Re: [PATCH] mm: Make sendfile(2) killable

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

On Mon, 12 Oct 2015 14:45:23 +0200 Jan Kara <jack@suse.com> wrote:

> Currently a simple program below issues a sendfile(2) system call which
> takes about 62 days to complete in my test KVM instance.

Geeze some people are impatient.

>         int fd;
>         off_t off = 0;
> 
>         fd = open("file", O_RDWR | O_TRUNC | O_SYNC | O_CREAT, 0644);
>         ftruncate(fd, 2);
>         lseek(fd, 0, SEEK_END);
>         sendfile(fd, fd, &off, 0xfffffff);
> 
> Now you should not ask kernel to do a stupid stuff like copying 256MB in
> 2-byte chunks and call fsync(2) after each chunk but if you do, sysadmin
> should have a way to stop you.
> 
> We actually do have a check for fatal_signal_pending() in
> generic_perform_write() which triggers in this path however because we
> always succeed in writing something before the check is done, we return
> value > 0 from generic_perform_write() and thus the information about
> signal gets lost.

ah.

> Fix the problem by doing the signal check before writing anything. That
> way generic_perform_write() returns -EINTR, the error gets propagated up
> and the sendfile loop terminates early.
>
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2488,6 +2488,11 @@ again:
>  			break;
>  		}
>  
> +		if (fatal_signal_pending(current)) {
> +			status = -EINTR;
> +			break;
> +		}
> +
>  		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
>  						&page, &fsdata);
>  		if (unlikely(status < 0))
> @@ -2525,10 +2530,6 @@ again:
>  		written += copied;
>  
>  		balance_dirty_pages_ratelimited(mapping);
> -		if (fatal_signal_pending(current)) {
> -			status = -EINTR;
> -			break;
> -		}
>  	} while (iov_iter_count(i));
>  
>  	return written ? written : status;

This won't work, will it?  If user hits ^C after we've written a few
pages, `written' is non-zero and the same thing happens?

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