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Re: [PATCH v3 2/6] block: block-status cache for data regions
From: |
Eric Blake |
Subject: |
Re: [PATCH v3 2/6] block: block-status cache for data regions |
Date: |
Mon, 16 Aug 2021 16:38:23 -0500 |
User-agent: |
NeoMutt/20210205-719-68949a |
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
- [PATCH v3 0/6] block-status cache for data regions, Hanna Reitz, 2021/08/12
- [PATCH v3 3/6] block: Clarify that @bytes is no limit on *pnum, Hanna Reitz, 2021/08/12
- [PATCH v3 1/6] block: Drop BDS comment regarding bdrv_append(), Hanna Reitz, 2021/08/12
- [PATCH v3 2/6] block: block-status cache for data regions, Hanna Reitz, 2021/08/12
- [PATCH v3 4/6] block/file-posix: Do not force-cap *pnum, Hanna Reitz, 2021/08/12
- [PATCH v3 6/6] block/iscsi: Do not force-cap *pnum, Hanna Reitz, 2021/08/12
- [PATCH v3 5/6] block/gluster: Do not force-cap *pnum, Hanna Reitz, 2021/08/12