[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
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