Re: [PATCH v3 2/6] block: block-status cache for data regions

[Eric Blake <eblake@redhat.com>]

On Thu, Aug 12, 2021 at 10:41:44AM +0200, Hanna Reitz wrote:
> As we have attempted before
> (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html,
> "file-posix: Cache lseek result for data regions";
> https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html,
> "file-posix: Cache next hole"), this patch seeks to reduce the number of
> SEEK_DATA/HOLE operations the file-posix driver has to perform.  The
> main difference is that this time it is implemented as part of the
> general block layer code.
> 

> We only use the cache for nodes without children (i.e. protocol nodes),
> because that is where the problem is: Drivers that rely on block-status
> implementations outside of qemu (e.g. SEEK_DATA/HOLE).
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---

> +++ b/block.c

> +/**
> + * Check whether [offset, offset + bytes) overlaps with the cached
> + * block-status data region.
> + *
> + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`,
> + * which is what bdrv_bsc_is_data()'s interface needs.
> + * Otherwise, *pnum is not touched.

Why duplicate this comment,...

> + */
> +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs,
> +                                           int64_t offset, int64_t bytes,
> +                                           int64_t *pnum)
> +{
> +    BdrvBlockStatusCache *bsc = qatomic_rcu_read(&bs->block_status_cache);
> +    bool overlaps;
> +
> +    overlaps =
> +        qatomic_read(&bsc->valid) &&
> +        ranges_overlap(offset, bytes, bsc->data_start,
> +                       bsc->data_end - bsc->data_start);
> +
> +    if (overlaps && pnum) {
> +        *pnum = bsc->data_end - offset;
> +    }
> +
> +    return overlaps;
> +}
> +
> +/**
> + * See block_int.h for this function's documentation.
> + */
> +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum)
> +{
> +    RCU_READ_LOCK_GUARD();
> +
> +    return bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum);
> +}
> +
> +/**
> + * See block_int.h for this function's documentation.

...but not these?

But that's minor.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org