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From: Chandan Babu R <chandanrlinux@gmail.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-xfs@vger.kernel.org, darrick.wong@oracle.com, david@fromorbit.com
Subject: Re: [PATCH V6 08/11] xfs: Check for extent overflow when remapping an extent
Date: Thu, 15 Oct 2020 15:31:26 +0530	[thread overview]
Message-ID: <1680655.hsWa3aTUJI@garuda> (raw)
In-Reply-To: <20201015083945.GH5902@infradead.org>

On Thursday 15 October 2020 2:09:45 PM IST Christoph Hellwig wrote:
> This patch demonstrates very well why I think having these magic
> defines and the comments in a header makes no sense.
> 
> > +/*
> > + * Remapping an extent involves unmapping the existing extent and mapping in the
> > + * new extent.
> > + *
> > + * When unmapping, an extent containing the entire unmap range can be split into
> > + * two extents,
> > + * i.e. | Old extent | hole | Old extent |
> > + * Hence extent count increases by 1.
> > + *
> > + * Mapping in the new extent into the destination file can increase the extent
> > + * count by 1.
> > + */
> > +#define XFS_IEXT_REFLINK_REMAP_CNT(smap_real, dmap_written) \
> > +	(((smap_real) ? 1 : 0) + ((dmap_written) ? 1 : 0))
> > +
> >  /*
> >   * Fork handling.
> >   */
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index 4f0198f636ad..c9f9ff68b5bb 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -1099,6 +1099,11 @@ xfs_reflink_remap_extent(
> >  			goto out_cancel;
> >  	}
> >  
> > +	error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK,
> > +			XFS_IEXT_REFLINK_REMAP_CNT(smap_real, dmap_written));
> > +	if (error)
> > +		goto out_cancel;
> > +
> 
> This is a completely mess.
> 
> If OTOH xfs_reflink_remap_extent had a local variable for the potential
> max number of extents, which is incremented near the initialization
> of smap_real and dmap_written, with a nice comment near to each
> increment it would make complete sense to the reader.
>

How about following the traits of XFS_IEXT_WRITE_UNWRITTEN_CNT (writing
to unwritten extent) and XFS_IEXT_REFLINK_END_COW_CNT (moving an extent
from cow fork to data fork) and setting XFS_IEXT_REFLINK_REMAP_CNT to a
worst case value of 2? A write spanning the entirety of an unwritten extent
does not change the extent count. Similarly, If there are no extents in the
data fork spanning the file range mapped by an extent in the cow
fork, moving the extent from cow fork to data fork increases the extent count
by just 1 and not by the worst case count of 2.


-- 
chandan




  reply	other threads:[~2020-10-15 10:01 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-12  9:29 [PATCH V6 00/11] Bail out if transaction can cause extent count to overflow Chandan Babu R
2020-10-12  9:29 ` [PATCH V6 01/11] xfs: Add helper for checking per-inode extent count overflow Chandan Babu R
2020-10-15  8:34   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 02/11] xfs: Check for extent overflow when trivally adding a new extent Chandan Babu R
2020-10-15  8:34   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 03/11] xfs: Check for extent overflow when punching a hole Chandan Babu R
2020-10-15  8:35   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 04/11] xfs: Check for extent overflow when adding/removing xattrs Chandan Babu R
2020-10-15  8:36   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 05/11] xfs: Check for extent overflow when adding/removing dir entries Chandan Babu R
2020-10-15  8:36   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 06/11] xfs: Check for extent overflow when writing to unwritten extent Chandan Babu R
2020-10-15  8:36   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 07/11] xfs: Check for extent overflow when moving extent from cow to data fork Chandan Babu R
2020-10-15  8:37   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 08/11] xfs: Check for extent overflow when remapping an extent Chandan Babu R
2020-10-15  8:39   ` Christoph Hellwig
2020-10-15 10:01     ` Chandan Babu R [this message]
2020-10-15 18:45       ` Darrick J. Wong
2020-10-16  4:27         ` Chandan Babu R
2020-10-16  7:04       ` Christoph Hellwig
2020-10-16 11:28         ` Chandan Babu R
2020-10-16 15:29           ` Darrick J. Wong
2020-10-17  2:55             ` Chandan Babu R
2020-10-12  9:29 ` [PATCH V6 09/11] xfs: Check for extent overflow when swapping extents Chandan Babu R
2020-10-12  9:29 ` [PATCH V6 10/11] xfs: Introduce error injection to reduce maximum inode fork extent count Chandan Babu R
2020-10-15  8:40   ` Christoph Hellwig
2020-10-12  9:29 ` [PATCH V6 11/11] xfs: Introduce error injection to allocate only minlen size extents for files Chandan Babu R
2020-10-15  8:41   ` Christoph Hellwig
2020-10-15 10:02     ` Chandan Babu R
2020-10-15 18:41       ` Darrick J. Wong
2020-10-16 11:31         ` Chandan Babu R

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