Re: [PATCH] um: Fix WRITE_ZEROES in the UBD Driver

[Anton Ivanov <anton.ivanov@cambridgegreys.com>]

On 25/01/2022 17:14, Frédéric Danis wrote:
> Call to fallocate with FALLOC_FL_PUNCH_HOLE on a device backed by a sparse
> file can end up by missing data, zeroes data range, if the underlying file
> is used with a tool like bmaptool which will referenced only used spaces.

Can you elaborate on when do you miss data.

A file with a hole created using FALLOC_FL_PUNCH_HOLE when reading from 
the hole should return zeroes.

If it returns anything else or if the zeroed range exceeds the one 
specified in falloc - that is an underlying fs bug.

>
> Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
> ---
>   arch/um/drivers/ubd_kern.c  | 8 +++++++-
>   arch/um/include/shared/os.h | 1 +
>   arch/um/os-Linux/file.c     | 9 +++++++++
>   3 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
> index 69d2d0049a61..b03269faef71 100644
> --- a/arch/um/drivers/ubd_kern.c
> +++ b/arch/um/drivers/ubd_kern.c
> @@ -1526,13 +1526,19 @@ static void do_io(struct io_thread_req *req, struct io_desc *desc)
>   			}
>   			break;
>   		case REQ_OP_DISCARD:
> -		case REQ_OP_WRITE_ZEROES:
>   			n = os_falloc_punch(req->fds[bit], off, len);
>   			if (n) {
>   				req->error = map_error(-n);
>   				return;
>   			}
>   			break;
> +		case REQ_OP_WRITE_ZEROES:
> +			n = os_falloc_zeroes(req->fds[bit], off, len);
> +			if (n) {
> +				req->error = map_error(-n);
> +				return;
> +			}
> +			break;
>   		default:
>   			WARN_ON_ONCE(1);
>   			req->error = BLK_STS_NOTSUPP;
> diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
> index 00214059d9ec..fafde1d5416e 100644
> --- a/arch/um/include/shared/os.h
> +++ b/arch/um/include/shared/os.h
> @@ -168,6 +168,7 @@ extern unsigned os_major(unsigned long long dev);
>   extern unsigned os_minor(unsigned long long dev);
>   extern unsigned long long os_makedev(unsigned major, unsigned minor);
>   extern int os_falloc_punch(int fd, unsigned long long offset, int count);
> +extern int os_falloc_zeroes(int fd, unsigned long long offset, int count);
>   extern int os_eventfd(unsigned int initval, int flags);
>   extern int os_sendmsg_fds(int fd, const void *buf, unsigned int len,
>   			  const int *fds, unsigned int fds_num);
> diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
> index e4421dbc4c36..fc4450db59bd 100644
> --- a/arch/um/os-Linux/file.c
> +++ b/arch/um/os-Linux/file.c
> @@ -625,6 +625,15 @@ int os_falloc_punch(int fd, unsigned long long offset, int len)
>   	return n;
>   }
>   
> +int os_falloc_zeroes(int fd, unsigned long long offset, int len)
> +{
> +	int n = fallocate(fd, FALLOC_FL_ZERO_RANGE|FALLOC_FL_KEEP_SIZE, offset, len);
> +
> +	if (n < 0)
> +		return -errno;
> +	return n;
> +}
> +
>   int os_eventfd(unsigned int initval, int flags)
>   {
>   	int fd = eventfd(initval, flags);


-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/


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