From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794232966; b=1RoyedZZgvazkBDoK2F/cIXu/vNkVRbFCmfLBjtCa8letlHtym2OF0nhkLsJBqKY2tMC OnjDu3Pjge1cRyTkHwyk+GvNZSiN2GPj1eDEw+z06uJpCYWwx5k/b2U5Fbdw0vqRAhGBI xy+ujSW4clRi0OSqlYWUj9URU9Sy8EDi3UL2oZLQ84moUI903ciQx4W6y/yr6dd7ktnK9 Ncq3C/vpCsFgRGs8RQurbQeVAYx3LqmuLWEw9l+jkOiAQkUUHsdMDL79/SSU2pd6J2vOp CvPVtvRYUNkiZYlgxaF+Bu6c5+zv0ufP/eSZwQ6AVyS2acLoYrOAB4wc7bL6S/F5mUA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794232966; h=Received:Message-ID:Date:MIME-Version:User-Agent:Subject:To:Cc: References:Content-Language:From:In-Reply-To:Content-Type: Content-Transfer-Encoding; bh=MBu8kJlSTZ+tXQZadvNH/wWLKbOoYGqZ6MrFHv8UHLI=; b=BeAbkZNHczqMYfjCMOqO6Aq5imIB/yFw5agt1QEZ7/S3wYcUk9MZVwLEIAoh6VTEOfTg cZzHjJXiAvsEzhtiUPVd4pahIhXE+1Tjf3pMqHIjkkK68KhwfK7Ic77uPBmodq9BkCgU3 cTsCkCfAG+0GHqRTzvpt22lyl9fhqpozSrxzAaLxQzwXKIM5Lnis7z7TFOhIMM/ImAc5R TiNzVCQ56GZJ8Ly/ws8gqiGz9pRgkkmwWIn/q/0/8UMezmZh1BFEZtqQxEWBe4sKbXB1N 40ZA/Hd5d8Nhf7IL8/0JSxCHXUtK4Plo+z6yrIg3At/YdEpon4rjMR2ej5f/cvxis4A== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E29AF606A321 for ; Mon, 9 Nov 2026 14:02:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4A5B1F0089F; Mon, 9 Nov 2026 14:02:41 +0000 (UTC) Message-ID: <6f01317a-1536-4747-ba11-23e27461a694@linux-m68k.org> Date: Tue, 10 Nov 2026 00:02:39 +1000 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/memory: use pmdp_get_lockless() for the pmd read in do_fault_around() To: Hui Zhu , "David Hildenbrand (Arm)" , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Geert Uytterhoeven , Sam Creasey Cc: Hui Zhu , stable+noautosel@kernel.org, linux-m68k@lists.linux-m68k.org References: <20261104091050.117076-1-hui.zhu@linux.dev> <88a6bf17-2189-44a8-8255-50b7ee1d80f2@linux.dev> Content-Language: en-US From: Greg Ungerer In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Hui, On 9/11/26 15:50, Hui Zhu wrote: > 2026/11/6 20:08, Greg Ungerer: > > Hi Greg, > >> Hi Hui, >> >> On 6/11/26 13:03, Hui Zhu wrote: >>> 2026/11/5 20:00, David Hildenbrand (Arm) : >>>> On 11/4/26 10:10, Hui Zhu wrote: >>>>> From: Hui Zhu >>>>> >>>>> KCSAN reports a data-race in do_read_fault / pmd_install; the same code >>>>> is in mainline: >>>>> >>>>>     write (marked) to 0xffff8d52a75fb810 of 8 bytes by task 27487 on cpu 5: >>>>>      native_set_pmd arch/x86/include/asm/pgtable_64.h:83 [inline] >>>>>      pmd_populate arch/x86/include/asm/pgalloc.h:84 [inline] >>>>>      pmd_install+0xe0/0x120 mm/memory.c:444 >>>>>      finish_fault+0x5bc/0xb00 mm/memory.c:5204 >>>>>      do_cow_fault mm/memory.c:5432 [inline] >>>>>      do_fault+0x5d0/0x6e0 mm/memory.c:5526 >>>>>      handle_mm_fault+0x211/0x4c0 mm/memory.c:6218 >>>>>      __mm_populate+0x144/0x270 mm/gup.c:2207 >>>>>      __do_sys_mlockall+0x2cd/0x380 mm/mlock.c:764 >>>>> >>>>>     read to 0xffff8d52a75fb810 of 8 bytes by task 27527 on cpu 9: >>>>>      do_fault_around mm/memory.c:5341 [inline] >>>>>      do_read_fault+0x162/0x670 mm/memory.c:5381 >>>>>      do_fault+0x31a/0x6e0 mm/memory.c:5524 >>>>>      handle_mm_fault+0x211/0x4c0 mm/memory.c:6218 >>>>>      faultin_page_range+0x227/0x530 mm/gup.c:2156 >>>>>      __x64_sys_madvise+0x67/0x80 mm/madvise.c:1479 >>>>> >>>>>     value changed: 0x0000000000000000 -> 0x000000001386c067 >>>>> >>>>> Both paths only hold mmap_lock for read, so do_fault_around() tests >>>>> vmf->pmd while pmd_install() populates it under the pmd lock.  The race is >>>>> benign: a stale pmd_none() only wastes the prealloc_pte, which do_fault() >>>>> frees again, and a pmd seen as populated only makes ->map_pages() find no >>>>> PTE and do_read_fault() fall back to __do_fault(). >>>>> >>>>> The load still has to be lockless-safe.  The store side already is: on >>>>> i386-PAE, pmd_install() publishes both halves of the pmd with a single >>>>> cmpxchg8b.  The load side is not.  pmdp_get() is a plain READ_ONCE() over >>>>> a pmd_t that is wider than a word there, so it becomes two 32-bit loads >>>>> and can return the low half of one pmd together with the high half of >>>>> another.  Use pmdp_get_lockless(), the accessor for pmd reads that do not >>>>> hold the pmd lock: it re-reads until both halves come from the same pmd. >>>>> It is also what __pte_offset_map() already uses on this very pmd a few >>>>> instructions later. >>>>> >>>>> Only X86_32+X86_PAE and SH+X2TLB compile the retry loop, these being the >>>>> GUP_GET_PXX_LOW_HIGH selectors that have more than two page table levels. >>>>> mips32 with 64-bit physical addresses selects GUP_GET_PXX_LOW_HIGH too, >>>>> but is 2-level and so falls back to pmdp_get().  On every other >>>>> configuration, x86-64 included, pmdp_get_lockless() is pmdp_get(): the >>>>> pmd is loaded in one go, the READ_ONCE() silences the report the same >>>>> way, and there is no functional change. >>>>> >>>>> Fixes: 7267ec008b5c ("mm: postpone page table allocation until we have page to map") >>>>> Cc: stable+noautosel@kernel.org # KCSAN annotation only, no functional change >>>>> Signed-off-by: Hui Zhu >>>>> Acked-by: Lorenzo Stoakes >>>>> --- >>>>> Changelog: >>>>> v4: >>>>> Commit message only, the diff is identical to v3.  Spell out the affected >>>>> arches and add the lore links for v1-v3 (Lorenzo Stoakes).  Reword the >>>>> cmpxchg8b sentence: the store is already atomic, it is the load that tears. >>>>> Add the Fixes: tag (David Hildenbrand). >>>>> v3: >>>>> https://lore.kernel.org/all/20261103014749.96764-1-hui.zhu@linux.dev/ >>>>> Shorten the comment to one line (David Hildenbrand). >>>>> v2: >>>>> https://lore.kernel.org/all/20261102093120.92654-1-hui.zhu@linux.dev/ >>>>> Use pmdp_get_lockless() instead of pmdp_get() (David Hildenbrand). >>>>> v1: >>>>> https://lore.kernel.org/all/20261102031308.66177-1-hui.zhu@linux.dev/ >>>>> >>>>>    mm/memory.c | 3 ++- >>>>>    1 file changed, 2 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>> index 0fb676dd2bd63..f25c2ce0ef153 100644 >>>>> --- a/mm/memory.c >>>>> +++ b/mm/memory.c >>>>> @@ -6017,7 +6017,8 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) >>>>>        to_pte = min3(from_pte + nr_pages, (pgoff_t)PTRS_PER_PTE, >>>>>                  pte_off + vma_pages(vmf->vma) - vma_off) - 1; >>>>>    -    if (pmd_none(*vmf->pmd)) { >>>>> +    /* pmd_install() rechecks pmd_none() under the pmd lock. */ >>>>> +    if (pmd_none(pmdp_get_lockless(vmf->pmd))) { >>>>>            vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm); >>>>>            if (!vmf->prealloc_pte) >>>>>                return VM_FAULT_OOM; >>>> Fun, this breaks some odd m68k configs because they do some legacy weird stuff with pmd_none(). >>>> >>>> I'll throw in the following cleanup. CCing m68k maintainers: I would drag >>>> this along with this fix here unless you disagree. >>>> >>>>   From e3c739933a04743e3923f6f39a603e93f2700304 Mon Sep 17 00:00:00 2001 >>>> From: "David Hildenbrand (Arm)" >>>> Date: Thu, 5 Nov 2026 12:49:38 +0100 >>>> Subject: [PATCH] m68k/mm: simplify >>>>    pmd_none()/pmd_val()/pmd_present()/pmd_bad() >>>> >>>> pmd_t on m68k used to contain 16 words. Commit ef22d8abd876 ("m68k: >>>> mm: Restructure Motorola MMU page-table layout") reduced Motorola PMDs >>>> to one word; Sun3 and ColdFire use word-sized folded PMDs. >>>> >>>> Use direct member access in pmd_val() and replace the Sun3 and ColdFire >>>> pointer-taking predicate wrappers with inline functions taking values. >>>> >>>> This allows us to write something like: >>>> >>>>     if (pmd_none(pmdp_get(pmdp))) >>>> >>>> While at it remove unused pmd_present2() and commented-out pmd_bad() >>>> on sun3. >>>> >>>> Signed-off-by: David Hildenbrand (Arm) >>>> --- >>>>    arch/m68k/include/asm/mcf_pgtable.h  | 20 +++++++++++++++----- >>>>    arch/m68k/include/asm/page.h         |  2 +- >>>>    arch/m68k/include/asm/sun3_pgtable.h | 23 +++++++++++++++-------- >>>>    3 files changed, 31 insertions(+), 14 deletions(-) >>>> >>>> diff --git a/arch/m68k/include/asm/mcf_pgtable.h b/arch/m68k/include/asm/mcf_pgtable.h >>>> index f45a882238dbb..4efa8e3e1e98a 100644 >>>> --- a/arch/m68k/include/asm/mcf_pgtable.h >>>> +++ b/arch/m68k/include/asm/mcf_pgtable.h >>>> @@ -130,11 +130,21 @@ static inline void pte_clear(struct mm_struct *mm, unsigned long addr, >>>>      #define pte_page(pte)    virt_to_page(__pte_page(pte)) >>>>    -static inline int pmd_none2(pmd_t *pmd) { return !pmd_val(*pmd); } >>>> -#define pmd_none(pmd) pmd_none2(&(pmd)) >>>> -static inline int pmd_bad2(pmd_t *pmd) { return 0; } >>>> -#define pmd_bad(pmd) pmd_bad2(&(pmd)) >>>> -#define pmd_present(pmd) (!pmd_none2(&(pmd))) >>>> +static inline int pmd_none(pmd_t pmd) >>>> +{ >>>> +    return !pmd_val(pmd); >>>> +} >>>> + >>>> +static inline int pmd_bad(pmd_t pmd) >>>> +{ >>>> +    return 0; >>>> +} >>>> + >>>> +static inline int pmd_present(pmd_t pmd) >>>> +{ >>>> +    return !pmd_none(pmd); >>>> +} >>>> + >>>>    static inline void pmd_clear(pmd_t *pmdp) { pmd_val(*pmdp) = 0; } >>>>      diff --git a/arch/m68k/include/asm/page.h b/arch/m68k/include/asm/page.h >>>> index d30f8b2f15927..09a2c020ec992 100644 >>>> --- a/arch/m68k/include/asm/page.h >>>> +++ b/arch/m68k/include/asm/page.h >>>> @@ -17,7 +17,7 @@ >>>>     */ >>>>    #if !defined(CONFIG_MMU) || CONFIG_PGTABLE_LEVELS == 3 >>>>    typedef struct { unsigned long pmd; } pmd_t; >>>> -#define pmd_val(x)    ((&x)->pmd) >>>> +#define pmd_val(x)    ((x).pmd) >>>>    #define __pmd(x)    ((pmd_t) { (x) } ) >>>>    #endif >>>>    diff --git a/arch/m68k/include/asm/sun3_pgtable.h b/arch/m68k/include/asm/sun3_pgtable.h >>>> index 704442a391fd7..5867d24032655 100644 >>>> --- a/arch/m68k/include/asm/sun3_pgtable.h >>>> +++ b/arch/m68k/include/asm/sun3_pgtable.h >>>> @@ -109,14 +109,21 @@ static inline void pte_clear (struct mm_struct *mm, unsigned long addr, pte_t *p >>>>    #define pmd_page(pmd)        virt_to_page((void *)pmd_page_vaddr(pmd)) >>>>      -static inline int pmd_none2 (pmd_t *pmd) { return !pmd_val (*pmd); } >>>> -#define pmd_none(pmd) pmd_none2(&(pmd)) >>>> -//static inline int pmd_bad (pmd_t pmd) { return (pmd_val (pmd) & SUN3_PMD_MASK) != SUN3_PMD_MAGIC; } >>>> -static inline int pmd_bad2 (pmd_t *pmd) { return 0; } >>>> -#define pmd_bad(pmd) pmd_bad2(&(pmd)) >>>> -static inline int pmd_present2 (pmd_t *pmd) { return pmd_val (*pmd) & SUN3_PMD_VALID; } >>>> -/* #define pmd_present(pmd) pmd_present2(&(pmd)) */ >>>> -#define pmd_present(pmd) (!pmd_none2(&(pmd))) >>>> +static inline int pmd_none(pmd_t pmd) >>>> +{ >>>> +    return !pmd_val(pmd); >>>> +} >>>> + >>>> +static inline int pmd_bad(pmd_t pmd) >>>> +{ >>>> +    return 0; >>>> +} >>>> + >>>> +static inline int pmd_present(pmd_t pmd) >>>> +{ >>>> +    return !pmd_none(pmd); >>>> +} >>>> + >>>>    static inline void pmd_clear (pmd_t *pmdp) { pmd_val (*pmdp) = 0; } >>>>      /* >>> Hi David, >>> >>> Thanks for picking this up, and for the m68k cleanup! >>> >>> Tested-by: Hui Zhu >>> >>> Tested with gcc-m68k-linux-gnu 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04), on >>> three configs that between them cover all three m68k families: >>> >>>    amiga_defconfig                    Motorola MMU, PGTABLE_LEVELS=3 >>>    sun3_defconfig                     SUN3,  PGTABLE_LEVELS=2 >>>    m5208evb_defconfig + CONFIG_MMU=y  ColdFire,  PGTABLE_LEVELS=2 >>> >>> Without your patch none of the three can build mm/memory.o, and each fails >>> at a different site: >>> >>>    arch/m68k/include/asm/page.h:20:27: error: lvalue required as unary '&' operand >>>    arch/m68k/include/asm/sun3_pgtable.h:113:33: error: lvalue required as unary '&' operand >>>    arch/m68k/include/asm/mcf_pgtable.h:134:33: error: lvalue required as unary '&' operand >>> >>> With it, all three build a full vmlinux with zero warnings. >>> >>> Two things that might save you some time: >>> >>> - m68k looks like the only arch with this pattern. Grepping arch/ and >>>    include/ for '(&x)->pmd' and '(&(pmd))' finds nothing outside arch/m68k/, >>>    and pmd_none2()/pmd_bad2()/pmd_present2() only exist in mcf_pgtable.h and >>>    sun3_pgtable.h. So your three hunks look complete to me. >>> >>> - No m52xx/m54xx defconfig enables CONFIG_MMU, so the ColdFire path through >>>    mcf_pgtable.h has no defconfig coverage at all; you have to flip it on by >>>    hand (scripts/config -e MMU && make olddefconfig) to reach it. That is >>>    probably how this went unnoticed for so long - my patch has been breaking >>>    m68k since v1, because pmdp_get() returns by value just like >>>    pmdp_get_lockless() does. >> The m5475evb_defconfig is supposed to have MMU enabled: >> >>       $ ARCH=m68k CROSS_COMPILE=m68k-linux-uclibc- make m5475evb_defconfig >>         HOSTCC  scripts/basic/fixdep >>         [snip] >>         HOSTLD  scripts/kconfig/conf >>       # >>       # configuration written to .config >>       # >>       $ >>       $ grep CONFIG_MMU .config >>       CONFIG_MMU=y >>       CONFIG_MMU_COLDFIRE=y >>       CONFIG_MMU_GATHER_NO_RANGE=y >>       CONFIG_MMU_GATHER_MERGE_VMAS=y >>       CONFIG_MMU_LAZY_TLB_REFCOUNT=y >> >> But something ends up disabling it when you compile. >> I'll fix this. >> >> >>> I also checked whether your patch changes code generation, by building the >>> same tree with and without it and comparing every object with objdump -d >>> (plain cmp is useless here, timestamps and link layout shift everything). >>> Counting only objects whose disassembly actually differs: >>> >>>    amiga      3733 objects, 0 changed >>>    sun3       3553 objects, 3 changed: mm/memory.o, mm/pgtable-generic.o, >>>                                        mm/vmalloc.o >>>    m5208evb    716 objects, 1 changed: mm/pgtable-generic.o >>> >>> amiga is the interesting one. It is the only config where pmd_val() itself >>> changed, and it comes out bit-for-bit identical, which also says the build >>> is reproducible enough for this comparison to mean anything. sun3 and >>> ColdFire differ because pmd_none() stops taking the address of its >>> argument, and the direction is an improvement. In mm/vmalloc.o on sun3 one >>> function loses a forced spill: >>> >>>    -  subql #4,%sp >>>    -  movel %d0,%sp@(12)     /* spill the pmd value */ >>>    -  lea %sp@(12),%a1       /* &(pmd) */ >>>    -  moveal %a1@,%a0        /* read it back */ >>> >>> and turns into a 13 instruction function with no stack slot at all. >>> Instruction counts are otherwise flat (sun3 mm/memory.o 7373 -> 7373, >>> mm/vmalloc.o 5514 -> 5515), so it is not a size regression. >>> >>> Dragging the cleanup along with the fix sounds right to me. It is a hard >>> prerequisite, and sent on its own it would just look like a cleanup with no >>> urgency. I assume it goes in ahead of 6ab55cfd in core-7.5.misc, so that >>> linux-next never sees a broken m68k in between. >>> >>> One red herring in case anyone else pokes at this: m5208evb with its stock >>> defconfig (CONFIG_MMU not set) fails to build mm/memory.o for reasons that >>> have nothing to do with any of this - implicit declarations of >>> pte_is_uffd_wp_marker() and pmd_pgtable(), plus a parse error in >>> include/linux/mm.h. Pre-existing, and it goes away once MMU is on. >> You should not be able to set MMU=y if configured for the M5208. >> It doesn't have an MMU. You want the 5475, 5485 or 54418 to test >> valid MMU=y configurations. >> >> Regards >> Greg >> >> >> > > You are right on both counts, and that mail of mine had three things > wrong. > > 1. "No m52xx/m54xx defconfig enables CONFIG_MMU" - wrong.  I grepped the > defconfig fragments for CONFIG_MMU=y, but MMU is 'default y', so an > enabled MMU never appears in the fragment.  m5475evb and m54418evb > both give CONFIG_MMU=y, as you show. > > 2. My "m5208evb + CONFIG_MMU=y" test config was not a valid config. > M520x is 'depends on !MMU', so forcing MMU on leaves the SoC choice > unsatisfiable and olddefconfig silently drops it: > > before:  # CONFIG_MMU is not set / CONFIG_M520x=y > after:   CONFIG_MMU=y / CONFIG_MMU_COLDFIRE=y / CONFIG_M520x gone > > Please disregard the m5208evb row in my codegen numbers. > > 3. The "red herring" I flagged was my own doing, not a pre-existing > problem.  mm/Makefile has 'mmu-$(CONFIG_MMU) := ... memory.o', so a > NOMMU config never builds mm/memory.o at all; I asked for a target > that config does not have.  Sorry for the noise on that one. > > Retested with the configs you named (7.4-rc1 plus the do_fault_around > change plus David's cleanup, gcc-m68k-linux-gnu 13.3.0): > > m5475evb_defconfig   without David's patch: mm/memory.o fails at > mcf_pgtable.h:134 with it: full -j24 build ok, vmlinux 4215344 > bytes, 0 warnings > > m54418evb_defconfig  without David's patch: same mcf_pgtable.h:134 > failure > > So the Tested-by I sent covers amiga, sun3 and m5475evb.  It is > build-tested only - I have no m68k hardware here to boot. Thanks for retesting. Looks good. > On MMU being disabled when you compile: I cannot reproduce that. Here > m5475evb_defconfig gives CONFIG_MMU=y, olddefconfig reports no change, > two consecutive defconfig runs produce an identical .config, and a full > build finishes with CONFIG_MMU=y and CONFIG_M547x=y still set and > .config untouched.  Two differences from your setup that might matter: > I am on a glibc toolchain rather than uclibc, and building out of tree > with O= rather than in tree. I can't reproduce either now. I must have done something silly when quickly double checking that config. Multiple configures and builds and I think it is all doing the right things at configuration and build. > One observation, which may well be unrelated.  m5475evb_defconfig never > sets CONFIG_M547x - it only ends up with it because the "ColdFire SoC > type" choice defaults to M520x, which is unavailable under MMU=y, so > kconfig falls back to the first usable member. m54418evb_defconfig sets > CONFIG_M5441x=y explicitly.  If that fallback is what breaks on your > side, making m5475evb explicit would sidestep it. FWIW, running "make savedefconfig" on a fully built m5475evb configuration results in an identical defconfig to arch/m68k/configs/m5475evb_defconfig. Anyway, I applied both changes and tested on real hardware. All seems to work fine. So for ColdFire: Tested-by: Greg Ungerer Regards Greg