Re: [Devel] [PATCH hci-8.1 v3 1/3] dm-qcow2: add qcow2_md_page_read_sync() helper

["Denis V. Lunev" <den@virtuozzo.com>]

On 10/8/26 12:04, Alexander Ivanov wrote:
> Add a helper, which returns an up to date md page from md cache,
> or allocates a new one, inserts it and reads it synchronously.
> On read failure the page is erased from cache and freed.
>
> Use it in qcow2_parse_header() instead of open-coded allocation
> and read of page 0. It will also be used by the next patch.
>
> https://virtuozzo.atlassian.net/browse/VSTOR-148171
> Feature: dm-qcow2: block device over QCOW2 files driver
>
> Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
> ---
>  drivers/md/dm-qcow2-target.c | 36 +++++++++++++++++++++++++++++++-----
>  drivers/md/dm-qcow2.h        |  2 ++
>  2 files changed, 33 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
> index be09615eb8e2..3e47e66b4007 100644
> --- a/drivers/md/dm-qcow2-target.c
> +++ b/drivers/md/dm-qcow2-target.c
> @@ -418,6 +418,36 @@ int qcow2_alloc_and_insert_md_page(struct qcow2 *qcow2, u64 index, struct md_pag
>  }
>  ALLOW_ERROR_INJECTION(qcow2_alloc_and_insert_md_page, ERRNO);
>  
> +/*
> + * Returns up to date md page from cache, or allocates
> + * a new one and reads it synchronously.
> + */
> +int qcow2_md_page_read_sync(struct qcow2 *qcow2, u64 id, struct md_page **md)
> +{
> +	int ret;
> +
> +	spin_lock_irq(&qcow2->md_pages_lock);
> +	*md = qcow2_md_page_find_or_postpone(qcow2, id, NULL);
> +	spin_unlock_irq(&qcow2->md_pages_lock);
> +	if (*md)
> +		return 0;
> +
> +	ret = qcow2_alloc_and_insert_md_page(qcow2, id, md);
> +	if (ret)
> +		return ret;
> +
> +	ret = qcow2_rw_page_sync(READ, qcow2, id, (*md)->page);
> +	spin_lock_irq(&qcow2->md_pages_lock);
> +	if (ret)
> +		qcow2_md_page_erase(qcow2, *md);
> +	else
> +		(*md)->status |= MD_UPTODATE;
> +	spin_unlock_irq(&qcow2->md_pages_lock);
> +	if (ret)
> +		qcow2_free_md_page(*md);
> +	return ret;
> +}
> +

The comment on qcow2_md_page_read_sync() says it returns
an up-to-date page, but that depends on no read being in flight.
If a page is cached but not MD_UPTODATE,
qcow2_md_page_find_or_postpone() returns NULL and the insert
that follows fails.

This can't happen on a suspended device, but
the comment should state the assumption.
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