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

["Aneesh Kumar K.V" <aneeshkumar.opensource@gmail.com>]

On 9/11/18 4:04 PM, 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@linux.intel.com>
> Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
> Cc: <stable@vger.kernel.org> [v4.14+]
> Cc: Zi Yan <zi.yan@cs.rutgers.edu>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> ---
>   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);
> +

Can you explain this more? I am confused by the usage of 'new' and 
'page' there. I guess the idea is if we are removing the migration pte 
at level 4 table, and if we found the backing page compound don't mark 
the page Mlocked?


-aneesh