From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Chandan Babu R <chandanrlinux@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>,
linux-xfs@vger.kernel.org, david@fromorbit.com
Subject: Re: [PATCH V6 08/11] xfs: Check for extent overflow when remapping an extent
Date: Thu, 15 Oct 2020 11:45:45 -0700 [thread overview]
Message-ID: <20201015184545.GC9832@magnolia> (raw)
In-Reply-To: <1680655.hsWa3aTUJI@garuda>
On Thu, Oct 15, 2020 at 03:31:26PM +0530, Chandan Babu R wrote:
> 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.
Probably not a huge deal, since at worst we bail out of reflink early
and userspace can just decide to fall back to a pagecache copy or
whatever. It'd be harder to deal with if this was the cow path where we
long ago returned from write() and even writeback...
...though now that I think about it, what /does/ happens if
_reflink_end_cow trips over this? I wonder if inodes need to be able to
reserve extent count for later, but ... hgnghghg that seems hard to
reason about.
--D
>
>
> --
> chandan
>
>
>
next prev parent reply other threads:[~2020-10-15 18:45 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
2020-10-15 18:45 ` Darrick J. Wong [this message]
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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