Re: [PATCH RFC] Btrfs: only subtract from len_to_oe_boundary when it is tracking an extent

[Christoph Hellwig <hch@lst.de>]

On Sun, Jul 30, 2023 at 12:02:26PM -0700, Chris Mason wrote:
> [ This is an RFC because Christoph switched us to almost always set
> len_to_oe_boundary in a patch in for-next  I think we still need this
> commit for strange corners, but it's already pretty hard to hit reliably
> so I wanted to toss it out for discussion. We should consider either
> Christoph's "btrfs: limit write bios to a single ordered extent" or this
> commit for 6.4 stable as well ]

I'm torn.  On the one hand "btrfs: limit write bios to a single ordered
extent" is a pretty significant behavior change, on the other hand
stable-only patches with totally different behavior are always a bit
strange.

Note that with my entire pending queue, len_to_oe_boundary goes away
entirely, but with the current speed of patch application it might take
another 6 to 8 month to get there.

> This is hard to trigger because bio_add_page() isn't going to make a bio
> of U32_MAX size unless you give it a perfect set of pages and fully
> contiguous extents on disk.  We can hit it pretty reliably while making
> large swapfiles during provisioning because the machine is freshly
> booted, mostly idle, and the disk is freshly formatted.

It might be useful to create and xfstests for that, even if it only
hits on a freshly booted machine, although we'll need some reordering
in the xfstests sequence to make sure it gets run early..