Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:
> David Chinner wrote:
> 
>> On Mon, Mar 19, 2007 at 05:37:03PM +1100, Nick Piggin wrote:
>>
>>> David Chinner wrote:
>>>
> 
>>>> +block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
>>>> +           get_block_t get_block)
>>>> +{
>>>> +    struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
>>>> +    unsigned long end;
>>>> +    loff_t size;
>>>> +    int ret = -EINVAL;
>>>> +
>>>> +    lock_page(page);
>>>> +    size = i_size_read(inode);
>>>> +    if ((page->mapping != inode->i_mapping) ||
>>>> +        ((page->index << PAGE_CACHE_SHIFT) > size)) {
>>>> +        /* page got truncated out from underneath us */
>>>> +        goto out_unlock;
>>>> +    }
>>>
>>>
>>> I see your explanation above, but I still don't see why this can't
>>> just follow the conventional if (!page->mapping) check for truncation.
>>> If the test happens to be performed after truncate concurrently
>>> decreases i_size, then the blocks are going to get truncated by the
>>> truncate afterwards anyway.
>>
>>
>>
>> We have to read the inode size in the normal case so that we know if
>> the page is at EOF and is a partial page so we don't allocate past EOF in
>> block_prepare_write().  Hence it seems like a no-brainer to me to check
>> and error out on a page that we *know* is beyond EOF.
>>
>> I can drop the check if you see no value in it - I just don't
>> like the idea of ignoring obvious boundary condition violations...
> 
> 
> I would prefer it dropped, to be honest. I can see how the check does
> pick up that corner case, however truncate is difficult enough (at
> least, it has been an endless source of problems) that we want to keep
> everyone else simple and have all the non-trivial stuff in truncate.
> 

Hmm, actually on second thoughts it probably is reasonable to recheck
i_size under the page lock... we need to do similar in the nopage path
to close the nopage vs invalidate race.

However, the already-truncated test I think can just be !page->mapping:
there should be no way for the page mapping to change to something
other than NULL.

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