On Thu, Sep 29, 2016 at 08:09:40PM -0700, Darrick J. Wong wrote:
> For O_DIRECT writes to shared blocks, we have to CoW them just like
> we would with buffered writes. For writes that are not block-aligned,
> just bounce them to the page cache.
>
> For block-aligned writes, however, we can do better than that. Use
> the same mechanisms that we employ for buffered CoW to set up a
> delalloc reservation, allocate all the blocks at once, issue the
> writes against the new blocks and use the same ioend functions to
> remap the blocks after the write. This should be fairly performant.
>
> Christoph discovered that xfs_reflink_allocate_cow_range may stumble
> over invalid entries in the extent array given that it drops the ilock
> but still expects the index to be stable. Simple fixing it to a new
> lookup for every iteration still isn't correct given that
> xfs_bmapi_allocate will trigger a BUG_ON() if hitting a hole, and
> there is nothing preventing a xfs_bunmapi_cow call removing extents
> once we dropped the ilock either.
>
> This patch duplicates the inner loop of xfs_bmapi_allocate into a
> helper for xfs_reflink_allocate_cow_range so that it can be done under
> the same ilock critical section as our CoW fork delayed allocation.
> The directio CoW warts will be revisited in a later patch.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> v2: Turns out that there's no way for xfs_end_io_direct_write to know
> if the write completed successfully. Therefore, do /not/ use the
> ioend for dio cow post-processing; instead, move it to xfs_vm_do_dio
> where we *can* tell if the write succeeded or not.
>
> v3: Update the file size if we do a directio CoW across EOF. This
> can happen if the last block is shared, the cowextsize hint is set,
> and we do a dio write past the end of the file.
>
> v4: Christoph rewrote the allocate code to fix some concurrency
> problems as part of migrating the code to support iomap.
> ---
> fs/xfs/xfs_aops.c | 91 +++++++++++++++++++++++++++++++++++++++----
> fs/xfs/xfs_file.c | 20 ++++++++-
> fs/xfs/xfs_reflink.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++++-
> fs/xfs/xfs_reflink.h | 2 +
> fs/xfs/xfs_trace.h | 1
> 5 files changed, 208 insertions(+), 13 deletions(-)
>
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 1d0435a..62a95e4 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -40,6 +40,7 @@
> /* flags for direct write completions */
> #define XFS_DIO_FLAG_UNWRITTEN (1 << 0)
> #define XFS_DIO_FLAG_APPEND (1 << 1)
> +#define XFS_DIO_FLAG_COW (1 << 2)
>
> /*
> * structure owned by writepages passed to individual writepage calls
> @@ -1191,18 +1192,24 @@ xfs_map_direct(
> struct inode *inode,
> struct buffer_head *bh_result,
> struct xfs_bmbt_irec *imap,
> - xfs_off_t offset)
> + xfs_off_t offset,
> + bool is_cow)
> {
> uintptr_t *flags = (uintptr_t *)&bh_result->b_private;
> xfs_off_t size = bh_result->b_size;
>
> trace_xfs_get_blocks_map_direct(XFS_I(inode), offset, size,
> - ISUNWRITTEN(imap) ? XFS_IO_UNWRITTEN : XFS_IO_OVERWRITE, imap);
> + ISUNWRITTEN(imap) ? XFS_IO_UNWRITTEN : is_cow ? XFS_IO_COW :
> + XFS_IO_OVERWRITE, imap);
>
> if (ISUNWRITTEN(imap)) {
> *flags |= XFS_DIO_FLAG_UNWRITTEN;
> set_buffer_defer_completion(bh_result);
> - } else if (offset + size > i_size_read(inode) || offset + size < 0) {
> + } else if (is_cow) {
> + *flags |= XFS_DIO_FLAG_COW;
> + set_buffer_defer_completion(bh_result);
> + }
> + if (offset + size > i_size_read(inode) || offset + size < 0) {
> *flags |= XFS_DIO_FLAG_APPEND;
> set_buffer_defer_completion(bh_result);
> }
> @@ -1248,6 +1255,44 @@ xfs_map_trim_size(
> bh_result->b_size = mapping_size;
> }
>
> +/* Bounce unaligned directio writes to the page cache. */
> +static int
> +xfs_bounce_unaligned_dio_write(
> + struct xfs_inode *ip,
> + xfs_fileoff_t offset_fsb,
> + struct xfs_bmbt_irec *imap)
> +{
> + struct xfs_bmbt_irec irec;
> + xfs_fileoff_t delta;
> + bool shared;
> + bool x;
> + int error;
> +
> + irec = *imap;
> + if (offset_fsb > irec.br_startoff) {
> + delta = offset_fsb - irec.br_startoff;
> + irec.br_blockcount -= delta;
> + irec.br_startblock += delta;
> + irec.br_startoff = offset_fsb;
> + }
> + error = xfs_reflink_trim_around_shared(ip, &irec, &x, &shared);
'shared' is the 3rd parameter.
> + if (error)
> + return error;
> + /*
> + * Are we doing a DIO write to a shared block? In
> + * the ideal world we at least would fork full blocks,
> + * but for now just fall back to buffered mode. Yuck.
> + * Use -EREMCHG ("remote address changed") to signal
> + * this, since in general XFS doesn't do this sort of
> + * fallback.
> + */
> + if (shared) {
> + trace_xfs_reflink_bounce_dio_write(ip, imap);
> + return -EREMCHG;
> + }
I get that this bumps the write back up to the buffered mechanism, but
the purpose is not very clear. We have the !unaligned check back up in
xfs_file_dio_aio_write(), in which case we do all of the cow allocation
stuff. I presume in that case we should never fail this check (?).
If the dio is unaligned, we skip that bit, get down to here and kick it
back only if the extent happens to be shared..? Unless I missed it, I
think this needs to be explained in the comments in both places,
including probably updating the comment at the end of dio_aio_write()
that states we don't fall back to buffered I/O on dio error.
> + return 0;
> +}
> +
> STATIC int
> __xfs_get_blocks(
> struct inode *inode,
> @@ -1267,6 +1312,8 @@ __xfs_get_blocks(
> xfs_off_t offset;
> ssize_t size;
> int new = 0;
> + bool is_cow = false;
> + bool need_alloc = false;
>
> BUG_ON(create && !direct);
>
> @@ -1292,8 +1339,27 @@ __xfs_get_blocks(
> end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + size);
> offset_fsb = XFS_B_TO_FSBT(mp, offset);
>
> - error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
> - &imap, &nimaps, XFS_BMAPI_ENTIRE);
> + if (create && direct) {
> + is_cow = xfs_reflink_find_cow_mapping(ip, offset, &imap,
> + &need_alloc);
> + }
Nits: no need for braces here and might be cleaner to check
xfs_is_reflink_inode(ip) here, since !reflink is probably the common
case.
> +
> + if (!is_cow) {
> + error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
> + &imap, &nimaps, XFS_BMAPI_ENTIRE);
> + /*
> + * Truncate an overwrite extent if there's a pending CoW
> + * reservation before the end of this extent. This forces us
> + * to come back to writepage to take care of the CoW.
writepage?
> + */
> + if (create && direct && nimaps &&
> + imap.br_startblock != HOLESTARTBLOCK &&
> + imap.br_startblock != DELAYSTARTBLOCK &&
> + !ISUNWRITTEN(&imap))
> + xfs_reflink_trim_irec_to_next_cow(ip, offset_fsb,
> + &imap);
> + }
> + ASSERT(!need_alloc);
> if (error)
> goto out_unlock;
>
> @@ -1345,6 +1411,13 @@ __xfs_get_blocks(
> if (imap.br_startblock != HOLESTARTBLOCK &&
> imap.br_startblock != DELAYSTARTBLOCK &&
> (create || !ISUNWRITTEN(&imap))) {
> + if (create && direct && !is_cow) {
> + error = xfs_bounce_unaligned_dio_write(ip, offset_fsb,
> + &imap);
> + if (error)
> + return error;
> + }
> +
> xfs_map_buffer(inode, bh_result, &imap, offset);
> if (ISUNWRITTEN(&imap))
> set_buffer_unwritten(bh_result);
> @@ -1353,7 +1426,8 @@ __xfs_get_blocks(
> if (dax_fault)
> ASSERT(!ISUNWRITTEN(&imap));
> else
> - xfs_map_direct(inode, bh_result, &imap, offset);
> + xfs_map_direct(inode, bh_result, &imap, offset,
> + is_cow);
> }
> }
>
> @@ -1479,7 +1553,10 @@ xfs_end_io_direct_write(
> trace_xfs_end_io_direct_write_unwritten(ip, offset, size);
>
> error = xfs_iomap_write_unwritten(ip, offset, size);
> - } else if (flags & XFS_DIO_FLAG_APPEND) {
> + }
> + if (flags & XFS_DIO_FLAG_COW)
> + error = xfs_reflink_end_cow(ip, offset, size);
> + if (flags & XFS_DIO_FLAG_APPEND) {
> trace_xfs_end_io_direct_write_append(ip, offset, size);
>
> error = xfs_setfilesize(ip, offset, size);
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index f99d7fa..025d52f 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -38,6 +38,7 @@
> #include "xfs_icache.h"
> #include "xfs_pnfs.h"
> #include "xfs_iomap.h"
> +#include "xfs_reflink.h"
>
> #include <linux/dcache.h>
> #include <linux/falloc.h>
> @@ -672,6 +673,13 @@ xfs_file_dio_aio_write(
>
> trace_xfs_file_direct_write(ip, count, iocb->ki_pos);
>
> + /* If this is a block-aligned directio CoW, remap immediately. */
> + if (xfs_is_reflink_inode(ip) && !unaligned_io) {
> + ret = xfs_reflink_allocate_cow_range(ip, iocb->ki_pos, count);
> + if (ret)
> + goto out;
> + }
Is the fact that we do this allocation up front rather than via
get_blocks() (like traditional direct write) one of the "warts" that
needs cleaning, or for some other reason?
> +
> data = *from;
> ret = __blockdev_direct_IO(iocb, inode, target->bt_bdev, &data,
> xfs_get_blocks_direct, xfs_end_io_direct_write,
> @@ -812,10 +820,18 @@ xfs_file_write_iter(
>
> if (IS_DAX(inode))
> ret = xfs_file_dax_write(iocb, from);
> - else if (iocb->ki_flags & IOCB_DIRECT)
> + else if (iocb->ki_flags & IOCB_DIRECT) {
> + /*
> + * Allow DIO to fall back to buffered *only* in the case
> + * that we're doing a reflink CoW.
> + */
> ret = xfs_file_dio_aio_write(iocb, from);
> - else
> + if (ret == -EREMCHG)
> + goto buffered;
> + } else {
> +buffered:
> ret = xfs_file_buffered_aio_write(iocb, from);
> + }
>
> if (ret > 0) {
> XFS_STATS_ADD(ip->i_mount, xs_write_bytes, ret);
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index d913ad1..c95cdc3 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -246,7 +246,8 @@ static int
> __xfs_reflink_reserve_cow(
> struct xfs_inode *ip,
> xfs_fileoff_t *offset_fsb,
> - xfs_fileoff_t end_fsb)
> + xfs_fileoff_t end_fsb,
> + bool *skipped)
> {
> struct xfs_bmbt_irec got, prev, imap;
> xfs_fileoff_t orig_end_fsb;
> @@ -279,8 +280,10 @@ __xfs_reflink_reserve_cow(
> end_fsb = orig_end_fsb = imap.br_startoff + imap.br_blockcount;
>
> /* Not shared? Just report the (potentially capped) extent. */
> - if (!shared)
> + if (!shared) {
> + *skipped = true;
> goto done;
> + }
>
> /*
> * Fork all the shared blocks from our write offset until the end of
> @@ -326,6 +329,7 @@ xfs_reflink_reserve_cow_range(
> {
> struct xfs_mount *mp = ip->i_mount;
> xfs_fileoff_t offset_fsb, end_fsb;
> + bool skipped = false;
> int error;
>
> trace_xfs_reflink_reserve_cow_range(ip, offset, count);
> @@ -335,7 +339,8 @@ xfs_reflink_reserve_cow_range(
>
> xfs_ilock(ip, XFS_ILOCK_EXCL);
> while (offset_fsb < end_fsb) {
> - error = __xfs_reflink_reserve_cow(ip, &offset_fsb, end_fsb);
> + error = __xfs_reflink_reserve_cow(ip, &offset_fsb, end_fsb,
> + &skipped);
> if (error) {
> trace_xfs_reflink_reserve_cow_range_error(ip, error,
> _RET_IP_);
> @@ -347,6 +352,102 @@ xfs_reflink_reserve_cow_range(
> return error;
> }
>
> +/* Allocate all CoW reservations covering a range of blocks in a file. */
> +static int
> +__xfs_reflink_allocate_cow(
> + struct xfs_inode *ip,
> + xfs_fileoff_t *offset_fsb,
> + xfs_fileoff_t end_fsb)
> +{
> + struct xfs_mount *mp = ip->i_mount;
> + struct xfs_bmbt_irec imap;
> + struct xfs_defer_ops dfops;
> + struct xfs_trans *tp;
> + xfs_fsblock_t first_block;
> + xfs_fileoff_t next_fsb;
> + int nimaps = 1, error;
> + bool skipped = false;
> +
> + xfs_defer_init(&dfops, &first_block);
> +
> + error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0, 0,
> + XFS_TRANS_RESERVE, &tp);
> + if (error)
> + return error;
> +
> + xfs_ilock(ip, XFS_ILOCK_EXCL);
> +
> + next_fsb = *offset_fsb;
> + error = __xfs_reflink_reserve_cow(ip, &next_fsb, end_fsb, &skipped);
> + if (error)
> + goto out_trans_cancel;
Do we really need to do the delayed allocation that results from this?
Couldn't we factor out the shared extent walking that allows us to just
perform the real allocations below?
It looks like speculative preallocation for dio is at least one strange
side effect that can result from this...
> +
> + if (skipped) {
> + *offset_fsb = next_fsb;
> + goto out_trans_cancel;
> + }
> +
> + xfs_trans_ijoin(tp, ip, 0);
> + error = xfs_bmapi_write(tp, ip, *offset_fsb, next_fsb - *offset_fsb,
> + XFS_BMAPI_COWFORK, &first_block,
> + XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK),
> + &imap, &nimaps, &dfops);
> + if (error)
> + goto out_trans_cancel;
Should we be using unwritten extents (BMAPI_PREALLOC) to avoid stale
data exposure similar to traditional direct write (or is the cow fork
extent never accessible until it is remapped)?
Brian
> +
> + /* We might not have been able to map the whole delalloc extent */
> + *offset_fsb = min(*offset_fsb + imap.br_blockcount, next_fsb);
> +
> + error = xfs_defer_finish(&tp, &dfops, NULL);
> + if (error)
> + goto out_trans_cancel;
> +
> + error = xfs_trans_commit(tp);
> +
> +out_unlock:
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> + return error;
> +out_trans_cancel:
> + xfs_defer_cancel(&dfops);
> + xfs_trans_cancel(tp);
> + goto out_unlock;
> +}
> +
> +/* Allocate all CoW reservations covering a part of a file. */
> +int
> +xfs_reflink_allocate_cow_range(
> + struct xfs_inode *ip,
> + xfs_off_t offset,
> + xfs_off_t count)
> +{
> + struct xfs_mount *mp = ip->i_mount;
> + xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset);
> + xfs_fileoff_t end_fsb = XFS_B_TO_FSB(mp, offset + count);
> + int error;
> +
> + ASSERT(xfs_is_reflink_inode(ip));
> +
> + trace_xfs_reflink_allocate_cow_range(ip, offset, count);
> +
> + /*
> + * Make sure that the dquots are there.
> + */
> + error = xfs_qm_dqattach(ip, 0);
> + if (error)
> + return error;
> +
> + while (offset_fsb < end_fsb) {
> + error = __xfs_reflink_allocate_cow(ip, &offset_fsb, end_fsb);
> + if (error) {
> + trace_xfs_reflink_allocate_cow_range_error(ip, error,
> + _RET_IP_);
> + break;
> + }
> + }
> +
> + return error;
> +}
> +
> /*
> * Find the CoW reservation (and whether or not it needs block allocation)
> * for a given byte offset of a file.
> diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
> index bffa4be..c0c989a 100644
> --- a/fs/xfs/xfs_reflink.h
> +++ b/fs/xfs/xfs_reflink.h
> @@ -28,6 +28,8 @@ extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip,
>
> extern int xfs_reflink_reserve_cow_range(struct xfs_inode *ip,
> xfs_off_t offset, xfs_off_t count);
> +extern int xfs_reflink_allocate_cow_range(struct xfs_inode *ip,
> + xfs_off_t offset, xfs_off_t count);
> extern bool xfs_reflink_find_cow_mapping(struct xfs_inode *ip, xfs_off_t offset,
> struct xfs_bmbt_irec *imap, bool *need_alloc);
> extern int xfs_reflink_trim_irec_to_next_cow(struct xfs_inode *ip,
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 7612096..8e89223 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -3332,7 +3332,6 @@ DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_enospc);
>
> DEFINE_RW_EVENT(xfs_reflink_reserve_cow_range);
> DEFINE_RW_EVENT(xfs_reflink_allocate_cow_range);
> -DEFINE_INODE_IREC_EVENT(xfs_reflink_allocate_cow_extent);
>
> DEFINE_INODE_IREC_EVENT(xfs_reflink_bounce_dio_write);
> DEFINE_IOMAP_EVENT(xfs_reflink_find_cow_mapping);
>
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