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Subject[PATCH v3 2/3] ext4: Wait for writeback to complete while making pages writable
In order to stabilize pages during disk writes, ext4_page_mkwrite must wait for
writeback operations to complete before making a page writable. Furthermore,
the function must return locked pages, and recheck the writeback status if the
page lock is ever dropped.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
---
fs/ext4/inode.c | 24 +++++++++++++++++++-----
1 files changed, 19 insertions(+), 5 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3db34b2..1d162a2 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5809,15 +5809,19 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
goto out_unlock;
}
ret = 0;
- if (PageMappedToDisk(page))
- goto out_unlock;
+
+ lock_page(page);
+ wait_on_page_writeback(page);
+ if (PageMappedToDisk(page)) {
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
+ }

if (page->index == size >> PAGE_CACHE_SHIFT)
len = size & ~PAGE_CACHE_MASK;
else
len = PAGE_CACHE_SIZE;

- lock_page(page);
/*
* return if we have all the buffers mapped. This avoid
* the need to call write_begin/write_end which does a
@@ -5827,8 +5831,8 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
if (page_has_buffers(page)) {
if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
ext4_bh_unmapped)) {
- unlock_page(page);
- goto out_unlock;
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
}
}
unlock_page(page);
@@ -5848,6 +5852,16 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
if (ret < 0)
goto out_unlock;
ret = 0;
+
+ /*
+ * write_begin/end might have created a dirty page and someone
+ * could wander in and start the IO. Make sure that hasn't
+ * happened.
+ */
+ lock_page(page);
+ wait_on_page_writeback(page);
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
out_unlock:
if (ret)
ret = VM_FAULT_SIGBUS;

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