Messages in this thread | | | Date | Wed, 8 May 2024 17:31:57 +0800 | Subject | Re: [PATCH 2/8] mm: memory: extend finish_fault() to support large folio | From | Baolin Wang <> |
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On 2024/5/8 16:53, Ryan Roberts wrote: > On 08/05/2024 04:44, Baolin Wang wrote: >> >> >> On 2024/5/7 18:37, Ryan Roberts wrote: >>> On 06/05/2024 09:46, Baolin Wang 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@linux.alibaba.com> >>>> --- >>>> mm/memory.c | 43 +++++++++++++++++++++++++++++++++---------- >>>> 1 file changed, 33 insertions(+), 10 deletions(-) >>>> >>>> diff --git a/mm/memory.c b/mm/memory.c >>>> index eea6e4984eae..936377220b77 100644 >>>> --- a/mm/memory.c >>>> +++ b/mm/memory.c >>>> @@ -4747,9 +4747,12 @@ 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) >>>> @@ -4780,24 +4783,44 @@ vm_fault_t finish_fault(struct vm_fault *vmf) >>>> return VM_FAULT_OOM; >>>> } >>>> + folio = page_folio(page); >>>> + nr_pages = folio_nr_pages(folio); >>>> + >>>> + if (unlikely(userfaultfd_armed(vma))) { >>>> + nr_pages = 1; >>>> + } else if (nr_pages > 1) { >>>> + unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE); >>>> + unsigned long end = start + nr_pages * PAGE_SIZE; >>>> + >>>> + /* In case the folio size in page cache beyond the VMA limits. */ >>>> + addr = max(start, vma->vm_start); >>>> + nr_pages = (min(end, vma->vm_end) - addr) >> PAGE_SHIFT; >>>> + >>>> + page = folio_page(folio, (addr - start) >> PAGE_SHIFT); >>> >>> I still don't really follow the logic in this else if block. Isn't it possible >>> that finish_fault() gets called with a page from a folio that isn't aligned with >>> vmf->address? >>> >>> For example, let's say we have a file who's size is 64K and which is cached in a >>> single large folio in the page cache. But the file is mapped into a process at >>> VA 16K to 80K. Let's say we fault on the first page (VA=16K). You will calculate >> >> For shmem, this doesn't happen because the VA is aligned with the hugepage size >> in the shmem_get_unmapped_area() function. See patch 7. > > Certainly agree that shmem can always make sure that it packs a vma in a way > such that its folios are naturally aligned in VA when faulting in memory. If you > mremap it, that alignment will be lost; I don't think that would be a problem
When mremap it, it will also call shmem_get_unmapped_area() to align the VA, but for mremap() with MAP_FIXED flag as David pointed out, yes, this patch may be not work perfectly.
> for a single process; mremap will take care of moving the ptes correctly and > this path is not involved. > > But what about the case when a process mmaps a shmem region, then forks, then > the child mremaps the shmem region. Then the parent faults in a THP into the > region (nicely aligned). Then the child faults in the same offset in the region > and gets the THP that the parent allocated; that THP will be aligned in the > parent's VM space but not in the child's.
Sorry, I did not get your point here. IIUC, the child's VA will also be aligned if the child mremap is not set MAP_FIXED, since the child's mremap will still call shmem_get_unmapped_area() to find an aligned new VA. Please correct me if I missed your point.
>>> start=0 and end=64K I think? >> >> Yes. Unfortunately, some file systems that support large mappings do not perform >> alignment for multi-size THP (non-PMD sized, for example: 64K). I think this >> requires modification to __get_unmapped_area--->thp_get_unmapped_area_vmflags() >> or file->f_op->get_unmapped_area() to align VA for multi-size THP in future. > > By nature of the fact that a file mapping is shared between multiple processes > and each process can map it where ever it wants down to 1 page granularity, its > impossible for any THP containing a part of that file to be VA-aligned in every > process it is mapped in.
Yes, so let me re-polish this patch. Thanks.
>> So before adding that VA alignment changes, only allow building the large folio >> mapping for anonymous shmem: >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 936377220b77..9e4d51826d23 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -4786,7 +4786,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) >> folio = page_folio(page); >> nr_pages = folio_nr_pages(folio); >> >> - if (unlikely(userfaultfd_armed(vma))) { >> + if (unlikely(userfaultfd_armed(vma)) || !vma_is_anon_shmem(vma)) { > > If the above theoretical flow for fork & mremap is valid, then I don't think > this is sufficient. > >> nr_pages = 1; >> } else if (nr_pages > 1) { >> unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * >> PAGE_SIZE); >> >>> Additionally, I think this path will end up mapping the entire folio (as long as >>> it fits in the VMA). But this bypasses the fault-around configuration. As I >>> think I mentioned against the RFC, this will inflate the RSS of the process and >>> can cause behavioural changes as a result. I believe the current advice is to >>> disable fault-around to prevent this kind of bloat when needed. >> >> With above change, I do not think this is a problem? since users already want to >> use mTHP for anonymous shmem. >> >>> It might be that you need a special variant of finish_fault() for shmem? >>> >>> >>>> + } >>>> 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)) { >>>> + for (i = 0; i < nr_pages; i++) >>>> + update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i); >>>> ret = VM_FAULT_NOPAGE; >>>> + goto unlock; >>>> } >>>> + set_pte_range(vmf, folio, page, nr_pages, addr); >>>> + type = is_cow ? MM_ANONPAGES : mm_counter_file(folio); >>>> + add_mm_counter(vma->vm_mm, type, nr_pages); >>>> + ret = 0; >>>> + >>>> +unlock: >>>> pte_unmap_unlock(vmf->pte, vmf->ptl); >>>> return ret; >>>> }
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