On Wed, May 13, 2020 at 05:03:03AM +0800, Yang Shi wrote:
> When running khugepaged with higher frequency (for example, set
> scan_sleep_millisecs to 0), the below error message was reported:
>
> khugepaged: expected_refcount (1024) > refcount (512)
> page:ffffd75784258000 count:511 mapcount:1 mapping:ffff968de06c7421 index:0x7fa288600
> compound_mapcount: 0
> flags: 0x17fffc00009003c(uptodate|dirty|lru|active|head|swapbacked)
> raw: 017fffc00009003c ffffd7578ba70788 ffffd7578bdb5148 ffff968de06c7421
> raw: 00000007fa288600 0000000000000000 000001ff00000000 ffff968e5e7d6000
> page dumped because: Unexpected refcount
> page->mem_cgroup:ffff968e5e7d6000
>
> This is introduced by allowing collapsing fork shared and PTE-mapped
> THPs. The check may run into the below race:
>
> Assuming parent process forked child process, then they do
>
> CPU A CPU B CPU C
> ----- ----- -----
> Parent Child khugepaged
>
> MADV_DONTNEED
> split huge pmd
> Double mapped
> MADV_DONTNEED
> zap_huge_pmd
> remove_page_rmap
> Clear double map
> khugepaged_scan_pmd(parent)
> check mapcount and refcount
> --> total_mapcount > refcount
> dec mapcount
>
> The issue can be reproduced by the below test program.
Good catch! Thanks. And the fix looks reasnable.
We might want to have a similar debug check in near !is_refcount_suitable()
case in __collapse_huge_page_isolate(). The function is called with
anon_vma lock taken on write and it should prevent the false-positive.
Anyway:
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> ---8<---
> void main()
> {
> void *addr;
> int ret;
> pid_t pid;
>
> addr = memalign(ALIGN, 2 * 1024 * 1024);
> if (!addr) {
> printf("malloc failed\n");
> return;
> }
>
> ret = madvise(addr, 2 * 1024 * 1024, MADV_HUGEPAGE);
> if (ret < 0) {
> printf("madvise failed\n");
> return;
> }
>
> memset(addr, 0xdeadbeef, 2 * 1024 * 1024);
>
> pid = fork();
>
> if (pid == 0) {
> /* Child process */
> ret = madvise(addr + (2 * 1024 * 1024) - 4096, 4096, MADV_DONTNEED);
> if (ret < 0) {
> printf("madvise failed in child\n");
> return;
> }
> sleep(120);
> } else if (pid > 0) {
> sleep(5);
> /* Parent process */
> ret = madvise(addr, 2 * 1024 * 1024, MADV_DONTNEED);
> if (ret < 0) {
> printf("madvise failed in parent\n");
> return;
> }
> } else {
> printf("fork failed\n");
> return;
> }
>
> sleep(120);
> }
> ---8<---
>
> So, total_mapcount > refcount seems not unexpected due to the inherent
> race. Removed the error message even though it is protected by
> CONFIG_VM_DEBUG since we have to live with the race and AFAIK some
> distros may have CONFIG_VM_DEBUG enabled dy default.
>
> Since such case is ephemeral we could always try collapse the area again
> later, so it sounds not harmful. But, it might report false positive if
> the page has excessive GUP pins (i.e. 512), however it might be not that
> bad since the same check will be done later. I didn't figure out a
> simple way to prevent the false positive.
>
> Added some notes to elaborate the race and the consequence as well.
>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> mm/khugepaged.c | 24 +++++++++++++++++-------
> 1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1fdd677..048f5d4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -602,12 +602,6 @@ static bool is_refcount_suitable(struct page *page)
> if (PageSwapCache(page))
> expected_refcount += compound_nr(page);
>
> - if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> - pr_err("expected_refcount (%d) > refcount (%d)\n",
> - expected_refcount, refcount);
> - dump_page(page, "Unexpected refcount");
> - }
> -
> return page_count(page) == expected_refcount;
> }
>
> @@ -1341,7 +1335,23 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> goto out_unmap;
> }
>
> - /* Check if the page has any GUP (or other external) pins */
> + /*
> + * Check if the page has any GUP (or other external) pins.
> + *
> + * Here the check is racy it may see totmal_mapcount > refcount
> + * in some cases.
> + * For example, one process with one forked child process.
> + * The parent has the PMD split due to MADV_DONTNEED, then
> + * the child is trying unmap the whole PMD, but khugepaged
> + * may be scanning the parent between the child has
> + * PageDoubleMap flag cleared and dec the mapcount. So
> + * khugepaged may see total_mapcount > refcount.
> + *
> + * But such case is ephemeral we could always retry collapse
> + * later. However it may report false positive if the page
> + * has excessive GUP pins (i.e. 512). Anyway the same check
> + * will be done again later the risk seems low.
> + */
> if (!is_refcount_suitable(page)) {
> result = SCAN_PAGE_COUNT;
> goto out_unmap;
> --
> 1.8.3.1
>
>
--
Kirill A. Shutemov
