Re: [PATCH v4] ceph: fix blindly expanding the readahead windows

On 5/10/23 18:01, 胡玮文 wrote:

On Tue, May 09, 2023 at 08:57:03AM +0800, xiubli@redhat.com wrote:

...

From: Xiubo Li xiubli@redhat.com

Blindly expanding the readahead windows will cause unneccessary pagecache thrashing and also will introdue the network workload.

                                 ^^^^^^^^
                                 introduce

...

We should disable expanding the windows if the readahead is disabled and also shouldn't expand the windows too much.

Expanding forward firstly instead of expanding backward for possible sequential reads.

Bound `rreq->len` to the actual file size to restore the previous page cache usage.

Cc: stable@vger.kernel.org Fixes: 49870056005c ("ceph: convert ceph_readpages to ceph_readahead") URL: https://lore.kernel.org/ceph-devel/20230504082510.247-1-sehuww@mail.scut.edu... URL: https://www.spinics.net/lists/ceph-users/msg76183.html Cc: Hu Weiwen sehuww@mail.scut.edu.cn Signed-off-by: Xiubo Li xiubli@redhat.com

---

V4:

• two small cleanup from Ilya's comments. Thanks

  fs/ceph/addr.c | 28 +++++++++++++++++++++------- 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c index ca4dc6450887..683ba9fbd590 100644 --- a/fs/ceph/addr.c +++ b/fs/ceph/addr.c @@ -188,16 +188,30 @@ static void ceph_netfs_expand_readahead(struct netfs_io_request *rreq) struct inode *inode = rreq->inode; struct ceph_inode_info *ci = ceph_inode(inode); struct ceph_file_layout *lo = &ci->i_layout;

• unsigned long max_pages = inode->i_sb->s_bdi->ra_pages;

I think it is better to use `ractl->ra->ra_pages' instead of `inode->i_sb->s_bdi->ra_pages'. So that we can consider per-request ra size config, e.g., posix_fadvise(POSIX_FADV_SEQUENTIAL) will double the ra_pages.

Yeah, good catch.

But `ractl' is not passed to this function. Can we just add this argument? ceph seems to be the only implementation of expand_readahead, so it should be easy. Or since this patch will be backported, maybe we should keep it simple, and write another patch for this?

I think we should fix it together with this. It should be easy.

We can just store the "file->f_ra->ra_pages" in ceph_init_request() instead, because each rreq will be related to a dedicated file.

...

• unsigned long max_len = max_pages << PAGE_SHIFT;
• loff_t end = rreq->start + rreq->len, new_end; u32 blockoff;

• u64 blockno;

• /* Expand the start downward */
• blockno = div_u64_rem(rreq->start, lo->stripe_unit, &blockoff);
• rreq->start = blockno * lo->stripe_unit;
• rreq->len += blockoff;

• /* Readahead is disabled */
• if (!max_pages)

• return;
  

If we have access to ractl here, we can also skip expanding on `ractl->file->f_mode & FMODE_RANDOM', which is set by `posix_fadvise(POSIX_FADV_RANDOM)'.

Correct, because the "page_cache_sync_ra()' will do the right thing and we can skip expanding it here in ceph.

Thanks

- Xiubo

...

• /* Now, round up the length to the next block */
• rreq->len = roundup(rreq->len, lo->stripe_unit);

• /*

• * Try to expand the length forward by rounding  up it to the next
  

                                                    ^^ an extra space

...

• * block, but do not exceed the file size, unless the original
  

• * request already exceeds it.
  

• */
  

• new_end = min(round_up(end, lo->stripe_unit), rreq->i_size);
• if (new_end > end && new_end <= rreq->start + max_len)

• rreq->len = new_end - rreq->start;
  

• /* Try to expand the start downward */
• div_u64_rem(rreq->start, lo->stripe_unit, &blockoff);
• if (rreq->len + blockoff <= max_len) {

• rreq->start -= blockoff;
  

• rreq->len += blockoff;
  

• } }

static bool ceph_netfs_clamp_length(struct netfs_io_subrequest *subreq) -- 2.40.0