Re: [PATCH] mm, thp: Fix mlocking THP page with migration enabled

[Zi Yan]

Hi Kirill,

On 11 Sep 2018, at 6:34, Kirill A. Shutemov wrote:

> A transparent huge page is represented by a single entry on an LRU list.
> Therefore, we can only make unevictable an entire compound page, not
> individual subpages.
>
> If a user tries to mlock() part of a huge page, we want the rest of the
> page to be reclaimable.
>
> We handle this by keeping PTE-mapped huge pages on normal LRU lists: the
> PMD on border of VM_LOCKED VMA will be split into PTE table.
>
> Introduction of THP migration breaks the rules around mlocking THP
> pages. If we had a single PMD mapping of the page in mlocked VMA, the
> page will get mlocked, regardless of PTE mappings of the page.
>
> For tmpfs/shmem it's easy to fix by checking PageDoubleMap() in
> remove_migration_pmd().
>
> Anon THP pages can only be shared between processes via fork(). Mlocked
> page can only be shared if parent mlocked it before forking, otherwise
> CoW will be triggered on mlock().
>
> For Anon-THP, we can fix the issue by munlocking the page on removing PTE
> migration entry for the page. PTEs for the page will always come after
> mlocked PMD: rmap walks VMAs from oldest to newest.
>
> Test-case:
>
> 	#include <unistd.h>
> 	#include <sys/mman.h>
> 	#include <sys/wait.h>
> 	#include <linux/mempolicy.h>
> 	#include <numaif.h>
>
> 	int main(void)
> 	{
> 	        unsigned long nodemask = 4;
> 	        void *addr;
>
> 		addr = mmap((void *)0x20000000UL, 2UL << 20, PROT_READ | PROT_WRITE,
> 			MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, -1, 0);
>
> 	        if (fork()) {
> 			wait(NULL);
> 			return 0;
> 	        }
>
> 	        mlock(addr, 4UL << 10);
> 	        mbind(addr, 2UL << 20, MPOL_PREFERRED | MPOL_F_RELATIVE_NODES,
> 	                &nodemask, 4, MPOL_MF_MOVE | MPOL_MF_MOVE_ALL);
>
> 	        return 0;
> 	}
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>
> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
> Cc: <stable@xxxxxxxxxxxxxxx> [v4.14+]
> Cc: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 2 +-
>  mm/migrate.c     | 3 +++
>  2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 533f9b00147d..00704060b7f7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2931,7 +2931,7 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>  	else
>  		page_add_file_rmap(new, true);
>  	set_pmd_at(mm, mmun_start, pvmw->pmd, pmde);
> -	if (vma->vm_flags & VM_LOCKED)
> +	if ((vma->vm_flags & VM_LOCKED) && !PageDoubleMap(new))
>  		mlock_vma_page(new);
>  	update_mmu_cache_pmd(vma, address, pvmw->pmd);
>  }
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d6a2e89b086a..01dad96b25b5 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -275,6 +275,9 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
>  		if (vma->vm_flags & VM_LOCKED && !PageTransCompound(new))
>  			mlock_vma_page(new);
>
> +		if (PageTransCompound(new) && PageMlocked(page))
> +			clear_page_mlock(page);
> +
>  		/* No need to invalidate - it was non-present before */
>  		update_mmu_cache(vma, pvmw.address, pvmw.pte);
>  	}
> -- 
> 2.18.0

Thanks for your patch. It fixes the mlock problem demonstrated by your test program.

I want to understand the Anon THP part of the problem more clearly. For Anon THPs,
you said, PTEs for the page will always come after mlocked PMD. I just wonder that if
a process forks a child1 which forks its own child2 and the child1 mlocks a subpage causing
split_pmd_page() and migrates its PTE-mapped THP, will the kernel see the sequence of PMD-mapped THP,
PTE-mapped THP, and PMD-mapped THP while walking VMAs? Will the second PMD-mapped THP
reset the mlock on the page?

In addition, I also discover that PageDoubleMap is not set for double mapped Anon THPs after migration,
the following patch fixes it. Do you want me to send it separately or you can merge it
with your patch?

diff --git a/mm/rmap.c b/mm/rmap.c
index eb477809a5c0..defe8fc265e3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1121,6 +1121,16 @@ void do_page_add_anon_rmap(struct page *page,
                 */
                if (compound)
                        __inc_node_page_state(page, NR_ANON_THPS);
+               else {
+                       if (PageTransCompound(page) && compound_mapcount(page) > 0) {
+                               struct page *head = compound_head(page);
+                               int i;
+
+                               SetPageDoubleMap(head);
+                               for (i = 0; i < HPAGE_PMD_NR; i++)
+                                       atomic_inc(&head[i]._mapcount);
+                       }
+               }
                __mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
        }
        if (unlikely(PageKsm(page)))

â
Best Regards,
Yan Zi

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