linux-kernel - Re: [PATCH v3 1/6] mm: memory: extend finish

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Date: Mon, 3 Jun 2024 16:29:29 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, willy@...radead.org,
 david@...hat.com, wangkefeng.wang@...wei.com, ying.huang@...el.com,
 ryan.roberts@....com, shy828301@...il.com, ziy@...dia.com,
 ioworker0@...il.com, da.gomez@...sung.com, p.raghav@...sung.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/6] mm: memory: extend finish_fault() to support large
 folio



On 2024/6/3 13:28, Barry Song wrote:
> On Thu, May 30, 2024 at 2:04 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
>>
>> Add large folio mapping establishment support for finish_fault() as a preparation,
>> to support multi-size THP allocation of anonymous shmem pages in the following
>> patches.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>>   mm/memory.c | 58 ++++++++++++++++++++++++++++++++++++++++++++---------
>>   1 file changed, 48 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index eef4e482c0c2..435187ff7ea4 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4831,9 +4831,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>   {
>>          struct vm_area_struct *vma = vmf->vma;
>>          struct page *page;
>> +       struct folio *folio;
>>          vm_fault_t ret;
>>          bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>>                        !(vma->vm_flags & VM_SHARED);
>> +       int type, nr_pages, i;
>> +       unsigned long addr = vmf->address;
>>
>>          /* Did we COW the page? */
>>          if (is_cow)
>> @@ -4864,24 +4867,59 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>                          return VM_FAULT_OOM;
>>          }
>>
>> +       folio = page_folio(page);
>> +       nr_pages = folio_nr_pages(folio);
>> +
>> +       /*
>> +        * Using per-page fault to maintain the uffd semantics, and same
>> +        * approach also applies to non-anonymous-shmem faults to avoid
>> +        * inflating the RSS of the process.
> 
> I don't feel the comment explains the root cause.
> For non-shmem, anyway we have allocated the memory? Avoiding inflating
> RSS seems not so useful as we have occupied the memory. the memory footprint

This is also to keep the same behavior as before for non-anon-shmem, and 
will be discussed in the future.

> is what we really care about. so we want to rely on read-ahead hints of subpage
> to determine read-ahead size? that is why we don't map nr_pages for non-shmem
> files though we can potentially reduce nr_pages - 1 page faults?

IMHO, there is 2 cases for non-anon-shmem:
(1) read mmap() faults: we can rely on the 'fault_around_bytes' 
interface to determin what size of mapping to build.
(2) writable mmap() faults: I want to keep the same behavior as before 
(per-page fault), but we can talk about this when I send new patches to 
use mTHP to control large folio allocation for writable mmap().

>> +        */
>> +       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
>> +               nr_pages = 1;
>> +       } else if (nr_pages > 1) {
>> +               pgoff_t idx = folio_page_idx(folio, page);
>> +               /* The page offset of vmf->address within the VMA. */
>> +               pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
>> +
>> +               /*
>> +                * Fallback to per-page fault in case the folio size in page
>> +                * cache beyond the VMA limits.
>> +                */
>> +               if (unlikely(vma_off < idx ||
>> +                            vma_off + (nr_pages - idx) > vma_pages(vma))) {
>> +                       nr_pages = 1;
>> +               } else {
>> +                       /* Now we can set mappings for the whole large folio. */
>> +                       addr = vmf->address - idx * PAGE_SIZE;
>> +                       page = &folio->page;
>> +               }
>> +       }
>> +
>>          vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>> -                                     vmf->address, &vmf->ptl);
>> +                                      addr, &vmf->ptl);
>>          if (!vmf->pte)
>>                  return VM_FAULT_NOPAGE;
>>
>>          /* Re-check under ptl */
>> -       if (likely(!vmf_pte_changed(vmf))) {
>> -               struct folio *folio = page_folio(page);
>> -               int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
>> -
>> -               set_pte_range(vmf, folio, page, 1, vmf->address);
>> -               add_mm_counter(vma->vm_mm, type, 1);
>> -               ret = 0;
>> -       } else {
>> -               update_mmu_tlb(vma, vmf->address, vmf->pte);
>> +       if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
>> +               update_mmu_tlb(vma, addr, vmf->pte);
>>                  ret = VM_FAULT_NOPAGE;
>> +               goto unlock;
>> +       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> 
> In what case we can't use !pte_range_none(vmf->pte, 1) for nr_pages == 1
> then unify the code for nr_pages==1 and nr_pages > 1?
> 
> It seems this has been discussed before, but I forget the reason.

IIUC, this is for uffd case, which is not a none pte entry.