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Re: SEEK_DATA/SEEK_HOLE support

To: Jeff Liu <jeff.liu@xxxxxxxxxx>
Subject: Re: SEEK_DATA/SEEK_HOLE support
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Sun, 2 Oct 2011 11:42:59 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4E887D7F.2010306@xxxxxxxxxx>
References: <4E887D7F.2010306@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sun, Oct 02, 2011 at 11:04:31PM +0800, Jeff Liu wrote:
> Dear developer,
> 
> Does anyone already worked on SEEK_DATA/SEEK_HOLE for XFS? I'd like to
> implement it if not. :)

Dave mentioned he had a basic implementation, he might have some code
that you can improve on.

Did we get consensus about the the semantics of them for unwritten
extents?  If we want them to be exact in the fact of unwritten extents
that is going to be the most work, e.g. if we find an unwritten extent
we'll then have to do a pagecache lookup and check if pages are dirty
and in that case not treat them as a hole.

The rest of the implementation should be easy, e.g. doing something like
the following pseudo-code which missed all the proper error codes and
conversions from the lseek arguments to filesystem blocks and the
required locking around it:


seek_hole()
{
        xfs_bmap_search_extents(ip, bno, XFS_DATA_FORK, &eof, &lastx, &got,
                        &prev);

        for (;;) {
                if (eof) {
                        /* past the file, nothing do here */
                        return;
                }

                if (got.br_startoff > bno)
                        /* found hole, return it */
                        return;
                }

                if (++lastx == ifp->if_bytes / sizeof(xfs_bmbt_rec_t)) {
                        /* reached EOF */
                        return;
                }
                xfs_bmbt_get_all(xfs_iext_get_ext(ifp, lastx), &got);
        }
}

seek_data()
{
        xfs_bmap_search_extents(ip, bno, XFS_DATA_FORK, &eof, &lastx, &got,
                        &prev);

        for (;;) {
                if (eof) {
                        /* past the file, nothing do here */
                        return;
                }

                /* next data extent */
                return got.br_startoff < bno ? bno : got.br_startof;
        }
}

        

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