linux-kernel - [v4 PATCH 8/8] mm: mmap: register suitable readonly file vmas for khugepaged

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Date:   Tue, 10 May 2022 13:32:22 -0700
From:   Yang Shi <shy828301@...il.com>
To:     vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
        linmiaohe@...wei.com, songliubraving@...com, riel@...riel.com,
        willy@...radead.org, ziy@...dia.com, tytso@....edu,
        akpm@...ux-foundation.org
Cc:     shy828301@...il.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [v4 PATCH 8/8] mm: mmap: register suitable readonly file vmas for khugepaged

The readonly FS THP relies on khugepaged to collapse THP for suitable
vmas.  But the behavior is inconsistent for "always" mode (https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@suse.cz/).

The "always" mode means THP allocation should be tried all the time and
khugepaged should try to collapse THP all the time. Of course the
allocation and collapse may fail due to other factors and conditions.

Currently file THP may not be collapsed by khugepaged even though all
the conditions are met. That does break the semantics of "always" mode.

So make sure readonly FS vmas are registered to khugepaged to fix the
break.

Registering suitable vmas in common mmap path, that could cover both
readonly FS vmas and shmem vmas, so removed the khugepaged calls in
shmem.c.

Still need to keep the khugepaged call in vma_merge() since vma_merge()
is called in a lot of places, for example, madvise, mprotect, etc.

Reported-by: Vlastimil Babka <vbabka@...e.cz>
Acked-by: Vlastmil Babka <vbabka@...e.cz>
Signed-off-by: Yang Shi <shy828301@...il.com>
---
 mm/mmap.c  | 6 ++++++
 mm/shmem.c | 4 ----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 34ff1600426c..6d7a6c7b50bb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2745,6 +2745,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		i_mmap_unlock_write(vma->vm_file->f_mapping);
 	}

+	/*
+	 * vma_merge() calls khugepaged_enter_vma() either, the below
+	 * call covers the non-merge case.
+	 */
+	khugepaged_enter_vma(vma, vma->vm_flags);
+
 	/* Once vma denies write, undo our temporary denial count */
 unmap_writable:
 	if (file && vm_flags & VM_SHARED)
diff --git a/mm/shmem.c b/mm/shmem.c
index 89f6f4fec3f9..67a3f3b05fb2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -34,7 +34,6 @@
 #include <linux/export.h>
 #include <linux/swap.h>
 #include <linux/uio.h>
-#include <linux/khugepaged.h>
 #include <linux/hugetlb.h>
 #include <linux/fs_parser.h>
 #include <linux/swapfile.h>
@@ -2232,7 +2231,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)

 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
-	khugepaged_enter_vma(vma, vma->vm_flags);
 	return 0;
 }

@@ -4133,8 +4131,6 @@ int shmem_zero_setup(struct vm_area_struct *vma)
 	vma->vm_file = file;
 	vma->vm_ops = &shmem_vm_ops;

-	khugepaged_enter_vma(vma, vma->vm_flags);
-
 	return 0;
 }

-- 
2.26.3