Re: [PATCH v2 4/4] mm/mlock: migrate folios out of CMA when mlocking a range

[Wandun <chenwandun1@gmail.com>]

On 7/7/26 20:59, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
> 
> The region covered by mlock[all] may contain CMA pages. cma_alloc installs
> migration entries in the page table, if a memory access occurs at this
> point, it must wait for the migration to complete, which may cause
> latency spikes on the RT kernels.
> 
> Try to move the migration cost into the mlock[all] caller, which is
> typically a setup path. So reduce the chance of latency spikes on RT
> kernels by migrating the currently mapped CMA pages out of CMA region.
> 
> Suggested-by: Frank van der Linden <fvdl@google.com>
> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
> Link: https://lore.kernel.org/all/CAPTztWZpnX1j8-7yeppVUsxE=O9hbVeqricDjZt8_pnN7a-kBQ@mail.gmail.com/#t
> ---
>  mm/mlock.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 118 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index ac65de40b22b..f56c685533f5 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -25,6 +25,7 @@
>  #include <linux/memcontrol.h>
>  #include <linux/mm_inline.h>
>  #include <linux/secretmem.h>
> +#include <linux/migrate.h>
>  #include <linux/compaction.h>
>  
>  #include "internal.h"
> @@ -428,6 +429,119 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
>  	return 0;
>  }
>  
> +#ifdef CONFIG_CMA
> +static int mlock_collect_migratable_pte_range(pmd_t *pmd, unsigned long addr,
> +			unsigned long end, struct mm_walk *walk)
> +{
> +	struct vm_area_struct *vma = walk->vma;
> +	struct list_head *folio_list = walk->private;
> +	spinlock_t *ptl;
> +	pte_t *start_pte, *pte;
> +	pte_t ptent;
> +	struct folio *folio;
> +	unsigned int step = 1;
> +
> +	if (!(vma->vm_flags & VM_LOCKED))
> +		return 0;
> +
> +	ptl = pmd_trans_huge_lock(pmd, vma);
> +	if (ptl) {
> +		if (!pmd_present(*pmd)) {
> +			if (unlikely(softleaf_is_migration(softleaf_from_pmd(*pmd)))) {
> +				spin_unlock(ptl);
> +				pmd_migration_entry_wait(vma->vm_mm, pmd);
> +				walk->action = ACTION_AGAIN;
> +				return 0;
> +			}
> +			goto out;
> +		}
> +		if (is_huge_zero_pmd(*pmd))
> +			goto out;
> +		folio = pmd_folio(*pmd);
> +		if (folio_is_zone_device(folio))
> +			goto out;
> +		if (is_migrate_cma_page(&folio->page))
> +			isolate_folio_to_list(folio, folio_list);
> +		goto out;
> +	}
> +
> +	start_pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> +	if (!start_pte) {
> +		walk->action = ACTION_AGAIN;
> +		return 0;
> +	}
> +
> +	for (pte = start_pte; addr != end; pte += step, addr += step * PAGE_SIZE) {
> +		step = 1;
> +		ptent = ptep_get(pte);
> +		if (!pte_present(ptent)) {
> +			if (unlikely(softleaf_is_migration(softleaf_from_pte(ptent)))) {
> +				pte_unmap_unlock(start_pte, ptl);
> +				migration_entry_wait(vma->vm_mm, pmd, addr);
> +				walk->action = ACTION_AGAIN;
> +				return 0;
> +			}
> +			continue;
> +		}
> +		folio = vm_normal_folio(vma, addr, ptent);
> +		if (!folio || folio_is_zone_device(folio))
> +			continue;
> +		step = folio_mlock_step(folio, pte, addr, end);
> +		if (is_migrate_cma_page(&folio->page))
Here should be, sorry about this.
	if (!is_migrate_cma_page(&folio->page))
		continue;

Best regards
Wandun
> +			continue;
> +		isolate_folio_to_list(folio, folio_list);
> +	}
> +	pte_unmap(start_pte);
> +out:
> +	spin_unlock(ptl);
> +	cond_resched();
> +	return 0;
> +}
> +
> +static const struct mm_walk_ops mlock_collect_migratable_ops = {
> +	.pmd_entry	= mlock_collect_migratable_pte_range,
> +	.walk_lock	= PGWALK_RDLOCK,
> +};
> +
> +static void mlock_migrate_cma_range(unsigned long start, unsigned long len)
> +{
> +	struct mm_struct *mm = current->mm;
> +	unsigned long end = start + len;
> +	LIST_HEAD(folio_list);
> +	struct migration_target_control mtc = {
> +		.nid = NUMA_NO_NODE,
> +		.gfp_mask = GFP_HIGHUSER | __GFP_NOWARN,
> +		.reason = MR_SYSCALL,
> +	};
> +
> +	if (compaction_allow_unevictable())
> +		return;
> +
> +	lru_cache_disable();
> +
> +	if (mmap_read_lock_killable(mm))
> +		goto out;
> +
> +	walk_page_range(mm, start, end, &mlock_collect_migratable_ops,
> +			&folio_list);
> +	mmap_read_unlock(mm);
> +
> +	if (list_empty(&folio_list))
> +		goto out;
> +
> +	if (migrate_pages(&folio_list, alloc_migration_target, NULL,
> +			  (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL, NULL))
> +		putback_movable_pages(&folio_list);
> +out:
> +	lru_cache_enable();
> +}
> +#else
> +static inline void mlock_migrate_cma_range(unsigned long start,
> +					   unsigned long len)
> +{
> +}
> +#endif /* CONFIG_CMA */
> +
>  /*
>   * mlock_vma_pages_range() - mlock any pages already in the range,
>   *                           or munlock all pages in the range.
> @@ -678,6 +792,7 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
>  	error = __mm_populate(start, len, 0);
>  	if (error)
>  		return __mlock_posix_error_return(error);
> +	mlock_migrate_cma_range(start, len);
>  	return 0;
>  }
>  
> @@ -790,8 +905,10 @@ SYSCALL_DEFINE1(mlockall, int, flags)
>  	    capable(CAP_IPC_LOCK))
>  		ret = apply_mlockall_flags(flags);
>  	mmap_write_unlock(current->mm);
> -	if (!ret && (flags & MCL_CURRENT))
> +	if (!ret && (flags & MCL_CURRENT)) {
>  		mm_populate(0, TASK_SIZE);
> +		mlock_migrate_cma_range(0, TASK_SIZE);
> +	}
>  
>  	return ret;
>  }