linux-ext4 - Re: [PATCH] ext4: Convert int to vm_fault

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Date:   Wed, 1 Aug 2018 18:34:45 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Matthew Wilcox <willy@...radead.org>, adilger.kernel@...ger.ca,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>, kemi.wang@...el.com,
        Sabyasachi Gupta <sabyasachi.linux@...il.com>,
        Brajeswar Ghosh <brajeswar.linux@...il.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: Convert int to vm_fault_t type

On Wed, Aug 1, 2018 at 6:25 PM, Theodore Y. Ts'o <tytso@....edu> 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@...il.com>
>
> 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.

>
> 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