On Thu, Aug 24, 2017 at 8:22 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> Return IOMAP_F_NEEDDSYNC from xfs_file_iomap_begin() for a synchronous
> write fault when inode is pinned, and has dirty fields other than the
> timestamps. In xfs_filemap_huge_fault() we then detect this case and
> call dax_finish_sync_fault() to make sure all metadata is committed, and
> to insert the page table entry.
>
> Note that this will also dirty corresponding radix tree entry which is
> what we want - fsync(2) will still provide data integrity guarantees for
> applications not using userspace flushing. And applications using
> userspace flushing can avoid calling fsync(2) and thus avoid the
> performance overhead.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/xfs_file.c | 6 +++++-
> fs/xfs/xfs_iomap.c | 5 +++++
> 2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 460ca9639e66..5c3597b7dbf9 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1031,7 +1031,11 @@ __xfs_filemap_fault(
>
> xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> if (IS_DAX(inode)) {
> - ret = dax_iomap_fault(vmf, pe_size, NULL, &xfs_iomap_ops);
> + pfn_t pfn;
> +
> + ret = dax_iomap_fault(vmf, pe_size, &pfn, &xfs_iomap_ops);
> + if (ret & VM_FAULT_NEEDDSYNC)
> + ret = dax_finish_sync_fault(vmf, pe_size, pfn);
> } else {
> if (write_fault)
> ret = iomap_page_mkwrite(vmf, &xfs_iomap_ops);
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 813394c62849..e7c762c8fe27 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -33,6 +33,7 @@
> #include "xfs_error.h"
> #include "xfs_trans.h"
> #include "xfs_trans_space.h"
> +#include "xfs_inode_item.h"
> #include "xfs_iomap.h"
> #include "xfs_trace.h"
> #include "xfs_icache.h"
> @@ -1085,6 +1086,10 @@ xfs_file_iomap_begin(
> trace_xfs_iomap_found(ip, offset, length, 0, &imap);
> }
>
> + if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) &&
> + (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
> + iomap->flags |= IOMAP_F_NEEDDSYNC;
> +
So this behaves differently than ext4 that returns EOPNOTSUP in the !DAX case.
Are we generally documenting MAP_SYNC to be ignored in the pagecache
case? Or should we explicitly fail MAP_SYNC for the !DAX case on all
filesystems?
Another option is to finish block allocations at fault time in the
pagecache+MAP_SYNC case, but still require fsync to writeback dirty
pages, but that seems pointless.
Whatever we do I think all implementations should agree.
