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Date: Thu, 14 May 2015 19:31:19 +0200
From: Andrea Arcangeli <aarcange@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
qemu-devel@...gnu.org, kvm@...r.kernel.org,
linux-api@...r.kernel.org
Cc: Pavel Emelyanov <xemul@...allels.com>,
Sanidhya Kashyap <sanidhya.gatech@...il.com>,
zhang.zhanghailiang@...wei.com,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Andres Lagar-Cavilla <andreslc@...gle.com>,
Dave Hansen <dave.hansen@...el.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
Andy Lutomirski <luto@...capital.net>,
Hugh Dickins <hughd@...gle.com>,
Peter Feiner <pfeiner@...gle.com>,
"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: [PATCH 22/23] userfaultfd: avoid mmap_sem read recursion in mcopy_atomic
If the rwsem starves writers it wasn't strictly a bug but lockdep
doesn't like it and this avoids depending on lowlevel implementation
details of the lock.
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
mm/userfaultfd.c | 92 ++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 66 insertions(+), 26 deletions(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c54c761..f8fe494 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -21,26 +21,39 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
pmd_t *dst_pmd,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
- unsigned long src_addr)
+ unsigned long src_addr,
+ struct page **pagep)
{
struct mem_cgroup *memcg;
pte_t _dst_pte, *dst_pte;
spinlock_t *ptl;
- struct page *page;
void *page_kaddr;
int ret;
+ struct page *page;
- ret = -ENOMEM;
- page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, dst_vma, dst_addr);
- if (!page)
- goto out;
-
- page_kaddr = kmap(page);
- ret = -EFAULT;
- if (copy_from_user(page_kaddr, (const void __user *) src_addr,
- PAGE_SIZE))
- goto out_kunmap_release;
- kunmap(page);
+ if (!*pagep) {
+ ret = -ENOMEM;
+ page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, dst_vma, dst_addr);
+ if (!page)
+ goto out;
+
+ page_kaddr = kmap_atomic(page);
+ ret = copy_from_user(page_kaddr,
+ (const void __user *) src_addr,
+ PAGE_SIZE);
+ kunmap_atomic(page_kaddr);
+
+ /* fallback to copy_from_user outside mmap_sem */
+ if (unlikely(ret)) {
+ ret = -EFAULT;
+ *pagep = page;
+ /* don't free the page */
+ goto out;
+ }
+ } else {
+ page = *pagep;
+ *pagep = NULL;
+ }
/*
* The memory barrier inside __SetPageUptodate makes sure that
@@ -82,9 +95,6 @@ out_release_uncharge_unlock:
out_release:
page_cache_release(page);
goto out;
-out_kunmap_release:
- kunmap(page);
- goto out_release;
}
static int mfill_zeropage_pte(struct mm_struct *dst_mm,
@@ -139,7 +149,8 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
ssize_t err;
pmd_t *dst_pmd;
unsigned long src_addr, dst_addr;
- long copied = 0;
+ long copied;
+ struct page *page;
/*
* Sanitize the command parameters:
@@ -151,6 +162,11 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
BUG_ON(src_start + len <= src_start);
BUG_ON(dst_start + len <= dst_start);
+ src_addr = src_start;
+ dst_addr = dst_start;
+ copied = 0;
+ page = NULL;
+retry:
down_read(&dst_mm->mmap_sem);
/*
@@ -160,10 +176,10 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
err = -EINVAL;
dst_vma = find_vma(dst_mm, dst_start);
if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
- goto out;
+ goto out_unlock;
if (dst_start < dst_vma->vm_start ||
dst_start + len > dst_vma->vm_end)
- goto out;
+ goto out_unlock;
/*
* Be strict and only allow __mcopy_atomic on userfaultfd
@@ -175,14 +191,14 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
* belonging to the userfaultfd and not syscalls.
*/
if (!dst_vma->vm_userfaultfd_ctx.ctx)
- goto out;
+ goto out_unlock;
/*
* FIXME: only allow copying on anonymous vmas, tmpfs should
* be added.
*/
if (dst_vma->vm_ops)
- goto out;
+ goto out_unlock;
/*
* Ensure the dst_vma has a anon_vma or this page
@@ -191,12 +207,13 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
*/
err = -ENOMEM;
if (unlikely(anon_vma_prepare(dst_vma)))
- goto out;
+ goto out_unlock;
- for (src_addr = src_start, dst_addr = dst_start;
- src_addr < src_start + len; ) {
+ while (src_addr < src_start + len) {
pmd_t dst_pmdval;
+
BUG_ON(dst_addr >= dst_start + len);
+
dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);
if (unlikely(!dst_pmd)) {
err = -ENOMEM;
@@ -229,13 +246,33 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
if (!zeropage)
err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
- dst_addr, src_addr);
+ dst_addr, src_addr, &page);
else
err = mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma,
dst_addr);
cond_resched();
+ if (unlikely(err == -EFAULT)) {
+ void *page_kaddr;
+
+ BUILD_BUG_ON(zeropage);
+ up_read(&dst_mm->mmap_sem);
+ BUG_ON(!page);
+
+ page_kaddr = kmap(page);
+ err = copy_from_user(page_kaddr,
+ (const void __user *) src_addr,
+ PAGE_SIZE);
+ kunmap(page);
+ if (unlikely(err)) {
+ err = -EFAULT;
+ goto out;
+ }
+ goto retry;
+ } else
+ BUG_ON(page);
+
if (!err) {
dst_addr += PAGE_SIZE;
src_addr += PAGE_SIZE;
@@ -248,8 +285,11 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
break;
}
-out:
+out_unlock:
up_read(&dst_mm->mmap_sem);
+out:
+ if (page)
+ page_cache_release(page);
BUG_ON(copied < 0);
BUG_ON(err > 0);
BUG_ON(!copied && !err);
--
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