Re: [RFC PATCH] xfs: check shared state of when CoW, update reflink flag when io ends

[Darrick J. Wong]

On Fri, Mar 17, 2023 at 03:59:48AM +0000, Shiyang Ruan wrote:
> As is mentioned[1] before, the generic/388 will randomly fail with dmesg
> warning.  This case uses fsstress with a lot of random operations.  It is hard
> to  reproduce.  Finally I found a 100% reproduce condition, which is setting
> the seed to 1677104360.  So I changed the generic/388 code: removed the loop
> and used the code below instad:
> ```
> ($FSSTRESS_PROG $FSSTRESS_AVOID -d $SCRATCH_MNT -v -s 1677104360 -n 221 -p 1 >> $seqres.full) > /dev/null 2>&1
> ($FSSTRESS_PROG $FSSTRESS_AVOID -d $SCRATCH_MNT -v -s 1677104360 -n 221 -p 1 >> $seqres.full) > /dev/null 2>&1
> _check_dmesg_for dax_insert_entry
> ```
> 
> According to the operations log, and kernel debug log I added, I found that
> the reflink flag of one inode won't be unset even if there's no more shared
> extents any more.
>   Then write to this file again.  Because of the reflink flag, xfs thinks it
>     needs cow, and extent(called it extA) will be CoWed to a new
>     extent(called it extB) incorrectly.  And extA is not used any more,
>     but didn't be unmapped (didn't do dax_disassociate_entry()).

IOWs, dax_iomap_copy_around (or something very near it) should be
calling dax_disassociate_entry on the source range after copying extA's
contents to extB to drop its page->shared count?

>   The next time we mapwrite to another file, xfs will allocate extA for it,
>     page fault handler do dax_associate_entry().  BUT bucause the extA didn't
>     be unmapped, it still stores old file's info in page->mapping,->index.
>     Then, It reports dmesg warning when it try to sotre the new file's info.
> 
> So, I think:
>   1. reflink flag should be updated after CoW operations.
>   2. xfs_reflink_allocate_cow() should add "if extent is shared" to determine
>      xfs do CoW or not.
> 
> I made the fix patch, it can resolve the fail of generic/388.  But it causes
> other cases fail: generic/127, generic/263, generic/616, xfs/315 xfs/421. I'm
> not sure if the fix is right, or I have missed something somewhere.  Please
> give me some advice.
> 
> Thank you very much!!
> 
> [1]: https://lore.kernel.org/linux-xfs/1669908538-55-1-git-send-email-ruansy.fnst@xxxxxxxxxxx/
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> ---
>  fs/xfs/xfs_reflink.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_reflink.h |  2 ++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index f5dc46ce9803..a6b07f5c1db2 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -154,6 +154,40 @@ xfs_reflink_find_shared(
>  	return error;
>  }
>  
> +int xfs_reflink_extent_is_shared(
> +	struct xfs_inode	*ip,
> +	struct xfs_bmbt_irec	*irec,
> +	bool			*shared)
> +{
> +	struct xfs_mount	*mp = ip->i_mount;
> +	struct xfs_perag	*pag;
> +	xfs_agblock_t		agbno;
> +	xfs_extlen_t		aglen;
> +	xfs_agblock_t		fbno;
> +	xfs_extlen_t		flen;
> +	int			error = 0;
> +
> +	*shared = false;
> +
> +	/* Holes, unwritten, and delalloc extents cannot be shared */
> +	if (!xfs_bmap_is_written_extent(irec))
> +		return 0;
> +
> +	pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, irec->br_startblock));
> +	agbno = XFS_FSB_TO_AGBNO(mp, irec->br_startblock);
> +	aglen = irec->br_blockcount;
> +	error = xfs_reflink_find_shared(pag, NULL, agbno, aglen, &fbno, &flen,
> +			true);
> +	xfs_perag_put(pag);
> +	if (error)
> +		return error;
> +
> +	if (fbno != NULLAGBLOCK)
> +		*shared = true;
> +
> +	return 0;
> +}
> +
>  /*
>   * Trim the mapping to the next block where there's a change in the
>   * shared/unshared status.  More specifically, this means that we
> @@ -533,6 +567,12 @@ xfs_reflink_allocate_cow(
>  		xfs_ifork_init_cow(ip);
>  	}
>  
> +	error = xfs_reflink_extent_is_shared(ip, imap, shared);
> +	if (error)
> +		return error;
> +	if (!*shared)
> +		return 0;
> +
>  	error = xfs_find_trim_cow_extent(ip, imap, cmap, shared, &found);
>  	if (error || !*shared)
>  		return error;
> @@ -834,6 +874,10 @@ xfs_reflink_end_cow_extent(
>  	/* Remove the mapping from the CoW fork. */
>  	xfs_bmap_del_extent_cow(ip, &icur, &got, &del);
>  
> +	error = xfs_reflink_clear_inode_flag(ip, &tp);

This will disable COW on /all/ blocks in the entire file, including the
shared ones.  At a bare minimum you'd have to scan the entire data fork
to ensure there are no shared extents.  That's probably why doing this
causes so many new regressions.

--D

> +	if (error)
> +		goto out_cancel;
> +
>  	error = xfs_trans_commit(tp);
>  	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>  	if (error)
> diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
> index 65c5dfe17ecf..d5835814bce6 100644
> --- a/fs/xfs/xfs_reflink.h
> +++ b/fs/xfs/xfs_reflink.h
> @@ -16,6 +16,8 @@ static inline bool xfs_is_cow_inode(struct xfs_inode *ip)
>  	return xfs_is_reflink_inode(ip) || xfs_is_always_cow_inode(ip);
>  }
>  
> +int xfs_reflink_extent_is_shared(struct xfs_inode *ip,
> +		struct xfs_bmbt_irec *irec, bool *shared);
>  extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip,
>  		struct xfs_bmbt_irec *irec, bool *shared);
>  int xfs_bmap_trim_cow(struct xfs_inode *ip, struct xfs_bmbt_irec *imap,
> -- 
> 2.39.2
>