From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8C5C3158DAE for ; Wed, 24 Apr 2024 09:57:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713952632; cv=none; b=A5qEXDpTcxyWjTNWHr03qQNPgK55eGyrRcCJwlnejWe5WMei9ug/xtrN+PRQyXvs49LbOVrxUWd9gKhz8GFiTwAaOtpOP93J5sXdVVDlWuVeGhTl2Vb0kcfBZ7eSjr+M9NidJHSFbjGxf30XtpOV3U7WbDvjsOiHH9QdzC9yGz8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713952632; c=relaxed/simple; bh=xElqm0mzanKgyLcHnkXCZf7n1tImjyjJ3L0Bvz1st3E=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Cfy3Syjt35mvDe9Jx+bcVAvnTJnuf1/HsceBY6FCGallYaKCqs0KFdlk0imRfcoweP/W/IxY8TCuePSfIU0Gvi10RghyDJSbi44X+jn77/VD7i42fo1FJRHoQnapGzobnP9fa2o/Q/E7ZE8gUhq2OzBAHQ9J8rRbXTXh3S4GYY0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 8B02D339; Wed, 24 Apr 2024 02:57:37 -0700 (PDT) Received: from [10.1.25.156] (unknown [10.1.25.156]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6C4A93F73F; Wed, 24 Apr 2024 02:57:08 -0700 (PDT) Message-ID: <7ae62c3f-918a-4778-badb-8f7ca74328d8@arm.com> Date: Wed, 24 Apr 2024 10:57:07 +0100 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: [RFC PATCH 1/5] mm: memory: extend finish_fault() to support large folio Content-Language: en-GB To: Baolin Wang , akpm@linux-foundation.org, hughd@google.com Cc: willy@infradead.org, david@redhat.com, wangkefeng.wang@huawei.com, 21cnbao@gmail.com, ying.huang@intel.com, shy828301@gmail.com, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <358aefb1858b63164894d7d8504f3dae0b495366.1713755580.git.baolin.wang@linux.alibaba.com> <6aa25e2a-a6b6-4ab7-8300-053ca3c0d748@arm.com> <6c418d70-a75d-4019-a0f5-56a61002d37a@arm.com> <5f949c1f-c56e-4227-af60-05a2a19f4c2e@linux.alibaba.com> From: Ryan Roberts In-Reply-To: <5f949c1f-c56e-4227-af60-05a2a19f4c2e@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 24/04/2024 10:26, Baolin Wang wrote: > > > On 2024/4/24 16:07, Ryan Roberts wrote: >> On 24/04/2024 04:23, Baolin Wang wrote: >>> >>> >>> On 2024/4/23 19:03, Ryan Roberts wrote: >>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>> Add large folio mapping establishment support for finish_fault() as a >>>>> preparation, >>>>> to support multi-size THP allocation of anonymous shared pages in the >>>>> following >>>>> patches. >>>>> >>>>> Signed-off-by: Baolin Wang >>>>> --- >>>>>    mm/memory.c | 25 ++++++++++++++++++------- >>>>>    1 file changed, 18 insertions(+), 7 deletions(-) >>>>> >>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>> index b6fa5146b260..094a76730776 100644 >>>>> --- a/mm/memory.c >>>>> +++ b/mm/memory.c >>>>> @@ -4766,7 +4766,10 @@ 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; >>>>> +    int nr_pages, i; >>>>> +    unsigned long addr; >>>>>          /* Did we COW the page? */ >>>>>        if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) >>>>> @@ -4797,22 +4800,30 @@ vm_fault_t finish_fault(struct vm_fault *vmf) >>>>>                return VM_FAULT_OOM; >>>>>        } >>>>>    +    folio = page_folio(page); >>>>> +    nr_pages = folio_nr_pages(folio); >>>>> +    addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE); >>>> >>>> I'm not sure this is safe. IIUC, finish_fault() is called for any file-backed >>>> mapping. So you could have a situation where part of a (regular) file is mapped >>>> in the process, faults and hits in the pagecache. But the folio returned by the >>>> pagecache is bigger than the portion that the process has mapped. So you now >>>> end >>>> up mapping beyond the VMA limits? In the pagecache case, you also can't assume >>>> that the folio is naturally aligned in virtual address space. >>> >>> Good point. Yes, I think you are right, I need consider the VMA limits, and I >>> should refer to the calculations of the start pte and end pte in >>> do_fault_around(). >> >> You might also need to be careful not to increase reported RSS. I have a vague >> recollection that David once mentioned a problem with fault-around because it >> causes the reported RSS to increase for the process and this could lead to >> different decisions in other places. IIRC Redhat had an advisory somewhere with >> suggested workaround being to disable fault-around. For the anon-shared memory >> case, it shouldn't be a problem because the user has opted into allocating >> bigger blocks, but there may be a need to ensure we don't also start eagerly >> mapping regular files beyond what fault-around is configured for. > > Thanks for reminding. And I also agree with you that this should not be a > problem since user has selected the larger folio, which is not the same as > fault-around. > >>>>>        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); >>>>> - >>>>> -        set_pte_range(vmf, folio, page, 1, vmf->address); >>>>> -        ret = 0; >>>>> -    } else { >>>>> +    if (nr_pages == 1 && vmf_pte_changed(vmf)) { >>>>>            update_mmu_tlb(vma, vmf->address, vmf->pte); >>>>>            ret = VM_FAULT_NOPAGE; >>>>> +        goto unlock; >>>>> +    } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) { >>>> >>>> I think you have grabbed this from do_anonymous_page()? But I'm not sure it >>>> works in the same way here as it does there. For the anon case, if userfaultfd >>>> is armed, alloc_anon_folio() will only ever allocate order-0. So we end up in >>> >>> IMO, the userfaultfd validation should do in the vma->vm_ops->fault() callback, >>> to make sure the nr_pages is always 1 if userfaultfd is armed. >> >> OK. Are you saying there is already logic to do that today? Great! > > I mean I should add the userfaultfd validation in shmem_fault(), and may be need > add a warning in finish_fault() to catch this issue if other > vma->vm_ops->fault() will support large folio allocation? > > WARN_ON(nr_pages > 1 && userfaultfd_armed(vma)); That adds quite a subtle requirement to vm_ops::fault() though, which I guess is implemented in a lot of places. It would be better if it could be handled centrally - i.e. that all the ptes are either none or a uffd marker? I'm sure there would be corner cases to think about if taking that route.