On Thu, May 31, 2018 at 08:07:48PM +0200, Christoph Hellwig wrote:
> We can handle the existing cow mapping case as a special case directly
> in xfs_writepage_map, and share code for allocating delalloc blocks
> with regular I/O in xfs_map_blocks. This means we need to always
> call xfs_map_blocks for reflink inodes, but we can still skip most of
> the work if it turns out that there is no COW mapping overlapping the
> current block.
>
> As a subtle detail we need to start caching holes in the wpc to deal
> with the case of COW reservations between EOF. But we'll need that
> infrastructure later anyway, so this is no big deal.
>
> Based on a patch from Dave Chinner.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/xfs_aops.c | 184 ++++++++++++++++++++++------------------------
> fs/xfs/xfs_aops.h | 4 +-
> 2 files changed, 90 insertions(+), 98 deletions(-)
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 9b5a4b6e268c..d536509ca05b 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -375,70 +375,107 @@ xfs_end_bio(
>
> STATIC int
> xfs_map_blocks(
> + struct xfs_writepage_ctx *wpc,
> struct inode *inode,
> - loff_t offset,
> - struct xfs_bmbt_irec *imap,
> - int type)
> + loff_t offset)
> {
> struct xfs_inode *ip = XFS_I(inode);
> struct xfs_mount *mp = ip->i_mount;
> ssize_t count = i_blocksize(inode);
> - xfs_fileoff_t offset_fsb, end_fsb;
> + xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset), end_fsb;
> + struct xfs_bmbt_irec imap;
> + int whichfork = XFS_DATA_FORK;
> int error = 0;
> int nimaps = 1;
>
> if (XFS_FORCED_SHUTDOWN(mp))
> return -EIO;
>
> - /*
> - * Truncate can race with writeback since writeback doesn't take the
> - * iolock and truncate decreases the file size before it starts
> - * truncating the pages between new_size and old_size. Therefore, we
> - * can end up in the situation where writeback gets a CoW fork mapping
> - * but the truncate makes the mapping invalid and we end up in here
> - * trying to get a new mapping. Bail out here so that we simply never
> - * get a valid mapping and so we drop the write altogether. The page
> - * truncation will kill the contents anyway.
> - */
> - if (type == XFS_IO_COW && offset > i_size_read(inode))
> - return 0;
> -
> - ASSERT(type != XFS_IO_COW);
> -
> xfs_ilock(ip, XFS_ILOCK_SHARED);
> ASSERT(ip->i_d.di_format != XFS_DINODE_FMT_BTREE ||
> (ip->i_df.if_flags & XFS_IFEXTENTS));
> ASSERT(offset <= mp->m_super->s_maxbytes);
>
> + if (xfs_is_reflink_inode(ip) &&
> + xfs_reflink_find_cow_mapping(ip, offset, &imap)) {
> + xfs_iunlock(ip, XFS_ILOCK_SHARED);
> + /*
> + * Truncate can race with writeback since writeback doesn't
> + * take the iolock and truncate decreases the file size before
> + * it starts truncating the pages between new_size and old_size.
> + * Therefore, we can end up in the situation where writeback
> + * gets a CoW fork mapping but the truncate makes the mapping
> + * invalid and we end up in here trying to get a new mapping.
> + * bail out here so that we simply never get a valid mapping
> + * and so we drop the write altogether. The page truncation
> + * will kill the contents anyway.
> + */
> + if (offset > i_size_read(inode)) {
> + whichfork = XFS_IO_HOLE;
Not sure what the point of this assignment is if we're going to return,
but that's not a valid fork either way. Did you mean to assign
wpc->io_type?
> + return 0;
> + }
> + whichfork = XFS_COW_FORK;
> + wpc->io_type = XFS_IO_COW;
> + goto allocate_blocks;
> + }
> +
> + /*
> + * Map valid and no COW extent in the way? We're done.
> + */
> + if (wpc->imap_valid) {
> + xfs_iunlock(ip, XFS_ILOCK_SHARED);
> + return 0;
> + }
> +
> + /*
> + * If we don't have a valid map, now it's time to get a new one for this
> + * offset. This will convert delayed allocations (including COW ones)
> + * into real extents.
> + */
> if (offset > mp->m_super->s_maxbytes - count)
> count = mp->m_super->s_maxbytes - offset;
> end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
> offset_fsb = XFS_B_TO_FSBT(mp, offset);
> error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
> - imap, &nimaps, XFS_BMAPI_ENTIRE);
> + &imap, &nimaps, XFS_BMAPI_ENTIRE);
> xfs_iunlock(ip, XFS_ILOCK_SHARED);
> -
> if (error)
> return error;
>
> - if (type == XFS_IO_DELALLOC &&
> - (!nimaps || isnullstartblock(imap->br_startblock))) {
> - error = xfs_iomap_write_allocate(ip, XFS_DATA_FORK, offset,
> - imap);
> - if (!error)
> - trace_xfs_map_blocks_alloc(ip, offset, count, type, imap);
> - return error;
> + if (!nimaps) {
> + /*
> + * Lookup returns no match? Beyond eof? regardless,
> + * return it as a hole so we don't write it
> + */
> + imap.br_startoff = offset_fsb;
> + imap.br_blockcount = end_fsb - offset_fsb;
> + imap.br_startblock = HOLESTARTBLOCK;
> + wpc->io_type = XFS_IO_HOLE;
> + } else if (imap.br_startblock == HOLESTARTBLOCK) {
> + /* landed in a hole */
> + wpc->io_type = XFS_IO_HOLE;
> }
>
> + if (wpc->io_type == XFS_IO_DELALLOC &&
> + (!nimaps || isnullstartblock(imap.br_startblock)))
> + goto allocate_blocks;
> +
> #ifdef DEBUG
> - if (type == XFS_IO_UNWRITTEN) {
> + if (wpc->io_type == XFS_IO_UNWRITTEN) {
> ASSERT(nimaps);
> - ASSERT(imap->br_startblock != HOLESTARTBLOCK);
> - ASSERT(imap->br_startblock != DELAYSTARTBLOCK);
> + ASSERT(imap.br_startblock != HOLESTARTBLOCK);
> + ASSERT(imap.br_startblock != DELAYSTARTBLOCK);
> }
> #endif
> - if (nimaps)
> - trace_xfs_map_blocks_found(ip, offset, count, type, imap);
> + wpc->imap = imap;
> + trace_xfs_map_blocks_found(ip, offset, count, wpc->io_type, &imap);
> + return 0;
> +allocate_blocks:
> + error = xfs_iomap_write_allocate(ip, whichfork, offset, &imap);
> + if (error)
> + return error;
> + wpc->imap = imap;
> + trace_xfs_map_blocks_alloc(ip, offset, count, wpc->io_type, &imap);
> return 0;
> }
>
> @@ -759,56 +796,6 @@ xfs_aops_discard_page(
> xfs_vm_invalidatepage(page, 0, PAGE_SIZE);
> }
>
> -static int
> -xfs_map_cow(
> - struct xfs_writepage_ctx *wpc,
> - struct inode *inode,
> - loff_t offset,
> - unsigned int *new_type)
> -{
> - struct xfs_inode *ip = XFS_I(inode);
> - struct xfs_bmbt_irec imap;
> - bool is_cow = false;
> - int error;
> -
> - /*
> - * If we already have a valid COW mapping keep using it.
> - */
> - if (wpc->io_type == XFS_IO_COW) {
> - wpc->imap_valid = xfs_imap_valid(inode, &wpc->imap, offset);
> - if (wpc->imap_valid) {
> - *new_type = XFS_IO_COW;
> - return 0;
> - }
> - }
> -
> - /*
> - * Else we need to check if there is a COW mapping at this offset.
> - */
> - xfs_ilock(ip, XFS_ILOCK_SHARED);
> - is_cow = xfs_reflink_find_cow_mapping(ip, offset, &imap);
> - xfs_iunlock(ip, XFS_ILOCK_SHARED);
> -
> - if (!is_cow)
> - return 0;
> -
> - /*
> - * And if the COW mapping has a delayed extent here we need to
> - * allocate real space for it now.
> - */
> - if (isnullstartblock(imap.br_startblock)) {
> - error = xfs_iomap_write_allocate(ip, XFS_COW_FORK, offset,
> - &imap);
> - if (error)
> - return error;
> - }
> -
> - wpc->io_type = *new_type = XFS_IO_COW;
> - wpc->imap_valid = true;
> - wpc->imap = imap;
> - return 0;
> -}
> -
> /*
> * We implement an immediate ioend submission policy here to avoid needing to
> * chain multiple ioends and hence nest mempool allocations which can violate
> @@ -873,10 +860,13 @@ xfs_writepage_map(
> continue;
> }
>
> - if (xfs_is_reflink_inode(XFS_I(inode))) {
> - error = xfs_map_cow(wpc, inode, offset, &new_type);
> - if (error)
> - goto out;
> + /*
> + * If we already have a valid COW mapping keep using it.
> + */
> + if (wpc->io_type == XFS_IO_COW &&
> + xfs_imap_valid(inode, &wpc->imap, offset)) {
> + wpc->imap_valid = true;
> + new_type = XFS_IO_COW;
> }
So this lifts the cow I/O type check from xfs_map_cow(). That path would
set imap_valid and new_type and return, which essentially looks like a
"blocks found" case where we go ahead and add the buffer to the ioend.
Now we set the same values, but we have to call xfs_map_blocks() to make
sure we cover the cow blocks overlap case that xfs_map_cow() used to
handle.
Shouldn't passing this check effectively jump straight to to the buffer
add? Otherwise it looks like we unconditionally attempt to allocate
blocks in the cow fork.
Alternatively...
>
> if (wpc->io_type != new_type) {
> @@ -887,22 +877,22 @@ xfs_writepage_map(
> if (wpc->imap_valid)
> wpc->imap_valid = xfs_imap_valid(inode, &wpc->imap,
> offset);
> - if (!wpc->imap_valid) {
> - error = xfs_map_blocks(inode, offset, &wpc->imap,
> - wpc->io_type);
> + if (!wpc->imap_valid || xfs_is_reflink_inode(XFS_I(inode))) {
> + error = xfs_map_blocks(wpc, inode, offset);
... perhaps this could change to something like:
if (!wpc->imap_valid ||
(xfs_is_reflink_inode(XFS_I(inode)) &&
wpc->io_type != XFS_IO_COW))
error = ...
... so the logic is a bit more clear. E.g., we always need to look up in
the COW fork for reflink inodes unless we're already doing COW I/O. Hm?
Brian
> if (error)
> goto out;
> wpc->imap_valid = xfs_imap_valid(inode, &wpc->imap,
> offset);
> }
> - if (wpc->imap_valid) {
> - lock_buffer(bh);
> - if (wpc->io_type != XFS_IO_OVERWRITE)
> - xfs_map_at_offset(inode, bh, &wpc->imap, offset);
> - xfs_add_to_ioend(inode, bh, offset, wpc, wbc, &submit_list);
> - count++;
> - }
>
> + if (!wpc->imap_valid || wpc->io_type == XFS_IO_HOLE)
> + continue;
> +
> + lock_buffer(bh);
> + if (wpc->io_type != XFS_IO_OVERWRITE)
> + xfs_map_at_offset(inode, bh, &wpc->imap, offset);
> + xfs_add_to_ioend(inode, bh, offset, wpc, wbc, &submit_list);
> + count++;
> } while (offset += len, ((bh = bh->b_this_page) != head));
>
> ASSERT(wpc->ioend || list_empty(&submit_list));
> diff --git a/fs/xfs/xfs_aops.h b/fs/xfs/xfs_aops.h
> index 69346d460dfa..b2ef5b661761 100644
> --- a/fs/xfs/xfs_aops.h
> +++ b/fs/xfs/xfs_aops.h
> @@ -29,6 +29,7 @@ enum {
> XFS_IO_UNWRITTEN, /* covers allocated but uninitialized data */
> XFS_IO_OVERWRITE, /* covers already allocated extent */
> XFS_IO_COW, /* covers copy-on-write extent */
> + XFS_IO_HOLE, /* covers region without any block allocation */
> };
>
> #define XFS_IO_TYPES \
> @@ -36,7 +37,8 @@ enum {
> { XFS_IO_DELALLOC, "delalloc" }, \
> { XFS_IO_UNWRITTEN, "unwritten" }, \
> { XFS_IO_OVERWRITE, "overwrite" }, \
> - { XFS_IO_COW, "CoW" }
> + { XFS_IO_COW, "CoW" }, \
> + { XFS_IO_HOLE, "hole" }
>
> /*
> * Structure for buffered I/O completions.
> --
> 2.17.0
>
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