From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oa1-f48.google.com (mail-oa1-f48.google.com [209.85.160.48]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 78B7D38DDB for ; Tue, 20 Aug 2024 02:43:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.160.48 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724121788; cv=none; b=T2nJhX9fi/BL5+j++E0VpX5D0PYFji1Drs6goyaxdjh29Esht8ftUbGxb4A2adO3e358oMjXYwy8zRIeMLMZKwwBdtD9+iSke+aLS5Mw/tPedZ9LK45i0SyM6loV3pgPCqByxnYQchBQHtV+gQQZYwBB2fnjRcSLRJ5UVu5moO4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724121788; c=relaxed/simple; bh=MiM+Baco+drbu6/oLZP+MltqU1AZ1t/ILbuUxIiehqg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=osKSx8/o/l+7NvuzenCHxjauPhPckmH1lr6M5BZkUboph1M8qr3ytBxc1o/aXoMp5fodohraxaNr9nXlOeiOdVPUvqCcEkxJ8044E0KYH1o+7TmkwevE6/wpPEZBkK1bsl8fxFFl3F7Z0eNhyDFbJGE8QzBpZGXcV1a8aexVSYA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=rivosinc.com; spf=pass smtp.mailfrom=rivosinc.com; dkim=pass (2048-bit key) header.d=rivosinc-com.20230601.gappssmtp.com header.i=@rivosinc-com.20230601.gappssmtp.com header.b=SVsBefwg; arc=none smtp.client-ip=209.85.160.48 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=rivosinc.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=rivosinc.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=rivosinc-com.20230601.gappssmtp.com header.i=@rivosinc-com.20230601.gappssmtp.com header.b="SVsBefwg" Received: by mail-oa1-f48.google.com with SMTP id 586e51a60fabf-27010ae9815so3003380fac.2 for ; Mon, 19 Aug 2024 19:43:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=rivosinc-com.20230601.gappssmtp.com; s=20230601; t=1724121785; x=1724726585; darn=vger.kernel.org; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:from:to :cc:subject:date:message-id:reply-to; bh=IFOzHWvbak6LE8xz7ZLjSo/pB2FEXgmHkGdVOLCa8Qg=; b=SVsBefwgA/CSd0Ji5kQ2L9s3wC0qmFJCJ6wwf1ap+TLIUsawjaiwN5k9OIRJ+Z+y+J Su6Df1Sa1ssjirYjDq3GUsPZjS7tjYZncKKwysDGiGY+w6GV8jBwbWj7QIvcVh72FwVc H2abm3uvNvs4zG+p5j3osmGIB3pV5ETgwX3YLA0Gk7OIfouEXhwNWraUaqkoHXd0xhq2 lumGWOLQF3BK03bT3Zr6uh5M5xQ7U7utVrAxKIAHgm76OltUUTDTyNHEYOKLyopR8yLW jL+1yYazJ6zvD5Ar0tHwen0fUW7V1UNOXA6dqQAiqJCjs8vNgoLf5mJlvibglxA9iQaW Fu5w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1724121785; x=1724726585; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=IFOzHWvbak6LE8xz7ZLjSo/pB2FEXgmHkGdVOLCa8Qg=; b=qtt8fW697+9c+IUDb70uTQ8rsrD8ZJzvz3gsqGbJN1VlEncgZ/y8xs0IyBbcZDBEvd AAjHz+Z1u9BPXiOkPAGmc9eL7iHocZe2zjPliv3jefaF7fmfFs2j7YC3k0EdRYf01HCm uYgQztj7wp5iFRGwCzEiXSpVzeyKfLm5spIMlskHnjT05bAU7oy+dPLbh/ADEFWgxCUF p7D3yiEBJorFlJTS6EAn65/vqS9CCTUnCAJ59/9ecRfizBed5e8x0NtsCoYRGJEdWG4X 4qtXmdjhkwq5nH8tVvKuOAkpjaBbANpDguI/Z8JBbO5ZJvMvs6liXSjQ1h1Zd0HkP4iA 3Izw== X-Forwarded-Encrypted: i=1; AJvYcCVsf32+cRsqGDNMQ3AkueLFMPSbhZpgBhmDbL06nLTf2eJ7ZsdD4h0nbbIZdKTgRxmfIC9NgoP5KD4=@vger.kernel.org X-Gm-Message-State: AOJu0YwVK6UWg0JjMT++cu8eztUC/HqhCmTXgQaAv6OgOKSUepZZMOW5 cdvmPtY+CS+6OXiYqBoaA4b41E0p97Zwo8JtFPnQpfBXOTuXKb1dzDOkqpTTPrA= X-Google-Smtp-Source: AGHT+IFyL2JXMETglLC3qz3X/+8Y7jUI/5f0Wd58GItgifYO6esi1EP2LeOLUWy1KtEsosdOCx3dgA== X-Received: by 2002:a05:6870:e387:b0:24e:8987:6f34 with SMTP id 586e51a60fabf-2701c354855mr15623458fac.3.1724121785232; Mon, 19 Aug 2024 19:43:05 -0700 (PDT) Received: from ghost ([2601:647:6700:64d0:7ef:3ad1:b795:8617]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7127af1e415sm7473468b3a.175.2024.08.19.19.43.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Aug 2024 19:43:04 -0700 (PDT) Date: Mon, 19 Aug 2024 19:43:01 -0700 From: Charlie Jenkins To: Levi Zim Cc: Palmer Dabbelt , cyy@cyyself.name, alexghiti@rivosinc.com, Paul Walmsley , aou@eecs.berkeley.edu, shuah@kernel.org, corbet@lwn.net, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH v3 1/3] riscv: mm: Use hint address in mmap if available Message-ID: References: Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Aug 20, 2024 at 09:48:50AM +0800, Levi Zim wrote: > On 2024-08-20 01:00, Charlie Jenkins wrote: > > On Mon, Aug 19, 2024 at 01:55:57PM +0800, Levi Zim wrote: > > > On 2024-03-22 22:06, Palmer Dabbelt wrote: > > > > On Thu, 01 Feb 2024 18:28:06 PST (-0800), Charlie Jenkins wrote: > > > > > On Wed, Jan 31, 2024 at 11:59:43PM +0800, Yangyu Chen wrote: > > > > > > On Wed, 2024-01-31 at 22:41 +0800, Yangyu Chen wrote: > > > > > > > On Tue, 2024-01-30 at 17:07 -0800, Charlie Jenkins wrote: > > > > > > > > On riscv it is guaranteed that the address returned by mmap is less > > > > > > > > than > > > > > > > > the hint address. Allow mmap to return an address all the way up to > > > > > > > > addr, if provided, rather than just up to the lower address space. > > > > > > > > > > This provides a performance benefit as well, allowing > > > > > > mmap to exit > > > > > > > > after > > > > > > > > checking that the address is in range rather than searching for a > > > > > > > > valid > > > > > > > > address. > > > > > > > > > > It is possible to provide an address that uses at most the same > > > > > > > > number > > > > > > > > of bits, however it is significantly more computationally expensive > > > > > > > > to > > > > > > > > provide that number rather than setting the max to be the hint > > > > > > > > address. > > > > > > > > There is the instruction clz/clzw in Zbb that returns the highest > > > > > > > > set > > > > > > > > bit > > > > > > > > which could be used to performantly implement this, but it would > > > > > > > > still > > > > > > > > be slower than the current implementation. At worst case, half of > > > > > > > > the > > > > > > > > address would not be able to be allocated when a hint address is > > > > > > > > provided. > > > > > > > > > > Signed-off-by: Charlie Jenkins > > > > > > > > --- > > > > > > > >  arch/riscv/include/asm/processor.h | 27 +++++++++++--------------- > > > > > > > > - > > > > > > > >  1 file changed, 11 insertions(+), 16 deletions(-) > > > > > > > > > > diff --git a/arch/riscv/include/asm/processor.h > > > > > > > > b/arch/riscv/include/asm/processor.h > > > > > > > > index f19f861cda54..8ece7a8f0e18 100644 > > > > > > > > --- a/arch/riscv/include/asm/processor.h > > > > > > > > +++ b/arch/riscv/include/asm/processor.h > > > > > > > > @@ -14,22 +14,16 @@ > > > > > > > > > > > > > > > >  #include > > > > > > > > > > > > > > > > -#ifdef CONFIG_64BIT > > > > > > > > -#define DEFAULT_MAP_WINDOW    (UL(1) << (MMAP_VA_BITS - 1)) > > > > > > > > -#define STACK_TOP_MAX        TASK_SIZE_64 > > > > > > > > - > > > > > > > >  #define arch_get_mmap_end(addr, len, flags)            \ > > > > > > > >  ({                                \ > > > > > > > >      unsigned long > > > > > > > > mmap_end;                    \ > > > > > > > >      typeof(addr) _addr = (addr);                \ > > > > > > > > -    if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && > > > > > > > > is_compat_task())) \ > > > > > > > > +    if ((_addr) == 0 ||                    \ > > > > > > > > +        (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) ||    \ > > > > > > > > +        ((_addr + len) > BIT(VA_BITS - > > > > > > > > 1)))            \ > > > > > > > >          mmap_end = STACK_TOP_MAX;            \ > > > > > > > > -    else if ((_addr) >= VA_USER_SV57) \ > > > > > > > > -        mmap_end = STACK_TOP_MAX;            \ > > > > > > > > -    else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= > > > > > > > > VA_BITS_SV48)) \ > > > > > > > > -        mmap_end = VA_USER_SV48;            \ > > > > > > > >      else                            \ > > > > > > > > -        mmap_end = VA_USER_SV39;            \ > > > > > > > > +        mmap_end = (_addr + len);            \ > > > > > > > >      mmap_end;                        \ > > > > > > > >  }) > > > > > > > > > > > > > > > > @@ -39,17 +33,18 @@ > > > > > > > >      typeof(addr) _addr = (addr);                \ > > > > > > > >      typeof(base) _base = (base);                \ > > > > > > > >      unsigned long rnd_gap = DEFAULT_MAP_WINDOW - (_base);    \ > > > > > > > > -    if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && > > > > > > > > is_compat_task())) \ > > > > > > > > +    if ((_addr) == 0 ||                    \ > > > > > > > > +        (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) ||    \ > > > > > > > > +        ((_addr + len) > BIT(VA_BITS - > > > > > > > > 1)))            \ > > > > > > > >          mmap_base = (_base);                \ > > > > > > > > -    else if (((_addr) >= VA_USER_SV57) && (VA_BITS >= > > > > > > > > VA_BITS_SV57)) \ > > > > > > > > -        mmap_base = VA_USER_SV57 - rnd_gap; \ > > > > > > > > -    else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= > > > > > > > > VA_BITS_SV48)) \ > > > > > > > > -        mmap_base = VA_USER_SV48 - rnd_gap; \ > > > > > > > >      else                            \ > > > > > > > > -        mmap_base = VA_USER_SV39 - rnd_gap; \ > > > > > > > > +        mmap_base = (_addr + len) - rnd_gap; \ > > > > > > > >      mmap_base;                        \ > > > > > > > >  }) > > > > > > > > > > > > > > > > +#ifdef CONFIG_64BIT > > > > > > > > +#define DEFAULT_MAP_WINDOW    (UL(1) << (MMAP_VA_BITS - 1)) > > > > > > > > +#define STACK_TOP_MAX        TASK_SIZE_64 > > > > > > > >  #else > > > > > > > >  #define DEFAULT_MAP_WINDOW    TASK_SIZE > > > > > > > >  #define STACK_TOP_MAX        TASK_SIZE > > > > > > > > > > I have carefully tested your patch on qemu with sv57. A > > > > > > bug that > > > > > > > needs > > > > > > > to be solved is that mmap with the same hint address without > > > > > > > MAP_FIXED > > > > > > > set will fail the second time. > > > > > > > > Userspace code to reproduce the bug: > > > > > > > > #include > > > > > > > #include > > > > > > > #include > > > > > > > > void test(char *addr) { > > > > > > >     char *res = mmap(addr, 4096, PROT_READ | PROT_WRITE, > > > > > > > MAP_ANONYMOUS > > > > > > > > MAP_PRIVATE, -1, 0); > > > > > > >     printf("hint %p got %p.\n", addr, res); > > > > > > > } > > > > > > > > int main (void) { > > > > > > >     test(1<<30); > > > > > > >     test(1<<30); > > > > > > >     test(1<<30); > > > > > > >     return 0; > > > > > > > } > > > > > > > > output: > > > > > > > > hint 0x40000000 got 0x40000000. > > > > > > > hint 0x40000000 got 0xffffffffffffffff. > > > > > > > hint 0x40000000 got 0xffffffffffffffff. > > > > > > > > output on x86: > > > > > > > > hint 0x40000000 got 0x40000000. > > > > > > > hint 0x40000000 got 0x7f9171363000. > > > > > > > hint 0x40000000 got 0x7f9171362000. > > > > > > > > It may need to implement a special arch_get_unmapped_area and > > > > > > > arch_get_unmapped_area_topdown function. > > > > > > > > > > > > > This is because hint address < rnd_gap. I have tried to let mmap_base = > > > > > > min((_addr + len), (base) + TASK_SIZE - DEFAULT_MAP_WINDOW). However it > > > > > > does not work for bottom-up while ulimit -s is unlimited. You said this > > > > > > behavior is expected from patch v2 review. However it brings a new > > > > > > regression even on sv39 systems. > > > > > > > > > > > > I still don't know the reason why use addr+len as the upper-bound. I > > > > > > think solution like x86/arm64/powerpc provide two address space switch > > > > > > based on whether hint address above the default map window is enough. > > > > > > > > > > > Yep this is expected. It is up to the maintainers to decide. > > > > Sorry I forgot to reply to this, I had a buffer sitting around somewhere > > > > but I must have lost it. > > > > > > > > I think Charlie's approach is the right way to go.  Putting my userspace > > > > hat on, I'd much rather have my allocations fail rather than silently > > > > ignore the hint when there's memory pressure. > > > > > > > > If there's some real use case that needs these low hints to be silently > > > > ignored under VA pressure then we can try and figure something out that > > > > makes those applications work. > > > I could confirm that this patch has broken chromium's partition allocator on > > > riscv64. The minimal reproduction I use is chromium-mmap.c: > > > > > > #include > > > #include > > > > > > int main() { > > >     void* expected = (void*)0x400000000; > > >     void* addr = mmap(expected, 17179869184, PROT_NONE, > > > MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); > > >     if (addr != expected) { > > It is not valid to assume that the address returned by mmap will be the > > hint address. If the hint address is not available, mmap will return a > > different address. > > Oh, sorry I didn't make it clear what is the expected behavior. > The printf here is solely for debugging purpose and I don't mean that > chromium expect it will get the hint address. The expected behavior is that > both the two mmap calls will succeed. > > > >         printf("Not expected address: %p != %p\n", addr, expected); > > >     } > > >     expected = (void*)0x3fffff000; > > >     addr = mmap(expected, 17179873280, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, > > > -1, 0); > > >     if (addr != expected) { > > >         printf("Not expected address: %p != %p\n", addr, expected); > > >     } > > >     return 0; > > > } > > > > > > The second mmap fails with ENOMEM. Manually reverting this commit fixes the > > > issue for me. So I think it's clearly a regression and breaks userspace. > > > > > The issue here is that overlapping memory is being requested. This > > second mmap will never be able to provide an address at 0x3fffff000 with > > a size of 0x400001000 since mmap just provided an address at 0x400000000 > > with a size of 0x400000000. > > > > Before this patch, this request causes mmap to return a completely > > arbitrary value. There is no reason to use a hint address in this manner > > because the hint can never be respected. Since an arbitrary address is > > desired, a hint of zero should be used. > > > > This patch causes the behavior to be more deterministic. Instead of > > providing an arbitrary address, it causes the address to be less than or > > equal to the hint address. This allows for applications to make > > assumptions about the returned address. > > About the overlap, of course the partition allocator's request for > overlapped vma seems unreasonable. > > But I still don't quite understand why mmap cannot use an address higher > than the hint address. > The hint address, after all, is a hint, not a requirement. Yes that is fair. A "hint" that does not guarantee anything is useless so architectures have abused the term quite a bit. > > Quoting the man page: > > > If another mapping already exists there, the kernel picks > > a new address that may or may not depend on the hint. The > > address of the new mapping is returned as the result of the call. > So for casual programmers that only reads man page but not architecture > specific kernel > documentation, the current behavior of mmap on riscv64 failing on overlapped > address ranges > are quite surprising IMO. The man pages for riscv are in desperate need of attention. I have submitted a couple of updates to them recently, but there is a lot more work to be done to help developers. > > And quoting the man page again about the errno: > > >       ENOMEM No memory is available. > > > >       ENOMEM The process's maximum number of mappings would have been > >              exceeded.  This error can also occur for munmap(), when > >              unmapping a region in the middle of an existing mapping, > >              since this results in two smaller mappings on either side > >              of the region being unmapped. > > > >       ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, > >              described in getrlimit(2), would have been exceeded. > > > >       ENOMEM We don't like addr, because it exceeds the virtual address > >              space of the CPU. > > > > There's no matching description for the ENOMEM returned here. > I would suggest removing "because it exceeds the virtual address > space of the CPU." from the last item if the ENOMEM behavior here > is expected. This ENOMEM means something like "no memory available in the requested region". > > > This code is unfortunately relying on the previously mostly undefined > > behavior of the hint address in mmap. > Although I haven't read the code of chromium's partition allocator to judge > whether it should > be improved or fixed for riscv64, I do know that the kernel "don't break > userspace" and > "never EVER blame the user programs". The hint address design of mmap is a tricky one because it is largely implementation defined and what the man pages say is not how it is implemented in most architectures! > > The goal of this patch is to help > > developers have more consistent mmap behavior, but maybe it is necessary > > to hide this behavior behind an mmap flag. > Thank you for helping to shape a more consistent mmap behavior. > I think this should be fixed ASAP either by allowing the hint address to be > ignored > (as suggested by the Linux man page), or hide this behavior behind an mmap > flag as you said. Having a flag could also lead to a generic way of defining this behavior. Other architectures do not provide a way for applications to guarantee that some number of bits are left unused in a virtual address, and that was one of the motivating design goals here. - Charlie > > > - Charlie > > > > > See alsohttps://github.com/riscv-forks/electron/issues/4 > > > > > > > > - Charlie > > > Sincerely, > > > Levi > > >