[RFC PATCH v3 04/18] iomap: Add async buffered write support

Stefan Roesch
2022-05-18T23:37:29Z

This adds async buffered write support to iomap. The support is focused
on the changes necessary to support XFS with iomap.

Support for other filesystems might require additional changes.

Signed-off-by: Stefan Roesch <shr@fb.com>
---
fs/iomap/buffered-io.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 6b06fd358958..b029e2b10e07 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -580,12 +580,18 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
size_t from = offset_in_folio(folio, pos), to = from + len;
size_t poff, plen;
gfp_t gfp = GFP_NOFS | __GFP_NOFAIL;
+ bool no_wait = (iter->flags & IOMAP_NOWAIT);
+
+ if (no_wait)
+ gfp = GFP_NOWAIT;

if (folio_test_uptodate(folio))
return 0;
folio_clear_error(folio);

iop = iomap_page_create_gfp(iter->inode, folio, nr_blocks, gfp);
+ if (no_wait && !iop)
+ return -EAGAIN;

do {
iomap_adjust_read_range(iter->inode, folio, &block_start,
@@ -602,6 +608,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
if (WARN_ON_ONCE(iter->flags & IOMAP_UNSHARE))
return -EIO;
folio_zero_segments(folio, poff, from, to, poff + plen);
+ } else if (no_wait) {
+ return -EAGAIN;
} else {
int status = iomap_read_folio_sync(block_start, folio,
poff, plen, srcmap);
@@ -632,6 +640,9 @@ static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
int status = 0;

+ if (iter->flags & IOMAP_NOWAIT)
+ fgp |= FGP_NOWAIT;
+
BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
if (srcmap != &iter->iomap)
BUG_ON(pos + len > srcmap->offset + srcmap->length);
@@ -789,6 +800,10 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
* Otherwise there's a nasty deadlock on copying from the
* same page as we're writing to, without it being marked
* up-to-date.
+ *
+ * For async buffered writes the assumption is that the user
+ * page has already been faulted in. This can be optimized by
+ * faulting the user page in the prepare phase of io-uring.
*/
if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
status = -EFAULT;
@@ -844,6 +859,9 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
};
int ret;

+ if (iocb->ki_flags & IOCB_NOWAIT)
+ iter.flags |= IOMAP_NOWAIT;
+
while ((ret = iomap_iter(&iter, ops)) > 0)
iter.processed = iomap_write_iter(&iter, i);
if (iter.pos == iocb->ki_pos)
--
2.30.2