On Wed, Aug 1, 2018 at 6:34 PM, Souptick Joarder <jrdr.linux@xxxxxxxxx> wrote:
> On Wed, Aug 1, 2018 at 6:25 PM, Theodore Y. Ts'o <tytso@xxxxxxx> wrote:
>> 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,
>> ext4_da_get_block_prep);
>> } 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.
>
> We have also changed block_page_mkwrite() to return vm_fault_t, but in
> a different patch. Hopefully that patch will be in linux-next tree soon.
Link to other patch where block_page_mkwrite() is changed to return
vm_fault_t type.
https://www.spinics.net/lists/linux-fsdevel/msg130670.html
>
>>
>> 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
