On Mon, Sep 09, 2019 at 08:27:15PM +0200, Christoph Hellwig wrote:
> Replace our local hacks to report the source block in the main iomap
> with the proper scrmap reporting.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/xfs_iomap.c | 49 +++++++++++++++++++++++-----------------------
> 1 file changed, 24 insertions(+), 25 deletions(-)
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 18a0f8a5d8c9..d12eacdc9bba 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -527,7 +527,8 @@ xfs_file_iomap_begin_delay(
> loff_t offset,
> loff_t count,
> unsigned flags,
> - struct iomap *iomap)
> + struct iomap *iomap,
> + struct iomap *srcmap)
> {
> struct xfs_inode *ip = XFS_I(inode);
> struct xfs_mount *mp = ip->i_mount;
> @@ -721,11 +722,13 @@ xfs_file_iomap_begin_delay(
> found_cow:
> xfs_iunlock(ip, XFS_ILOCK_EXCL);
> if (imap.br_startoff <= offset_fsb) {
> - /* ensure we only report blocks we have a reservation for */
> - xfs_trim_extent(&imap, cmap.br_startoff, cmap.br_blockcount);
> - return xfs_bmbt_to_iomap(ip, iomap, &imap, IOMAP_F_SHARED);
> + error = xfs_bmbt_to_iomap(ip, srcmap, &imap, 0);
> + if (error)
> + return error;
> + } else {
> + xfs_trim_extent(&cmap, offset_fsb,
> + imap.br_startoff - offset_fsb);
> }
> - xfs_trim_extent(&cmap, offset_fsb, imap.br_startoff - offset_fsb);
> return xfs_bmbt_to_iomap(ip, iomap, &cmap, IOMAP_F_SHARED);
>
> out_unlock:
> @@ -933,7 +936,7 @@ xfs_file_iomap_begin(
> {
> struct xfs_inode *ip = XFS_I(inode);
> struct xfs_mount *mp = ip->i_mount;
> - struct xfs_bmbt_irec imap;
> + struct xfs_bmbt_irec imap, cmap;
> xfs_fileoff_t offset_fsb, end_fsb;
> int nimaps = 1, error = 0;
> bool shared = false;
> @@ -947,7 +950,7 @@ xfs_file_iomap_begin(
> !IS_DAX(inode) && !xfs_get_extsz_hint(ip)) {
> /* Reserve delalloc blocks for regular writeback. */
> return xfs_file_iomap_begin_delay(inode, offset, length, flags,
> - iomap);
> + iomap, srcmap);
> }
>
> /*
> @@ -987,9 +990,6 @@ xfs_file_iomap_begin(
> * been done up front, so we don't need to do them here.
> */
> if (xfs_is_cow_inode(ip)) {
> - struct xfs_bmbt_irec cmap;
> - bool directio = (flags & IOMAP_DIRECT);
> -
> /* if zeroing doesn't need COW allocation, then we are done. */
> if ((flags & IOMAP_ZERO) &&
> !needs_cow_for_zeroing(&imap, nimaps))
> @@ -997,23 +997,11 @@ xfs_file_iomap_begin(
>
> /* may drop and re-acquire the ilock */
> error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
> - &lockmode, directio);
> + &lockmode, flags & IOMAP_DIRECT);
> if (error)
> goto out_unlock;
> -
> - /*
> - * For buffered writes we need to report the address of the
> - * previous block (if there was any) so that the higher level
> - * write code can perform read-modify-write operations; we
> - * won't need the CoW fork mapping until writeback. For direct
> - * I/O, which must be block aligned, we need to report the
> - * newly allocated address. If the data fork has a hole, copy
> - * the COW fork mapping to avoid allocating to the data fork.
> - */
> - if (shared &&
> - (directio || imap.br_startblock == HOLESTARTBLOCK))
> - imap = cmap;
> -
> + if (shared)
> + goto out_found_cow;
> end_fsb = imap.br_startoff + imap.br_blockcount;
> length = XFS_FSB_TO_B(mp, end_fsb) - offset;
> }
> @@ -1067,6 +1055,17 @@ xfs_file_iomap_begin(
> trace_xfs_iomap_found(ip, offset, length, XFS_DATA_FORK, &imap);
> goto out_finish;
>
> +out_found_cow:
> + xfs_iunlock(ip, lockmode);
> + length = XFS_FSB_TO_B(mp, cmap.br_startoff + cmap.br_blockcount);
> + trace_xfs_iomap_found(ip, offset, length - offset, XFS_COW_FORK, &cmap);
> + if (imap.br_startblock != HOLESTARTBLOCK) {
TBH I've been wondering for a while now if it would make more sense to
do this in iomap_apply (and the open-coded versions in dax.c):
struct iomap srcmap = { .type = IOMAP_HOLE };
in the iomap_apply function (and change the "if (!srcmap.type)" checks
to "if (srcmap.type != IOMAP_HOLE)"). That would get rid of the weird
situation where iomap.h doesn't define an iomap type name corresponding
to 0 but clearly it has some special meaning because the iomap code
changes behavior based on that.
It also strikes me as weird that for the @imap parameter, type == 0
would be considered a coding error but for @srcmap, we use type == 0 to
mean "no mapping" but we don't do that for @srcmap.type == IOMAP_HOLE.
I mention that because, if some ->iomap_begin function returns
IOMAP_HOLE then iomap_apply will pass the (hole) srcmap as the second
parameter to the ->actor function. When that happens, iomap_write_begin
call will try to fill in the rest of the page from @srcmap (which is
hole), not the @iomap (which might not be a hole) which seems wrong.
As for this function, if we made the above change, then the conditional
becomes unneccessary -- we know this is a COW write, so we call
xfs_bmbt_to_iomap on both mappings and exit. No need for special
casing.
--D
> + error = xfs_bmbt_to_iomap(ip, srcmap, &imap, 0);
> + if (error)
> + return error;
> + }
> + return xfs_bmbt_to_iomap(ip, iomap, &cmap, IOMAP_F_SHARED);
> +
> out_unlock:
> xfs_iunlock(ip, lockmode);
> return error;
> --
> 2.20.1
>
