From: Taotao Chen <chentaotao@didiglobal.com>

Implement IOCB_DONTCACHE flag handling in ext4 write paths:
1. Check IOCB_DONTCACHE flag passed via fsdata
2. Propagate FGP_DONTCACHE to page cache operations

Existing write behavior remains unchanged when IOCB_DONTCACHE is not set.

Signed-off-by: Taotao Chen <chentaotao@didiglobal.com>
---
 fs/ext4/inode.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 94c7d2d828a6..787dd152a47e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1147,16 +1147,22 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
 {
 	struct inode *inode = mapping->host;
 	int ret, needed_blocks;
+	int iocb_flag;
 	handle_t *handle;
 	int retries = 0;
 	struct folio *folio;
 	pgoff_t index;
+	fgf_t fgp = FGP_WRITEBEGIN;
 	unsigned from, to;
 
 	ret = ext4_emergency_state(inode->i_sb);
 	if (unlikely(ret))
 		return ret;
 
+	iocb_flag = (int)(uintptr_t)(*fsdata);
+	if (iocb_flag & IOCB_DONTCACHE)
+		fgp |= FGP_DONTCACHE;
+
 	trace_ext4_write_begin(inode, pos, len);
 	/*
 	 * Reserve one block more for addition to orphan list in case
@@ -1184,7 +1190,7 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
 	 * the folio (if needed) without using GFP_NOFS.
 	 */
 retry_grab:
-	folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
+	folio = __filemap_get_folio(mapping, index, fgp,
 					mapping_gfp_mask(mapping));
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
@@ -2917,6 +2923,8 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 			       struct folio **foliop, void **fsdata)
 {
 	int ret, retries = 0;
+	int iocb_flag;
+	fgf_t fgp = FGP_WRITEBEGIN;
 	struct folio *folio;
 	pgoff_t index;
 	struct inode *inode = mapping->host;
@@ -2928,10 +2936,15 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 	index = pos >> PAGE_SHIFT;
 
 	if (ext4_nonda_switch(inode->i_sb) || ext4_verity_in_progress(inode)) {
+		ret = ext4_write_begin(file, mapping, pos, len, foliop, fsdata);
 		*fsdata = (void *)FALL_BACK_TO_NONDELALLOC;
-		return ext4_write_begin(file, mapping, pos,
-					len, foliop, fsdata);
+		return ret;
 	}
+
+	iocb_flag = (int)(uintptr_t)(*fsdata);
+	if (iocb_flag & IOCB_DONTCACHE)
+		fgp |= FGP_DONTCACHE;
+
 	*fsdata = (void *)0;
 	trace_ext4_da_write_begin(inode, pos, len);
 
@@ -2945,7 +2958,7 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 	}
 
 retry:
-	folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
+	folio = __filemap_get_folio(mapping, index, fgp,
 			mapping_gfp_mask(mapping));
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
-- 
2.34.1

On Mon, Apr 21, 2025 at 10:50:30AM +0000, 陈涛涛 Taotao Chen wrote:
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 94c7d2d828a6..787dd152a47e 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1147,16 +1147,22 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
>  {
>  	struct inode *inode = mapping->host;
>  	int ret, needed_blocks;
> +	int iocb_flag;
>  	handle_t *handle;
>  	int retries = 0;
>  	struct folio *folio;
>  	pgoff_t index;
> +	fgf_t fgp = FGP_WRITEBEGIN;
>  	unsigned from, to;
>  
>  	ret = ext4_emergency_state(inode->i_sb);
>  	if (unlikely(ret))
>  		return ret;
>  
> +	iocb_flag = (int)(uintptr_t)(*fsdata);
> +	if (iocb_flag & IOCB_DONTCACHE)
> +		fgp |= FGP_DONTCACHE;
> +

See my comment against the first patch in this series.  It *should* be
possible to solve the problem just for ext4 by adding this line here:

	*fsdata = (void *)0;

The problem is that it's super-fragile, since how *fsdata gets used
changes at different points in time, so it makes code review and
maintenance more difficult.  (As evidenced by the fact that you missed
this; this is not a criticism on your programming ability, but rather
for the design choise of overloading the use of *fsdata.  This is a
trap that someone else might fall into when doing future code
changes.)

And of course, the question is whether PATCH 1/3 could potentially
break other file systems.  We would need audit all of the other
*_write_begin() functions, and then document this for the sake of
future file system developers that might want to change their
write_begin() function.

This is why my preference would be to add an extra flags paramter to
write_begin(), but that is going to be a lot more work.

Cheers,

						- Ted