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List:       linux-btrfs
Subject:    Re: [PATCH] btrfs: do not zero f_bavail if we have available space
From:       Martin Steigerwald <martin () lichtvoll ! de>
Date:       2020-01-31 20:06:03
Message-ID: 2687257.pSW7MMRxUZ () merkaba
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Josef Bacik - 31.01.20, 15:31:05 CET:
> There was some logic added a while ago to clear out f_bavail in
> statfs() if we did not have enough free metadata space to satisfy our
> global reserve.  This was incorrect at the time, however didn't
> really pose a problem for normal file systems because we would often
> allocate chunks if we got this low on free metadata space, and thus
> wouldn't really hit this case unless we were actually full.
> 
> Fast forward to today and now we are much better about not allocating
> metadata chunks all of the time.  Couple this with d792b0f19711 which
> now means we'll easily have a larger global reserve than our free
> space, we are now more likely to trip over this while still having
> plenty of space.
> 
> Fix this by skipping this logic if the global rsv's space_info is not
> full.  space_info->full is 0 unless we've attempted to allocate a
> chunk for that space_info and that has failed.  If this happens then
> the space for the global reserve is definitely sacred and we need to
> report b_avail == 0, but before then we can just use our calculated
> b_avail.

Thank you!

The fix works:

merkaba:~> LANG=en df -hT /daten
Filesystem             Type   Size  Used Avail Use% Mounted on
/dev/mapper/sata-daten btrfs  400G  311G   91G  78% /daten

Tested-By: Martin Steigerwald <martin@lichtvoll.de>

Thanks,
Martin

> 
> There are other cases where df isn't quite right, and Qu is addressing
> them in a more holistic way.  This simply fixes the users that are
> currently experiencing pain because of this problem.
> 
> Fixes: ca8a51b3a979 ("btrfs: statfs: report zero available if metadata
> are exhausted") Reported-by: Martin Steigerwald <martin@lichtvoll.de>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/super.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index d421884f0c23..42433ca822aa 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2143,7 +2143,15 @@ static int btrfs_statfs(struct dentry *dentry,
> struct kstatfs *buf) */
>  	thresh = SZ_4M;
> 
> -	if (!mixed && total_free_meta - thresh < block_rsv->size)
> +	/*
> +	 * We only want to claim there's no available space if we can no
> longer +	 * allocate chunks for our metadata profile and our 
global
> reserve will +	 * not fit in the free metadata space.  If we aren't
> ->full then we +	 * still can allocate chunks and thus are fine using
> the currently +	 * calculated f_bavail.
> +	 */
> +	if (!mixed && block_rsv->space_info->full &&
> +	    total_free_meta - thresh < block_rsv->size)
>  		buf->f_bavail = 0;
> 
>  	buf->f_type = BTRFS_SUPER_MAGIC;


-- 
Martin


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