From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Song Liu <songliubraving@fb.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <matthew.wilcox@oracle.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Kernel Team <Kernel-team@fb.com>,
William Kucharski <william.kucharski@oracle.com>,
"srikar@linux.vnet.ibm.com" <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP
Date: Fri, 16 Aug 2019 17:54:43 +0300 [thread overview]
Message-ID: <20190816145443.6ard3iilytc6jlgv@box> (raw)
In-Reply-To: <20190813162451.GD6971@redhat.com>
On Tue, Aug 13, 2019 at 06:24:51PM +0200, Oleg Nesterov wrote:
> > Let me see first that my explanation makes sense :P
>
> It does ;)
Does it look fine to you? It's on top of Song's patchset.
From 58834d6c1e63321af742b208558a6b5cb86fc7ec Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Fri, 16 Aug 2019 17:50:41 +0300
Subject: [PATCH] khugepaged: Add comments for retract_page_tables()
Oleg Nesterov pointed that logic behind checks in retract_page_tables()
are not obvious.
Add comments to clarify the reasoning for the checks and why they are
safe.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
mm/khugepaged.c | 28 +++++++++++++++++++++++-----
1 file changed, 23 insertions(+), 5 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5c0a5f0826b2..00cec6a127aa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1421,7 +1421,22 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
i_mmap_lock_write(mapping);
vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
- /* probably overkill */
+ /*
+ * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
+ * got written to. These VMAs are likely not worth investing
+ * down_write(mmap_sem) as PMD-mapping is likely to be split
+ * later.
+ *
+ * Not that vma->anon_vma check is racy: it can be set up after
+ * the check but before we took mmap_sem by the fault path.
+ * But page lock would prevent establishing any new ptes of the
+ * page, so we are safe.
+ *
+ * An alternative would be drop the check, but check that page
+ * table is clear before calling pmdp_collapse_flush() under
+ * ptl. It has higher chance to recover THP for the VMA, but
+ * has higher cost too.
+ */
if (vma->anon_vma)
continue;
addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
@@ -1434,9 +1449,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
continue;
/*
* We need exclusive mmap_sem to retract page table.
- * If trylock fails we would end up with pte-mapped THP after
- * re-fault. Not ideal, but it's more important to not disturb
- * the system too much.
+ *
+ * We use trylock due to lock inversion: we need to acquire
+ * mmap_sem while holding page lock. Fault path does it in
+ * reverse order. Trylock is a way to avoid deadlock.
*/
if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
@@ -1446,8 +1462,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
up_write(&vma->vm_mm->mmap_sem);
mm_dec_nr_ptes(vma->vm_mm);
pte_free(vma->vm_mm, pmd_pgtable(_pmd));
- } else
+ } else {
+ /* Try again later */
khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+ }
}
i_mmap_unlock_write(mapping);
}
--
Kirill A. Shutemov
next prev parent reply other threads:[~2019-08-16 14:54 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-07 23:37 [PATCH v12 0/6] THP aware uprobe Song Liu
2019-08-07 23:37 ` [PATCH v12 1/6] mm: move memcmp_pages() and pages_identical() Song Liu
2019-08-07 23:37 ` [PATCH v12 2/6] uprobe: use original page when all uprobes are removed Song Liu
2019-08-07 23:37 ` [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD Song Liu
2019-08-08 16:37 ` Oleg Nesterov
2019-08-08 17:16 ` Song Liu
2019-08-09 16:35 ` Oleg Nesterov
2019-08-09 16:50 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 4/6] uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT Song Liu
2019-08-07 23:37 ` [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP Song Liu
2019-08-08 16:33 ` Oleg Nesterov
2019-08-08 17:05 ` Song Liu
2019-08-09 15:24 ` Oleg Nesterov
2019-08-09 16:30 ` Song Liu
2019-08-09 18:01 ` Song Liu
2019-08-12 12:11 ` Kirill A. Shutemov
2019-08-12 13:22 ` Oleg Nesterov
2019-08-12 14:40 ` Kirill A. Shutemov
2019-08-12 21:04 ` Song Liu
2019-08-13 14:44 ` Song Liu
2019-08-15 10:16 ` Oleg Nesterov
2019-08-15 16:27 ` Song Liu
2019-08-13 13:30 ` Oleg Nesterov
2019-08-13 14:05 ` Oleg Nesterov
2019-08-13 15:05 ` Kirill A. Shutemov
2019-08-13 16:24 ` Oleg Nesterov
2019-08-16 14:54 ` Kirill A. Shutemov [this message]
2019-08-12 13:06 ` Oleg Nesterov
2019-08-12 14:36 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 6/6] uprobe: collapse THP pmd after removing all uprobes Song Liu
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