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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: D2F9E120017 X-Stat-Signature: 66zmnwpr8xgzc3p14i6yr65g5sfpez4o X-Rspam-User: X-HE-Tag: 1715158393-114397 X-HE-Meta: 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 tjcHuEJP fAF1EyBABC8E0Rn5HgJeAulAii1Ay3HKqHmsgxsBsVha0Lt62sJek8pPKdA5T4iQJabTH0fo6JUEf6xM4dU+Zs26/IDmFXJcfn2YRuNglESG5OlZ2gpmjwqyWk0foGiRxrMywqdCxXMZmH6wDyn9Ph99xvuhLuFoCKoDaBotrzqllN9oyIwZLEPfVAfd9zlsNPz0ilFnLNSD5mJregWI3qLfUsRUaNQDxja+I14wvl/C7RkVgnAqw5HMnug== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >>> --- >>>   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 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. > >> 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. > > 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; >>>   }