Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing

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On 01/04/2020 05.31, Huang, Ying wrote:
Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes:

On 31/03/2020 11.56, Huang, Ying wrote:
From: Huang Ying <ying.huang@xxxxxxxxx>

Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
ignored.  To improve the accuracy of /proc/PID/smaps, its parsing and processing
is added.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
   fs/proc/task_mmu.c | 16 ++++++++++++----
   1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 8d382d4ec067..b5b3aef8cb3b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -548,8 +548,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
   	bool locked = !!(vma->vm_flags & VM_LOCKED);
   	struct page *page;

         struct page *page = NULL;

Looks good.  Will do this in the next version.

   -	/* FOLL_DUMP will return -EFAULT on huge zero page */
-	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+	if (pmd_present(*pmd)) {
+		/* FOLL_DUMP will return -EFAULT on huge zero page */
+		page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+	} else if (unlikely(is_swap_pmd(*pmd))) {
+		swp_entry_t entry = pmd_to_swp_entry(*pmd);
+
+		VM_BUG_ON(!is_migration_entry(entry));
+		page = migration_entry_to_page(entry);

                 if (is_migration_entry(entry))
                         page = migration_entry_to_page(entry);

Seems safer and doesn't add much code.

With this, we lose an opportunity to capture some bugs during debugging.
Right?

You can keep VM_BUG_ON or VM_WARN_ON_ONCE

Off-by-page in statistics isn't a big deal and not a good reason to crash (even debug) kernel.
But for normal build should use safe behaviour if this isn't hard.


Best Regards,
Huang, Ying

+	} else {
+		return;
+	}
   	if (IS_ERR_OR_NULL(page))
   		return;
   	if (PageAnon(page))
@@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
     	ptl = pmd_trans_huge_lock(pmd, vma);
   	if (ptl) {
-		if (pmd_present(*pmd))
-			smaps_pmd_entry(pmd, addr, walk);
+		smaps_pmd_entry(pmd, addr, walk);
   		spin_unlock(ptl);
   		goto out;
   	}






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