[Devel] [vzlin-dev] [PATCH rh7] ploop: fix race on prealloc request

[Dmitry Monakhov]

Maxim Patlasov <MPatlasov at openvz.org> writes:

> kaio_submit_alloc() sometimes piggybacks ploop request with additional work:
> set preq->prealloc_size, pass preq to fsync_thread, perform preallocation
> there, get it back to kaio_submit_alloc(), move preallocation from
> preq->prealloc_size to io->prealloced_size.
>
> The problem happens when such a request does not get back to kaio_submit_alloc.
> For example, it may happen if ploop shrink or compact is in progress: by the
> time the request is re-scheduled after preallocation, free blocks emerges,
> and ploop_entry_request() use one of them calling kaio_submit intead of
> kaio_submit_alloc.
>
> To solve this case and other similar ones, the patch moves the code
> transferring preq->prealloc_size to ploop_req_state_process(). Now, this
> transfer will be done independently on how ploop state machine decides to
> handle that preq.
>
> https://jira.sw.ru/browse/PSBM-35095
ACK
> ---
>  drivers/block/ploop/dev.c     |   13 +++++++++++++
>  drivers/block/ploop/io_kaio.c |    9 +--------
>  2 files changed, 14 insertions(+), 8 deletions(-)
> kaio_submit_alloc() sometimes piggybacks ploop request with additional work:
> set preq->prealloc_size, pass preq to fsync_thread, perform preallocation
> there, get it back to kaio_submit_alloc(), move preallocation from
> preq->prealloc_size to io->prealloced_size.
>
> The problem happens when such a request does not get back to kaio_submit_alloc.
> For example, it may happen if ploop shrink or compact is in progress: by the
> time the request is re-scheduled after preallocation, free blocks emerges,
> and ploop_entry_request() use one of them calling kaio_submit intead of
> kaio_submit_alloc.
>
> To solve this case and other similar ones, the patch moves the code
> transferring preq->prealloc_size to ploop_req_state_process(). Now, this
> transfer will be done independently on how ploop state machine decides to
> handle that preq.
>
> https://jira.sw.ru/browse/PSBM-35095
> ---
>  drivers/block/ploop/dev.c     |   13 +++++++++++++
>  drivers/block/ploop/io_kaio.c |    9 +--------
>  2 files changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/block/ploop/dev.c b/drivers/block/ploop/dev.c
> index 30eb8a7..97e75a7 100644
> --- a/drivers/block/ploop/dev.c
> +++ b/drivers/block/ploop/dev.c
> @@ -2275,6 +2275,19 @@ static void ploop_req_state_process(struct ploop_request * preq)
>  			ploop_tracker_notify(plo, sec);
>  	}
>  
> +	/* trick: preq->prealloc_size is actually new pos of eof */
> +	if (unlikely(preq->prealloc_size && !preq->error)) {
> +		struct ploop_io *io = &ploop_top_delta(plo)->io;
> +		int log = preq->plo->cluster_log + 9;
> +
> +		BUG_ON(preq != io->prealloc_preq);
> +		io->prealloc_preq = NULL;
> +
> +		io->prealloced_size = preq->prealloc_size -
> +				      ((loff_t)io->alloc_head << log);
> +		preq->prealloc_size = 0; /* only for sanity */
> +	}
> +
>  restart:
>  	__TRACE("ST %p %u %lu\n", preq, preq->req_cluster, preq->eng_state);
>  	switch (preq->eng_state) {
> diff --git a/drivers/block/ploop/io_kaio.c b/drivers/block/ploop/io_kaio.c
> index ccdf1f4..db900a3 100644
> --- a/drivers/block/ploop/io_kaio.c
> +++ b/drivers/block/ploop/io_kaio.c
> @@ -460,14 +460,7 @@ kaio_submit_alloc(struct ploop_io *io, struct ploop_request * preq,
>  		return;
>  	}
>  
> -	/* trick: preq->prealloc_size is actually new pos of eof */
> -	if (unlikely(preq->prealloc_size)) {
> -		BUG_ON(preq != io->prealloc_preq);
> -		io->prealloc_preq = NULL;
> -
> -		io->prealloced_size = preq->prealloc_size - ((loff_t)iblk << log);
> -		preq->prealloc_size = 0; /* only for sanity */
> -	}
> +	BUG_ON(preq->prealloc_size);
>  
>  	if (unlikely(io->prealloced_size < clu_siz)) {
>  		if (!io->prealloc_preq) {
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