Re: [PATCH] ext4: Convert int to vm_fault_t type

From: Theodore Y. Ts'o
Date: Wed Aug 01 2018 - 08:55:34 EST

On Sat, Jul 28, 2018 at 02:20:00PM +0530, Souptick Joarder wrote:
> Use new return type vm_fault_t for ext4_page_mkwrite
> handler and block_page_mkwrite_return.
> Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>

FYI, this patch was very sloppy, and didn't do the right thing. That's
because of how you messed with the changing how the return codes are
now handled.

> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -6108,27 +6108,27 @@ static int ext4_bh_unmapped(handle_t *handle, struct buffer_head *bh)
> return !buffer_mapped(bh);
> }
> -int ext4_page_mkwrite(struct vm_fault *vmf)
> +vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> {
> struct vm_area_struct *vma = vmf->vma;
> struct page *page = vmf->page;
> loff_t size;
> unsigned long len;
> - int ret;
> + vm_fault_t ret;
> struct file *file = vma->vm_file;
> struct inode *inode = file_inode(file);
> struct address_space *mapping = inode->i_mapping;
> handle_t *handle;
> get_block_t *get_block;
> - int retries = 0;
> + int retries = 0, err;

OK, ret now is a vm_fault_t, and err is an error return....

> @@ -6138,9 +6138,9 @@ int ext4_page_mkwrite(struct vm_fault *vmf)
> do {
> ret = block_page_mkwrite(vma, vmf,
> ext4_da_get_block_prep);

But block_page_mkwrite() still returns an int, not a vm_fault_t....

> - } while (ret == -ENOSPC &&
> + } while (ret == VM_FAULT_SIGBUS &&
> ext4_should_retry_alloc(inode->i_sb, &retries));

So this is Just wrong, This needed to be:

do {
err = block_page_mkwrite(vma, vmf,
} while (err == -ENOSPC &&
ext4_should_retry_alloc(inode->i_sb, &retries));
goto out_ret;

That's because out_ret is what will translate the int error code to
the vm_fault_t via:

ret = block_page_mkwrite_return(err);

The fact that ext4_page_mkwrite() returns a vm_fault_t, while
block_page_mkwrite() returns an int which then has to get translated
into a vm_fault_t via block_page_mkwrite_return() is I suspect going
to confuse an awful lot of callers.

I'll fix up the patch, but I just wanted to call your attention to
this pitfall in the patch which confused even you as the patch author....

(BTW, the buggy patch triggered a new failure, ext4/307, which is how
I noticed that the patch was all wrong. If you had run any kind of
static code checker you would have noticed that block_page_mkwrite()
was returning an int and that was getting assigned into a variable of
type vm_fault_t. The fact that you *didn't* notice makes me worry
that all of this code churn may, in the end, not actually help us as
much as we thought. :-(

- Ted