Re: [PATCH RFC v3 3/3] iomap: bound ioend size to 4096 pages

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, May 17, 2021 at 01:17:22PM -0400, Brian Foster wrote:
> The iomap writeback infrastructure is currently able to construct
> extremely large bio chains (tens of GBs) associated with a single
> ioend. This consolidation provides no significant value as bio
> chains increase beyond a reasonable minimum size. On the other hand,
> this does hold significant numbers of pages in the writeback
> state across an unnecessarily large number of bios because the ioend
> is not processed for completion until the final bio in the chain
> completes. Cap an individual ioend to a reasonable size of 4096
> pages (16MB with 4k pages) to avoid this condition.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/iomap/buffered-io.c |  6 ++++--
>  include/linux/iomap.h  | 26 ++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 642422775e4e..f2890ee434d0 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1269,7 +1269,7 @@ iomap_chain_bio(struct bio *prev)
>  
>  static bool
>  iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
> -		sector_t sector)
> +		unsigned len, sector_t sector)
>  {
>  	if ((wpc->iomap.flags & IOMAP_F_SHARED) !=
>  	    (wpc->ioend->io_flags & IOMAP_F_SHARED))
> @@ -1280,6 +1280,8 @@ iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
>  		return false;
>  	if (sector != bio_end_sector(wpc->ioend->io_bio))
>  		return false;
> +	if (wpc->ioend->io_size + len > IOEND_MAX_IOSIZE)
> +		return false;
>  	return true;
>  }
>  
> @@ -1297,7 +1299,7 @@ iomap_add_to_ioend(struct inode *inode, loff_t offset, struct page *page,
>  	unsigned poff = offset & (PAGE_SIZE - 1);
>  	bool merged, same_page = false;
>  
> -	if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, offset, sector)) {
> +	if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, offset, len, sector)) {
>  		if (wpc->ioend)
>  			list_add(&wpc->ioend->io_list, iolist);
>  		wpc->ioend = iomap_alloc_ioend(inode, wpc, offset, sector, wbc);
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 07f3f4e69084..89b15cc236d5 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -203,6 +203,32 @@ struct iomap_ioend {
>  	struct bio		io_inline_bio;	/* MUST BE LAST! */
>  };
>  
> +/*
> + * Maximum ioend IO size is used to prevent ioends from becoming unbound in
> + * size. bios can reach 4GB in size if pages are contiguous, and bio chains are
> + * effectively unbound in length. Hence the only limits on the size of the bio
> + * chain is the contiguity of the extent on disk and the length of the run of
> + * sequential dirty pages in the page cache. This can be tens of GBs of physical
> + * extents and if memory is large enough, tens of millions of dirty pages.
> + * Locking them all under writeback until the final bio in the chain is
> + * submitted and completed locks all those pages for the legnth of time it takes

s/legnth/length/

> + * to write those many, many GBs of data to storage.
> + *
> + * Background writeback caps any single writepages call to half the device
> + * bandwidth to ensure fairness and prevent any one dirty inode causing
> + * writeback starvation. fsync() and other WB_SYNC_ALL writebacks have no such
> + * cap on wbc->nr_pages, and that's where the above massive bio chain lengths
> + * come from. We want large IOs to reach the storage, but we need to limit
> + * completion latencies, hence we need to control the maximum IO size we
> + * dispatch to the storage stack.
> + *
> + * We don't really have to care about the extra IO completion overhead here
> + * because iomap has contiguous IO completion merging. If the device can sustain

Assuming you're referring to iomap_finish_ioends, only XFS employs the
ioend completion merging, and only for ioends where it decides to
override the default bi_end_io.  iomap on its own never calls
iomap_ioend_try_merge.

This patch establishes a maximum ioend size of 4096 pages so that we
don't trip the lockup watchdog while clearing pagewriteback and also so
that we don't pin a large number of pages while constructing a big chain
of bios.  On gfs2 and zonefs, each ioend completion will now have to
clear up to 4096 pages from whatever context bio_endio is called.

For XFS it's a more complicated -- XFS already overrode the bio handler
for ioends that required further metadata updates (e.g. unwritten
conversion, eof extension, or cow) so that it could combine ioends when
possible.  XFS wants to combine ioends to amortize the cost of getting
the ILOCK and running transactions over a larger number of pages.

So I guess I see how the two changes dovetail nicely for XFS -- iomap
issues smaller write bios, and the xfs ioend worker can recombine
however many bios complete before the worker runs.  As a bonus, we don't
have to worry about situations like the device driver completing so many
bios from a single invocation of a bottom half handler that we run afoul
of the soft lockup timer.

Is that a correct understanding of how the two changes intersect with
each other?  TBH I was expecting the two thresholds to be closer in
value.

The other two users of iomap for buffered io (gfs2 and zonefs) don't
have a means to defer and combine ioends like xfs does.  Do you think
they should?  I think it's still possible to trip the softlockup there.

--D

> + * high throughput and large bios, the ioends are merged on completion and
> + * processed in large, efficient chunks with no additional IO latency.
> + */
> +#define IOEND_MAX_IOSIZE	(4096ULL << PAGE_SHIFT)
> +
>  struct iomap_writeback_ops {
>  	/*
>  	 * Required, maps the blocks so that writeback can be performed on
> -- 
> 2.26.3
>