present.
>>>
>>> There should be no functional change as a result of applying
>>> this patch.
>>>
>>
>> Hi,
>>
>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>> ---
>>> mm/huge_memory.c | 110
>>> +++++++++++++++++++++++++++++------------------
>>> 1 file changed, 67 insertions(+), 43 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 67c86a5d64a6..b96a1ff2bf40 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -943,47 +943,88 @@ unsigned long thp_get_unmapped_area(struct
>>> file *filp, unsigned long addr,
>>> }
>>> EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>>> -static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault
>>> *vmf,
>>> - struct page *page, gfp_t gfp)
>>> +static struct folio *pmd_thp_fault_alloc(gfp_t gfp, struct
>>> vm_area_struct *vma,
>>> + unsigned long haddr, unsigned long addr)
>>
>> I suggest calling this something like "vma_alloc_anon_folio_pmd()"? Then
>> it's more consistent with vma_alloc_folio().
>>
>> Also, likely we should just only pass in "addr" and calculate "haddr"
>> ourselves, it's cheap and reduces the number of function parameters.
>>
>>> {
>>> - struct vm_area_struct *vma = vmf->vma;
>>> - struct folio *folio = page_folio(page);
>>> - pgtable_t pgtable;
>>> - unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>> - vm_fault_t ret = 0;
>>> + const int order = HPAGE_PMD_ORDER;
>>> + struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr,
>>> true);
>>> - VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>> + if (unlikely(!folio)) {
>>> + count_vm_event(THP_FAULT_FALLBACK);
>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>> + goto out;
>>> + }
>>> + VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>> if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
>>> folio_put(folio);
>>> count_vm_event(THP_FAULT_FALLBACK);
>>> count_vm_event(THP_FAULT_FALLBACK_CHARGE);
>>> - count_mthp_stat(HPAGE_PMD_ORDER,
>>> MTHP_STAT_ANON_FAULT_FALLBACK);
>>> - count_mthp_stat(HPAGE_PMD_ORDER,
>>> MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>> - return VM_FAULT_FALLBACK;
>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>> + goto out;
>>> }
>>> folio_throttle_swaprate(folio, gfp);
>>> - pgtable = pte_alloc_one(vma->vm_mm);
>>> - if (unlikely(!pgtable)) {
>>> - ret = VM_FAULT_OOM;
>>> - goto release;
>>> - }
>>> -
>>> - folio_zero_user(folio, vmf->address);
>>> + folio_zero_user(folio, addr);
>>> /*
>>> * The memory barrier inside __folio_mark_uptodate makes sure
>>> that
>>> * folio_zero_user writes become visible before the set_pmd_at()
>>> * write.
>>> */
>>> __folio_mark_uptodate(folio);
>>> +out:
>>> + return folio;
>>> +}
>>> +
>>> +static void __pmd_thp_fault_success_stats(struct vm_area_struct *vma)
>>> +{
>>> + count_vm_event(THP_FAULT_ALLOC);
>>> + count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
>>> + count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>> +}
>>
>> Why isn't that moved into map_pmd_thp()
>>
>> Note that in this patch you do:
>>
>> map_pmd_thp(folio, vmf, vma, haddr);
>> spin_unlock(vmf->ptl);
>> __pmd_thp_fault_success_stats(vma);
>>
>> But in patch #2
>>
>> map_pmd_thp(folio, vmf, vma, haddr);
>> __pmd_thp_fault_success_stats(vma);
>> goto unlock;
>> release:
>> folio_put(folio);
>> unlock:
>> spin_unlock(vmf->ptl);
>>
>> Please make that consistent, meaning:
>>
>> 1) Inline __pmd_thp_fault_success_stats() into map_pmd_thp(). No need to
>> have the separated out.
>>
>> 2) Either do the PTL unlocking in __pmd_thp_fault_success_stats() or in
>> the caller. In the caller is likely easiest. Adjusting the counters
>> should be cheap, if not we could revisit this later with real data.
>>
>>> +
>>> +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>> + struct vm_area_struct *vma, unsigned long haddr)
>>> +{
>>> + pmd_t entry;
>>> +
>>> + entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>>> + entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>> + folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>> + folio_add_lru_vma(folio, vma);
>>> + set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>> + update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>
>> It's quite weird to see a mixture of haddr and vmf->address, and likely
>> this mixture is wrong or not not required.
>>
>> Looking at arc's update_mmu_cache_pmd() implementation, I cannot see how
>> passing in the unaligned address would do the right thing. But maybe arc
>> also doesn't trigger that code path ... who knows :)
>>
>>
>> Staring at some other update_mmu_cache_pmd() users, it's quite
>> inconsistent. Primarily only do_huge_pmd_numa_page() and
>> __do_huge_pmd_anonymous_page() use the unaligned address. The others
>> seem to use the aligned address ... as one would expect when modifying a
>> PMD.
>>
>>
>> I suggest to change this function to *not* pass in the vmf, and rename
>> it to something like:
>>
>> static void folio_map_anon_pmd(struct folio *folio, struct
>> vm_area_struct *vma, pmd_t *pmd, unsigned long haddr)
>
> ... or better "map_anon_folio_pmd" so it better matches
> vma_alloc_folio_pmd() suggested above.
I'll vote for this.
>
>
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