Re: [PATCH v3] ceph: fix blindly expanding the readahead windows

On 5/8/23 20:46, Ilya Dryomov wrote:

On Thu, May 4, 2023 at 1:11 PM 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. 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

---

V3:

• Folded Hu Weiwen's fix and bound `rreq->len` to the actual file size.

Hi Xiubo,

This looks much better! Just a couple of nits:

...

fs/ceph/addr.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c index ca4dc6450887..357d9d28f202 100644 --- a/fs/ceph/addr.c +++ b/fs/ceph/addr.c @@ -188,16 +188,32 @@ 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;
  

•   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;
  
  

•   /* 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
  

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

•    * request already exceeds it.
  

•    */
  

•   new_end = round_up(end, lo->stripe_unit);
  

•   new_end = min(new_end, rreq->i_size);
  

This can be done on single line:

     new_end = min(round_up(end, lo->stripe_unit), rreq->i_size);

Sure, will fix it.

...

•   if (new_end > end && new_end <= rreq->start + max_len)
  

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

•   /* Try to expand the start downward */
  

•   blockno = div_u64_rem(rreq->start, lo->stripe_unit, &blockoff);
  

•   if (rreq->len + blockoff <= max_len) {
  

•           rreq->start = blockno * lo->stripe_unit;
  

Can this be written as:

             rreq->start -= blockoff;

It seems like it would be easier to read and probably cheaper to compute too.

Yeah, this looks much easier to understand.

Thanks

- Xiubo

Thanks,

             Ilya