Re: [PATCH][v6] filemap: drop the mmap_sem for all blocking operations

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Dec 2018 10:27:57 -0500 Josef Bacik <josef@toxicpanda.com> wrote:

> v5->v6:
> - added more comments as per Andrew's suggestion.
> - fixed the fpin leaks in the two error paths that were pointed out.
> 

hm,

> --- a/mm/filemap.c~filemap-drop-the-mmap_sem-for-all-blocking-operations-v6
> +++ a/mm/filemap.c
> @@ -2461,7 +2476,8 @@ static struct file *do_sync_mmap_readahe
>  
>  /*
>   * Asynchronous readahead happens when we find the page and PG_readahead,
> - * so we want to possibly extend the readahead further..
> + * so we want to possibly extend the readahead further.  We return the file that
> + * was pinned if we have to drop the mmap_sem in order to do IO.
>   */
>  static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  					    struct page *page)
> @@ -2545,14 +2561,15 @@ retry_find:
>  		page = pagecache_get_page(mapping, offset,
>  					  FGP_CREAT|FGP_FOR_MMAP,
>  					  vmf->gfp_mask);
> -		if (!page)
> +		if (!page) {
> +			if (fpin)
> +				goto out_retry;

Is this right?  If pagecache_get_page() returns NULL we can now return
VM_FAULT_MAJOR|VM_FAULT_RETRY whereas we used to return ENOMEM.

>  			return vmf_error(-ENOMEM);
> +		}
>  	}
>  
> -	if (!lock_page_maybe_drop_mmap(vmf, page, &fpin)) {
> -		put_page(page);
> -		return ret | VM_FAULT_RETRY;
> -	}
> +	if (!lock_page_maybe_drop_mmap(vmf, page, &fpin))
> +		goto out_retry;
>  
>  	/* Did it get truncated? */
>  	if (unlikely(page->mapping != mapping)) {