[PATCH 08/15] mm: support async buffered reads in generic_file_buffered_read()

Jens Axboe
2020-06-18T14:45:31Z

Use the async page locking infrastructure, if IOCB_WAITQ is set in the
passed in iocb. The caller must expect an -EIOCBQUEUED return value,
which means that IO is started but not done yet. This is similar to how
O_DIRECT signals the same operation. Once the callback is received by
the caller for IO completion, the caller must retry the operation.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
mm/filemap.c | 38 +++++++++++++++++++++++++++-----------
1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index e8aaf43bee9f..a5b1fa8f7ce4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1210,6 +1210,14 @@ static int __wait_on_page_locked_async(struct page *page,
return ret;
}

+static int wait_on_page_locked_async(struct page *page,
+ struct wait_page_queue *wait)
+{
+ if (!PageLocked(page))
+ return 0;
+ return __wait_on_page_locked_async(compound_head(page), wait, false);
+}
+
/**
* put_and_wait_on_page_locked - Drop a reference and wait for it to be unlocked
* @page: The page to wait for.
@@ -2049,17 +2057,25 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
index, last_index - index);
}
if (!PageUptodate(page)) {
- if (iocb->ki_flags & IOCB_NOWAIT) {
- put_page(page);
- goto would_block;
- }
-
/*
* See comment in do_read_cache_page on why
* wait_on_page_locked is used to avoid unnecessarily
* serialisations and why it's safe.
*/
- error = wait_on_page_locked_killable(page);
+ if (iocb->ki_flags & IOCB_WAITQ) {
+ if (written) {
+ put_page(page);
+ goto out;
+ }
+ error = wait_on_page_locked_async(page,
+ iocb->ki_waitq);
+ } else {
+ if (iocb->ki_flags & IOCB_NOWAIT) {
+ put_page(page);
+ goto would_block;
+ }
+ error = wait_on_page_locked_killable(page);
+ }
if (unlikely(error))
goto readpage_error;
if (PageUptodate(page))
@@ -2147,7 +2163,10 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,

page_not_up_to_date:
/* Get exclusive access to the page ... */
- error = lock_page_killable(page);
+ if (iocb->ki_flags & IOCB_WAITQ)
+ error = lock_page_async(page, iocb->ki_waitq);
+ else
+ error = lock_page_killable(page);
if (unlikely(error))
goto readpage_error;

@@ -2190,10 +2209,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
}

if (!PageUptodate(page)) {
- if (iocb->ki_flags & IOCB_WAITQ)
- error = lock_page_async(page, iocb->ki_waitq);
- else
- error = lock_page_killable(page);
+ error = lock_page_killable(page);
if (unlikely(error))
goto readpage_error;
if (!PageUptodate(page)) {
--
2.27.0