Re: [PATCH v3] mm/memory: use pmdp_get_lockless() for the pmd read in do_fault_around()

[David Hildenbrand (Arm)]

On 11/3/26 02:47, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@xxxxxxxxxx>
> 
> 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.  pmdp_get() is a plain READ_ONCE()
> and can tear where pmd_t is wider than a word, as on i386-PAE where
> pmd_install() publishes two words with a single cmpxchg8b.  Use
> pmdp_get_lockless(), the accessor for pmd reads without the pmd lock and
> what __pte_offset_map() already uses on this very pmd a few instructions
> later.  It is pmdp_get() everywhere else, so the report is silenced the
> same way.
> 
> No functional change on architectures that can load a pmd in one go.
> 
> Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> ---
> Changelog:
> v3:
> Shorten the comment to one line (David Hildenbrand).
> v2:
> Use pmdp_get_lockless() instead of pmdp_get() (David Hildenbrand).
> 
>  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;

I'll wait for for review and then pick it up.

-- 
Cheers,

David