Re: [PATCH] mm/huge_memory: fix the memory leak due to the race

[Michal Hocko]

[CCing Kirill]

On Tue 21-06-16 22:05:56, zhongjiang wrote:
> From: zhong jiang <zhongjiang@xxxxxxxxxx>
> 
> with great pressure, I run some test cases. As a result, I found
> that the THP is not freed, it is detected by check_mm().
> 
> BUG: Bad rss-counter state mm:ffff8827edb70000 idx:1 val:512
> 
> Consider the following race :
> 
> 	CPU0                               CPU1
>   __handle_mm_fault()
>         wp_huge_pmd()
>    	    do_huge_pmd_wp_page()
> 		pmdp_huge_clear_flush_notify()
>                 (pmd_none = true)
> 					exit_mmap()
> 					   unmap_vmas()
> 					     zap_pmd_range()
> 						pmd_none_or_trans_huge_or_clear_bad()
> 						   (result in memory leak)
>                 set_pmd_at()
>
> because of CPU0 have allocated huge page before pmdp_huge_clear_notify,
> and it make the pmd entry to be null. Therefore, The memory leak can occur.

I do not understand this description. CPU1 is in the exit path with last
mm user gone. So CPU0 is a different process with its own mm. How can
they influence each other. But maybe I am just missing your point.
 
> The patch fix the scenario that the pmd entry can lead to be null.
> 
> Signed-off-by: zhong jiang <zhongjiang@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e10a4fe..ef04b94 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1340,11 +1340,11 @@ alloc:
>  		pmd_t entry;
>  		entry = mk_huge_pmd(new_page, vma->vm_page_prot);
>  		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> -		pmdp_huge_clear_flush_notify(vma, haddr, pmd);
> +		pmdp_invalidate(vma, haddr, pmd);	
>  		page_add_new_anon_rmap(new_page, vma, haddr, true);
>  		mem_cgroup_commit_charge(new_page, memcg, false, true);
>  		lru_cache_add_active_or_unevictable(new_page, vma);
> -		set_pmd_at(mm, haddr, pmd, entry);
> +		pmd_populate(mm, pmd, entry);
>  		update_mmu_cache_pmd(vma, address, pmd);
>  		if (!page) {
>  			add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs