From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com [115.124.30.130]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 358DA3613C for ; Wed, 8 May 2024 09:32:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715160728; cv=none; b=JDYMvNDxJmslZr4rxHJ+/x/dOiEasbeCqxuFjD+Lrk3CC31D6CFxWwb5eFL0seJ1x0gS4rmoApcVn7holTSYqwvuEFpVdm4APy0VnxgIfqaI90VuNQWAnJP5SK/pv9ZBweaD2C5BVN6FavrkIpY8LVth/6Bq+kUvk3UQr2jHBvA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715160728; c=relaxed/simple; bh=N2qCMFWNGt/971GXcPVaxJJfzsYq+ZVjHxs7EqIppY8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WadLL6loJCw4449uXLMAHpMHvL/ldRHrCeEArkpxh65ohPTuS5NH1MR/qGBHmqWJCpxpwSGJco875Yr6yDNSom5BuP9FBse/9FWc/Vo7zyrgB0lBWHm0KTqaPIHIFbm9bSZ3Hox02RpAMJHrhEYcvGzvLLWyj31va/MVfvTsq3U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=WCVhhFG6; arc=none smtp.client-ip=115.124.30.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="WCVhhFG6" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1715160720; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=X6ll+DleuF5VzRVyJHMdLVkT8eka2pdj3LoVi8Wvj6M=; b=WCVhhFG62S78M+b/R3RdUXRirlY90IjviSDTqPC2ZHEY1C+4ByEdXGLwMEEoDeZSJ/+RLptL/Th/EcAVQvWw9fX0rn0KYqFet15AlM+5qjMOaJVoGXsXW9eTXGzqEx4NP55PBcBMQhKaOploGvmwY7mNZ5AJXqySWwgkOy6wrW4= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033068173054;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0W63DC6R_1715160717; Received: from 30.97.56.69(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0W63DC6R_1715160717) by smtp.aliyun-inc.com; Wed, 08 May 2024 17:31:58 +0800 Message-ID: <3d87da24-7887-4912-abcf-14062e8514de@linux.alibaba.com> Date: Wed, 8 May 2024 17:31:57 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/8] mm: memory: extend finish_fault() to support large folio To: Ryan Roberts , akpm@linux-foundation.org, hughd@google.com Cc: willy@infradead.org, david@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ying.huang@intel.com, 21cnbao@gmail.com, shy828301@gmail.com, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <13939ade-a99a-4075-8a26-9be7576b7e03@arm.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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 >>>> --- >>>>   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; >>>>   }