On 2/1/20 2:43 AM, Andres Freund wrote:
> Hi
>
> Currently io_uring executes fadvise in submission context except for
> DONTNEED:
>
> static int io_fadvise(struct io_kiocb *req, struct io_kiocb **nxt,
> bool force_nonblock)
> {
> ...
> /* DONTNEED may block, others _should_ not */
> if (fa->advice == POSIX_FADV_DONTNEED && force_nonblock)
> return -EAGAIN;
>
> which makes sense for POSIX_FADV_{NORMAL, RANDOM, WILLNEED}, but doesn't
> seem like it's true for POSIX_FADV_WILLNEED?
>
> As far as I can tell POSIX_FADV_WILLNEED synchronously starts readahead,
> including page allocation etc, which of course might trigger quite
> blocking. The fs also quite possibly needs to read metadata.
>
>
> Seems like either WILLNEED would have to always be deferred, or
> force_page_cache_readahead, __do_page_cache_readahead would etc need to
> be wired up to know not to block. Including returning EAGAIN, despite
> force_page_cache_readahead and generic_readahead() intentially ignoring
> return values / errors.
>
> I guess it's also possible to just add a separate precheck that looks
> whether there's any IO needing to be done for the range. That could
> potentially also be used to make DONTNEED nonblocking in case everything
> is clean already, which seems like it could be nice. But that seems
> weird modularity wise.
Good point, we can block on the read-ahead. Which is counter intuitive,
but true.
I'll queue up the below for now, better safe than sorry.
diff --git a/fs/io_uring.c b/fs/io_uring.c
index fb5c5b3e23f4..1464e4c9b04c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2728,8 +2728,7 @@ static int io_fadvise(struct io_kiocb *req, struct io_kiocb **nxt,
struct io_fadvise *fa = &req->fadvise;
int ret;
- /* DONTNEED may block, others _should_ not */
- if (fa->advice == POSIX_FADV_DONTNEED && force_nonblock)
+ if (force_nonblock)
return -EAGAIN;
ret = vfs_fadvise(req->file, fa->offset, fa->len, fa->advice);
--
Jens Axboe
