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[PATCH 1/6] dax: don't abuse get_block mapping for endio callbacks

To: xfs@xxxxxxxxxxx
Subject: [PATCH 1/6] dax: don't abuse get_block mapping for endio callbacks
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 4 Mar 2015 10:30:22 +1100
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, jack@xxxxxxx, willy@xxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1425425427-16283-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1425425427-16283-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

dax_fault() currently relies on the get_block callback to attach an
io completion callback to the mapping buffer head so that it can
run unwritten extent conversion after zeroing allocated blocks.

Instead of this hack, pass the conversion callback directly into
dax_fault() similar to the get_block callback. When the filesystem
allocates unwritten extents, it will set the buffer_unwritten()
flag, and hence the dax_fault code can call the completion function
in the contexts where it is necessary without overloading the
mapping buffer head.

Note: The changes to ext4 to use this interface are suspect at best.
In fact, the way ext4 did this end_io assignment in the first place
looks suspect because it only set a completion callback when there
wasn't already some other write() call taking place on the same
inode. The ext4 end_io code looks rather intricate and fragile with
all it's reference counting and passing to different contexts for
modification via inode private pointers that aren't protected by
locks...

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/dax.c           | 15 ++++++++-------
 fs/ext2/file.c     |  4 ++--
 fs/ext4/file.c     | 16 ++++++++++++++--
 fs/ext4/inode.c    | 21 +++++++--------------
 include/linux/fs.h |  6 ++++--
 5 files changed, 35 insertions(+), 27 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index ed1619e..d7b4dba 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -269,7 +269,8 @@ static int copy_user_bh(struct page *to, struct buffer_head 
*bh,
 }
 
 static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
-                       struct vm_area_struct *vma, struct vm_fault *vmf)
+                       struct vm_area_struct *vma, struct vm_fault *vmf,
+                       dax_iodone_t complete_unwritten)
 {
        struct address_space *mapping = inode->i_mapping;
        sector_t sector = bh->b_blocknr << (inode->i_blkbits - 9);
@@ -310,14 +311,14 @@ static int dax_insert_mapping(struct inode *inode, struct 
buffer_head *bh,
  out:
        i_mmap_unlock_read(mapping);
 
-       if (bh->b_end_io)
-               bh->b_end_io(bh, 1);
+       if (buffer_unwritten(bh))
+               complete_unwritten(bh, 1);
 
        return error;
 }
 
 static int do_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
-                       get_block_t get_block)
+                       get_block_t get_block, dax_iodone_t complete_unwritten)
 {
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
@@ -418,7 +419,7 @@ static int do_dax_fault(struct vm_area_struct *vma, struct 
vm_fault *vmf,
                page_cache_release(page);
        }
 
-       error = dax_insert_mapping(inode, &bh, vma, vmf);
+       error = dax_insert_mapping(inode, &bh, vma, vmf, complete_unwritten);
 
  out:
        if (error == -ENOMEM)
@@ -446,7 +447,7 @@ static int do_dax_fault(struct vm_area_struct *vma, struct 
vm_fault *vmf,
  * fault handler for DAX files.
  */
 int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
-                       get_block_t get_block)
+             get_block_t get_block, dax_iodone_t complete_unwritten)
 {
        int result;
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;
@@ -455,7 +456,7 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault 
*vmf,
                sb_start_pagefault(sb);
                file_update_time(vma->vm_file);
        }
-       result = do_dax_fault(vma, vmf, get_block);
+       result = do_dax_fault(vma, vmf, get_block, complete_unwritten);
        if (vmf->flags & FAULT_FLAG_WRITE)
                sb_end_pagefault(sb);
 
diff --git a/fs/ext2/file.c b/fs/ext2/file.c
index e317017..8da747a 100644
--- a/fs/ext2/file.c
+++ b/fs/ext2/file.c
@@ -28,12 +28,12 @@
 #ifdef CONFIG_FS_DAX
 static int ext2_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
-       return dax_fault(vma, vmf, ext2_get_block);
+       return dax_fault(vma, vmf, ext2_get_block, NULL);
 }
 
 static int ext2_dax_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
-       return dax_mkwrite(vma, vmf, ext2_get_block);
+       return dax_mkwrite(vma, vmf, ext2_get_block, NULL);
 }
 
 static const struct vm_operations_struct ext2_dax_vm_ops = {
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 33a09da..f7dabb1 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -192,15 +192,27 @@ errout:
 }
 
 #ifdef CONFIG_FS_DAX
+static void ext4_end_io_unwritten(struct buffer_head *bh, int uptodate)
+{
+       struct inode *inode = bh->b_assoc_map->host;
+       /* XXX: breaks on 32-bit > 16GB. Is that even supported? */
+       loff_t offset = (loff_t)(uintptr_t)bh->b_private << inode->i_blkbits;
+       int err;
+       if (!uptodate)
+               return;
+       WARN_ON(!buffer_unwritten(bh));
+       err = ext4_convert_unwritten_extents(NULL, inode, offset, bh->b_size);
+}
+
 static int ext4_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
-       return dax_fault(vma, vmf, ext4_get_block);
+       return dax_fault(vma, vmf, ext4_get_block, ext4_end_io_unwritten);
                                        /* Is this the right get_block? */
 }
 
 static int ext4_dax_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
-       return dax_mkwrite(vma, vmf, ext4_get_block);
+       return dax_mkwrite(vma, vmf, ext4_get_block, ext4_end_io_unwritten);
 }
 
 static const struct vm_operations_struct ext4_dax_vm_ops = {
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5cb9a21..43433de 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -657,18 +657,6 @@ has_zeroout:
        return retval;
 }
 
-static void ext4_end_io_unwritten(struct buffer_head *bh, int uptodate)
-{
-       struct inode *inode = bh->b_assoc_map->host;
-       /* XXX: breaks on 32-bit > 16GB. Is that even supported? */
-       loff_t offset = (loff_t)(uintptr_t)bh->b_private << inode->i_blkbits;
-       int err;
-       if (!uptodate)
-               return;
-       WARN_ON(!buffer_unwritten(bh));
-       err = ext4_convert_unwritten_extents(NULL, inode, offset, bh->b_size);
-}
-
 /* Maximum number of blocks we map for direct IO at once. */
 #define DIO_MAX_BLOCKS 4096
 
@@ -706,10 +694,15 @@ static int _ext4_get_block(struct inode *inode, sector_t 
iblock,
 
                map_bh(bh, inode->i_sb, map.m_pblk);
                bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
-               if (IS_DAX(inode) && buffer_unwritten(bh) && !io_end) {
+               if (IS_DAX(inode) && buffer_unwritten(bh)) {
+                       /*
+                        * dgc: I suspect unwritten conversion on ext4+DAX is
+                        * fundamentally broken here when there are concurrent
+                        * read/write in progress on this inode.
+                        */
+                       WARN_ON_ONCE(io_end);
                        bh->b_assoc_map = inode->i_mapping;
                        bh->b_private = (void *)(unsigned long)iblock;
-                       bh->b_end_io = ext4_end_io_unwritten;
                }
                if (io_end && io_end->flag & EXT4_IO_END_UNWRITTEN)
                        set_buffer_defer_completion(bh);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 937e280..82100ae 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -70,6 +70,7 @@ typedef int (get_block_t)(struct inode *inode, sector_t 
iblock,
                        struct buffer_head *bh_result, int create);
 typedef void (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
                        ssize_t bytes, void *private);
+typedef void (dax_iodone_t)(struct buffer_head *bh_map, int uptodate);
 
 #define MAY_EXEC               0x00000001
 #define MAY_WRITE              0x00000002
@@ -2603,8 +2604,9 @@ ssize_t dax_do_io(int rw, struct kiocb *, struct inode *, 
struct iov_iter *,
 int dax_clear_blocks(struct inode *, sector_t block, long size);
 int dax_zero_page_range(struct inode *, loff_t from, unsigned len, 
get_block_t);
 int dax_truncate_page(struct inode *, loff_t from, get_block_t);
-int dax_fault(struct vm_area_struct *, struct vm_fault *, get_block_t);
-#define dax_mkwrite(vma, vmf, gb)      dax_fault(vma, vmf, gb)
+int dax_fault(struct vm_area_struct *, struct vm_fault *, get_block_t,
+               dax_iodone_t);
+#define dax_mkwrite(vma, vmf, gb, iod) dax_fault(vma, vmf, gb, iod)
 
 #ifdef CONFIG_BLOCK
 typedef void (dio_submit_t)(int rw, struct bio *bio, struct inode *inode,
-- 
2.0.0

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