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Re: [PATCH 2/3] xfs: Implement FALLOC_FL_COLLAPSE_RANGE

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 2/3] xfs: Implement FALLOC_FL_COLLAPSE_RANGE
From: Namjae Jeon <linkinjeon@xxxxxxxxx>
Date: Thu, 1 Aug 2013 14:33:09 +0900
Cc: tytso@xxxxxxx, adilger.kernel@xxxxxxxxx, bpm@xxxxxxx, elder@xxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, a.sangwan@xxxxxxxxxxx, Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
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In-reply-to: <20130801011812.GJ7118@dastard>
References: <1375281734-15874-1-git-send-email-linkinjeon@xxxxxxxxx> <20130801011812.GJ7118@dastard>
2013/8/1, Dave Chinner <david@xxxxxxxxxxxxx>:
> On Wed, Jul 31, 2013 at 11:42:14PM +0900, Namjae Jeon wrote:
>> From: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
>>
>> New fallocate flag FALLOC_FL_COLLAPSE_RANGE implementation for XFS.
>
Hi Dave.
> A good start, but there's lots of work needed to make this safe for
> general use.
First, Thanks for your advice and help.

>
>> Signed-off-by: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
>> Signed-off-by: Ashish Sangwan <a.sangwan@xxxxxxxxxxx>
>> ---
>>  fs/xfs/xfs_bmap.c     |   92
>> +++++++++++++++++++++++++++++++++++++++++++++++++
>>  fs/xfs/xfs_bmap.h     |    3 ++
>>  fs/xfs/xfs_file.c     |   26 ++++++++++++--
>>  fs/xfs/xfs_iops.c     |   35 +++++++++++++++++++
>>  fs/xfs/xfs_vnodeops.c |   62 +++++++++++++++++++++++++++++++++
>>  fs/xfs/xfs_vnodeops.h |    2 ++
>>  6 files changed, 217 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
>> index 05c698c..ae677b1 100644
>> --- a/fs/xfs/xfs_bmap.c
>> +++ b/fs/xfs/xfs_bmap.c
>> @@ -6145,3 +6145,95 @@ next_block:
>>
>>      return error;
>>  }
>> +
>> +/*
>> + * xfs_update_logical()
>> + *  Updates starting logical block of extents by subtracting
>> + *  shift from them. At max XFS_LINEAR_EXTS number extents
>> + *  will be updated and *current_ext is the extent number which
>> + *  is currently being updated.
>
> Single space after the "*" in the comments. Also, format them out to
> 80 columns.
>
Single space after * followed by a tab gives checkpath warning:
WARNING: please, no space before tabs
#29: FILE: fs/xfs/xfs_bmap.c:6151:
+ * ^IUpdates starting logical block of extents by subtracting$

>> + */
>> +int
>> +xfs_update_logical(
>> +    xfs_trans_t             *tp,
>> +    struct xfs_inode        *ip,
>> +    int                     *done,
>> +    xfs_fileoff_t           start_fsb,
>> +    xfs_fileoff_t           shift,
>> +    xfs_extnum_t            *current_ext)
>
> This belongs in xfs_bmap_btree.c, named something like
> xfs_bmbt_shift_rec_offsets().
Okay.

>
> Also, not typedefs for structures, please. (i.e. struct xfs_trans).
Okay.

>
>
>> +{
>> +    xfs_btree_cur_t         *cur;
>
>       struct xfs_btree_cur    *cur;
>
> And for all the other structures, too.
Okay.

>
>> +    xfs_bmbt_rec_host_t     *gotp;
>> +    xfs_mount_t             *mp;
>> +    xfs_ifork_t             *ifp;
>> +    xfs_extnum_t            nexts = 0;
>> +    xfs_fileoff_t           startoff;
>> +    int                     error = 0;
>> +    int                     i;
>> +
>> +    ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
>> +    mp = ip->i_mount;
>
> Do the assigment in the declaration on mp.
Okay.

>
>       struct xfs_mount        *mp = ip->i_mount;
>
>> +
>> +    if (!(ifp->if_flags & XFS_IFEXTENTS) &&
>> +            (error = xfs_iread_extents(tp, ip, XFS_DATA_FORK)))
>> +            return error;
>
> Hmmmm - that rings alarm bells.
ok, we will remove the alarm.

>
>> +
>> +    if (!*current_ext) {
>> +            gotp = xfs_iext_bno_to_ext(ifp, start_fsb, current_ext);
>> +            /*
>> +             * gotp can be null in 2 cases: 1) if there are no extents
>> +             * or 2) start_fsb lies in a hole beyond which there are
>> +             * no extents. Either way, we are done.
>> +             */
>> +            if (!gotp) {
>> +                    *done = 1;
>> +                    return 0;
>> +            }
>> +    }
>
> So, you do a lookup on an extent index, which returns a incore
> extent record.
>
>> +
>> +    if (ifp->if_flags & XFS_IFBROOT)
>> +            cur = xfs_bmbt_init_cursor(mp, tp, ip, XFS_DATA_FORK);
>> +    else
>> +            cur = NULL;
>> +
>> +    while (nexts++ < XFS_LINEAR_EXTS &&
>
> What has XFS_LINEAR_EXTS got to do with how many extents can be moved
> in a single transaction at a time?
We are also not sure how many extents to move in 1 transaction.
xfs_bunmapi deletes 3 extents in 1 iteration.
But deletion and updation are 2 different tasks.
BTW, we tested this patch with 10,000 extents and it is working fine.
Could you help us out here? What number would be appropriate?

>
>> +           *current_ext <  XFS_IFORK_NEXTENTS(ip, XFS_DATA_FORK)) {
>> +            gotp = xfs_iext_get_ext(ifp, (*current_ext)++);
>> +            startoff = xfs_bmbt_get_startoff(gotp);
>> +            if (cur) {
>> +                    if ((error = xfs_bmbt_lookup_eq(cur,
>> +                                    startoff,
>> +                                    xfs_bmbt_get_startblock(gotp),
>> +                                    xfs_bmbt_get_blockcount(gotp),
>> +                                    &i)))
>
> Please don't put assignments inside if() statements where possible.
> i.e.
>                       error = xfs_bmbt_lookup_eq(cur, ....
>                       if (error)
>
> Is the normal way of doing this.
Okay:)

>
>> +                            goto del_cursor;
>> +                    XFS_WANT_CORRUPTED_GOTO(i == 1, del_cursor);
>> +            }
>> +            startoff -= shift;
>> +            xfs_bmbt_set_startoff(gotp, startoff);
>
> So, we've looked up and extent, and reduced it's offset by shift.
>
>> +            if (cur) {
>> +                    error = xfs_bmbt_update(cur, startoff,
>> +                                            xfs_bmbt_get_startblock(gotp),
>> +                                            xfs_bmbt_get_blockcount(gotp),
>> +                                            xfs_bmbt_get_state(gotp));
>> +                    if (error)
>> +                            goto del_cursor;
>
> And then we update the btree record in place.
>
> Ok, major alarm bells are going off at this point. Is the record
> still in the right place? Where did you validate that the shift
> downwards didn't overlap the previous extent in the tree?
>
> What happens if the start or end of the range to be shifted partially
> overlaps an extent? The shift can only be done into a hole in the
> file offset, so any attempt to shift downwards that overlaps an
> existing extent is immediately invalid and indicates something is
> wrong (pssobily corruption).
>
> And if the end of the range being shifted partially overlaps an
> extent, then that extent needs to be split - it requires two records
> in the tree to track the part that was shifted and the part that was
> not.
Your concern is correct.
There can be 2 approach of updating the extent tree.

Approach1: (The way truncate works): We start from the last allocated
extent and move towards the last offset which was punched out.
It mean start updating extents from right side and move towards left
and update btree in between
Approach2: We start from the extent just after the punched area and
move towards the last extent of the file.
It means moving from left to right and update btree in between.
When we used approach 1, we were getting btree corruption but there
seems to be no problem in approach2.
And, that holds true even in the case where 2 extents have _exactly_
same starting offset and length.
So, in approach2, attempt to shift extent that overlaps an existing
extent is working correctly without requiring any extent split.
we will check the reason for it.

>
>> +            }
>> +    }
>> +
>> +    /* Check if we are done */
>> +    if (*current_ext ==  XFS_IFORK_NEXTENTS(ip, XFS_DATA_FORK))
>> +            *done = 1;
>> +
>> +del_cursor:
>> +    if (cur) {
>> +            if (!error)
>> +                    cur->bc_private.b.allocated = 0;
>
> Um, why?
Okay, will remove.
>
>> -    if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
>> +    if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |
>> +                 FALLOC_FL_COLLAPSE_RANGE))
>> +            return -EOPNOTSUPP;
>> +
>> +    /* not just yet for rt inodes */
>> +    if ((mode & FALLOC_FL_COLLAPSE_RANGE) && XFS_IS_REALTIME_INODE(ip))
>>              return -EOPNOTSUPP;
>
> Why not? The extent manipulations are identical....
Okay, Agree. we will remove this check in next release.
>
>> @@ -845,6 +850,21 @@ xfs_file_fallocate(
>>      if (error)
>>              goto out_unlock;
>>
>> +    if (mode & FALLOC_FL_COLLAPSE_RANGE) {
>> +            error = -xfs_collapse_file_space(ip, offset + len, len);
>> +            if (error || offset >= i_size_read(inode))
>> +                    goto out_unlock;
>> +
>> +            /* Shrink size in case of FALLOC_FL_COLLAPSE_RANGE */
>> +            if ((offset + len) > i_size_read(inode))
>> +                    new_size = offset;
>> +            else
>> +                    new_size = i_size_read(inode) - len;
>> +            error = -xfs_update_file_size(ip, new_size);
>> +
>> +            goto out_unlock;
>> +    }
>
> This needs to vector through xfs_change_file_space() - it already
> has code for doing offset/range validity checks, attaching
> appropriate dquots for accounting, post-op file size and timestamp
> updates, etc.
It already is going through xfs_change_file_space(). Check this=>
@@ -826,11 +831,11 @@ xfs_file_fallocate(

        xfs_ilock(ip, XFS_IOLOCK_EXCL);

-       if (mode & FALLOC_FL_PUNCH_HOLE)
+       if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_COLLAPSE_RANGE))
                cmd = XFS_IOC_UNRESVSP;
So rightly said, we do not need to do validity checks etc for collapse range.

>
>> +
>> +/*
>> + * xfs_update_file_size()
>> + *  Just a simple disk size and time update
>> + */
>> +int
>> +xfs_update_file_size(
>> +    struct xfs_inode        *ip,
>> +    loff_t                  newsize)
>
> This function should be nuked from orbit. I stopped looking at it
> when the bug count got past 5. If you use xfs_change_file_space,
> it's not necessary, either.
we are using xfs_change_file_space(). See my comment above. :)
But, xfs_change_file_space does not change logical file size.
Neither can we use xfs_setattr, because it will truncate the
preallocated extents beyond EOF.

>
>> diff --git a/fs/xfs/xfs_vnodeops.c b/fs/xfs/xfs_vnodeops.c
>> index dc730ac..95b46e7 100644
>> --- a/fs/xfs/xfs_vnodeops.c
>> +++ b/fs/xfs/xfs_vnodeops.c
>> @@ -1868,3 +1868,65 @@ xfs_change_file_space(
>>              xfs_trans_set_sync(tp);
>>      return xfs_trans_commit(tp, 0);
>>  }
>> +
>> +/*
>> + * xfs_collapse_file_space()
>> + *  This implements the fallocate collapse range functionlaity.
>> + *  It removes the hole from file by shifting all the extents which
>> + *  lies beyond start block.
>> + */
>> +int
>> +xfs_collapse_file_space(
>> +    xfs_inode_t     *ip,
>> +    loff_t          start,
>> +    loff_t          shift)
>> +{
>> +    int             done = 0;
>> +    xfs_mount_t     *mp = ip->i_mount;
>> +    uint            resblks;
>> +    xfs_trans_t     *tp;
>> +    int             error = 0;
>> +    xfs_extnum_t    current_ext = 0;
>> +    xfs_fileoff_t   start_fsb = XFS_B_TO_FSB(mp, start);
>> +    xfs_fileoff_t   shift_fsb = XFS_B_TO_FSB(mp, shift);
>> +    resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0);
>> +
>> +    while (!error && !done) {
>> +            tp = xfs_trans_alloc(mp, XFS_TRANS_DIOSTRAT);
>> +            tp->t_flags |= XFS_TRANS_RESERVE;
>> +            error = xfs_trans_reserve(tp, resblks, XFS_WRITE_LOG_RES(mp),
>> +                                      0, XFS_TRANS_PERM_LOG_RES,
>> +                                      XFS_WRITE_LOG_COUNT);
>
> Why a permanent log reservation for a single, non-nested transaction?
XFS transaction log reservation code is becoming our major problem.
Could you suggest a proper way?

>
>> +            if (error) {
>> +                    ASSERT(error == ENOSPC || XFS_FORCED_SHUTDOWN(mp));
>> +                    xfs_trans_cancel(tp, 0);
>> +                    break;
>> +            }
>> +
>> +            xfs_ilock(ip, XFS_ILOCK_EXCL);
>> +
>> +            error = xfs_trans_reserve_quota(tp, mp, ip->i_udquot,
>> +                                            ip->i_gdquot, ip->i_pdquot,
>> +                                            resblks, 0,
>> +                                            XFS_QMOPT_RES_REGBLKS);
>> +            if (error)
>> +                    goto out;
>> +
>> +            xfs_trans_ijoin(tp, ip, 0);
>> +
>> +            error = xfs_update_logical(tp, ip, &done, start_fsb,
>> +                                       shift_fsb, &current_ext);
>> +            if (error)
>> +                    goto out;
>> +
>> +            error = xfs_trans_commit(tp, XFS_TRANS_RELEASE_LOG_RES);
>> +            xfs_iunlock(ip, XFS_ILOCK_EXCL);
>> +    }
>
> Where are you punching out the blocks in the range we are shifting
> the down into? i.e. before you can do this shift operation, you have
We are calling xfs_free_file_space via xfs_change_file_space.

> to ensure that the range being shifted into has no extents in it.
> IOWs, the first operation that needs to be done is a hole punch of
> the range being removed - we need xfs_free_file_space() call on the
> range first.
>
> And as Ted pointed out, we also need to invalidate the page cache
> over the range being moved.
xfs_free_file_space does that for us.

Thanks Dave!
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
>

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