Re: [PATCH 3/3] mm: read remote memory without the mmap lock where possible

[David Hildenbrand (Arm)]

On 6/18/26 19:01, Usama Arif wrote:
> On Tue, 16 Jun 2026 15:03:00 -0400 Rik van Riel <riel@xxxxxxxxxxx> wrote:
> 
>> __access_remote_vm() takes mmap_read_lock() for the entire transfer and
>> uses get_user_pages_remote(), which faults pages in.  For the common
>> case of reading memory that is already resident -- /proc/PID/cmdline,
>> /proc/PID/environ, ptrace PEEK of resident pages -- the mmap lock is
>> unnecessary and is badly contended on large machines.
>>
>> Add an opportunistic, read-only fast path that transfers what it can
>> without the mmap lock.  For each address it takes the per-VMA lock with
>> lock_vma_under_rcu(), re-checks the read-side VMA permissions, and uses
>> folio_walk_start(..., FW_VMA_LOCKED) to grab a short-lived reference to
>> a present page before copying it out.  Anything non-trivial -- a not-
>> present page (needs faulting), a hugetlb or VM_IO/VM_PFNMAP mapping, or
>> a race with a VMA writer -- falls back to the existing mmap_lock path
>> for the remainder.
>>
>> untagged_addr_remote() asserts the mmap lock, so add an unlocked variant
>> for the fast path; the untag mask is a stable per-mm value.
>>
>> Only reads are handled here; writes keep using the slow path.
>>
>> Assisted-by: Claude:claude-opus-4-8
>> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/uaccess_64.h |  12 +++
>>  include/linux/uaccess.h           |  11 ++
>>  mm/memory.c                       | 166 +++++++++++++++++++++++++++++-
>>  3 files changed, 188 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
>> index 4a52497ba6a1..c6fac900a747 100644
>> --- a/arch/x86/include/asm/uaccess_64.h
>> +++ b/arch/x86/include/asm/uaccess_64.h
>> @@ -51,6 +51,18 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
>>  	(__force __typeof__(addr))__untagged_addr_remote(mm, __addr);	\
>>  })
>>  
>> +/* Same as __untagged_addr_remote(), but usable without the mmap lock held. */
>> +static inline unsigned long __untagged_addr_remote_unlocked(struct mm_struct *mm,
>> +							    unsigned long addr)
>> +{
>> +	return addr & READ_ONCE((mm)->context.untag_mask);
>> +}
>> +
>> +#define untagged_addr_remote_unlocked(mm, addr)	({			\
>> +	unsigned long __addr = (__force unsigned long)(addr);		\
>> +	(__force __typeof__(addr))__untagged_addr_remote_unlocked(mm, __addr); \
>> +})
>> +
>>  #endif
>>  
>>  #define valid_user_address(x) \
>> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
>> index 8a264662b242..c8c83372c9d8 100644
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> @@ -34,6 +34,17 @@
>>  })
>>  #endif
>>  
>> +/*
>> + * Like untagged_addr_remote(), but for callers that stabilize @mm by other
>> + * means (e.g. a per-VMA lock) and must not assert the mmap lock.
>> + */
>> +#ifndef untagged_addr_remote_unlocked
>> +#define untagged_addr_remote_unlocked(mm, addr)	({	\
>> +	(void)(mm);					\
>> +	untagged_addr(addr);				\
>> +})
>> +#endif
>> +
>>  #ifdef masked_user_access_begin
>>   #define can_do_masked_user_access() 1
>>  # ifndef masked_user_write_access_begin
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 86a973119bd4..0b23b82eaa18 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -42,6 +42,8 @@
>>  #include <linux/kernel_stat.h>
>>  #include <linux/mm.h>
>>  #include <linux/mm_inline.h>
>> +#include <linux/secretmem.h>
>> +#include <linux/pagewalk.h>
>>  #include <linux/sched/mm.h>
>>  #include <linux/sched/numa_balancing.h>
>>  #include <linux/sched/task.h>
>> @@ -7062,6 +7064,153 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
>>  EXPORT_SYMBOL_GPL(generic_access_phys);
>>  #endif
>>  
>> +/*
>> + * The fast path uses folio_walk_start(FW_VMA_LOCKED), which needs the per-VMA
>> + * lock and RCU-freed page tables to walk page tables without the mmap lock.
>> + */
>> +#if defined(CONFIG_PER_VMA_LOCK) && defined(CONFIG_MMU_GATHER_RCU_TABLE_FREE)
>> +/*
>> + * Opportunistic lockless fast path for __access_remote_vm() reads.
>> + *
>> + * Memory already resident in @mm can be read without taking the heavily
>> + * contended mmap_lock: a per-VMA lock stabilizes the VMA, and folio_walk_start()
>> + * with FW_VMA_LOCKED grabs a short-lived reference to a present page via an
>> + * RCU/PTL protected page table walk (relying on MMU_GATHER_RCU_TABLE_FREE).
>> + *
>> + * Anything that would require faulting a page in, touching a hugetlb or
>> + * VM_IO/VM_PFNMAP mapping, or that races a VMA writer is left to the mmap_lock
>> + * path in __access_remote_vm().  Only reads are handled here.
>> + *
>> + * Returns the number of bytes transferred via the fast path.
>> + */
>> +static int access_remote_vm_fast(struct mm_struct *mm, unsigned long addr,
>> +				 void *buf, int len, unsigned int gup_flags)
>> +{
>> +	void *old_buf = buf;
>> +
>> +	addr = untagged_addr_remote_unlocked(mm, addr);
>> +
>> +	while (len) {
>> +		struct vm_area_struct *vma;
>> +		vm_flags_t vm_flags;
>> +
>> +		vma = lock_vma_under_rcu(mm, addr);
>> +		if (!vma)
>> +			break;
>> +
>> +		/*
>> +		 * Mirror the read-side permission checks of check_vma_flags(),
>> +		 * and exclude what FW_VMA_LOCKED cannot handle (hugetlb) or what
>> +		 * needs the ->access() handler (VM_IO/VM_PFNMAP).  Checked once
>> +		 * per VMA; anything not positively allowed falls back to the
>> +		 * slow path, which re-validates everything.
>> +		 */
>> +		vm_flags = vma->vm_flags;
>> +		if ((vm_flags & (VM_IO | VM_PFNMAP)) ||
>> +		    is_vm_hugetlb_page(vma) || vma_is_secretmem(vma) ||
>> +		    (!(vm_flags & VM_READ) &&
>> +		     (!(gup_flags & FOLL_FORCE) || !(vm_flags & VM_MAYREAD)))) {
>> +			vma_end_read(vma);
>> +			break;
>> +		}
> 
> This should also do the FOLL_ANON check from check_vma_flags().
> 
> check_vma_flags() rejects non-anonymous VMAs when FOLL_ANON is set:
> 
> 	if ((gup_flags & FOLL_ANON) && !vma_anon)
> 		return -EFAULT;

Duplicating GUP logic in a non-GUP file. Splendid. :)

-- 
Cheers,

David