Re: [PATCH v4 2/3] btrfs: Split remaining space to discard in chunks

[Luca Stefani]

On 16/09/24 12:39, Qu Wenruo wrote:
> 在 2024/9/16 19:46, Luca Stefani 写道:
> > Per Qu Wenruo in case we have a very large disk, e.g. 8TiB device,
> > mostly empty although we will do the split according to our super block
> > locations, the last super block ends at 256G, we can submit a huge
> > discard for the range [256G, 8T), causing a super large delay.
> >
> > We now split the space left to discard based on BTRFS_MAX_DATA_CHUNK_SIZE
> > in preparation of introduction of cancellation signals handling.
> >
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=219180
> > Link: https://bugzilla.suse.com/show_bug.cgi?id=1229737
> > Signed-off-by: Luca Stefani <luca.stefani.ge1@xxxxxxxxx>
> > ---
> >   fs/btrfs/extent-tree.c | 24 +++++++++++++++++++-----
> >   1 file changed, 19 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index a5966324607d..cbe66d0acff8 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -1239,7 +1239,7 @@ static int btrfs_issue_discard(struct 
> > block_device *bdev, u64 start, u64 len,
> >                      u64 *discarded_bytes)
> >   {
> >       int j, ret = 0;
> > -    u64 bytes_left, end;
> > +    u64 bytes_left, bytes_to_discard, end;
> >       u64 aligned_start = ALIGN(start, 1 << SECTOR_SHIFT);
> >       /* Adjust the range to be aligned to 512B sectors if necessary. */
> > @@ -1300,13 +1300,27 @@ static int btrfs_issue_discard(struct 
> > block_device *bdev, u64 start, u64 len,
> >           bytes_left = end - start;
> >       }
> > -    if (bytes_left) {
> > +    while (bytes_left) {
> > +        if (bytes_left > BTRFS_MAX_DATA_CHUNK_SIZE)
> > +            bytes_to_discard = BTRFS_MAX_DATA_CHUNK_SIZE;
>
> That MAX_DATA_CHUNK_SIZE is only possible for RAID0/RAID10/RAID5/RAID6, 
> by spanning the device extents across multiple devices.
>
> For each device, the maximum size is limited to 1G (check 
> init_alloc_chunk_ctl_policy_regular()).
>
> So you can just limit it to 1G instead.
> (If you want, you can also extract that into a macro as a cleanup).
I think SZ_1G is enough for now.
> Furthermore, you can use min() instead of a if ().
>
> So you only need:
>
>          bytes_to_discard = min(SZ_1G, bytes_left);
>
> Otherwise this looks good enough to me.
> If the 1G size is not good enough, we can later tune it to smaller values.
>
> Personally speaking I think 1G would be enough.
>
> Thanks,
> Qu
Ack, done in v5
> > +        else
> > +            bytes_to_discard = bytes_left;
> > +
> >           ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
> > -                       bytes_left >> SECTOR_SHIFT,
> > +                       bytes_to_discard >> SECTOR_SHIFT,
> >                          GFP_NOFS);
> > -        if (!ret)
> > -            *discarded_bytes += bytes_left;
> > +
> > +        if (ret) {
> > +            if (ret != -EOPNOTSUPP)
> > +                break;
> > +            continue;
> > +        }
> > +
> > +        start += bytes_to_discard;
> > +        bytes_left -= bytes_to_discard;
> > +        *discarded_bytes += bytes_to_discard;
> >       }
> > +
> >       return ret;
> >   }