Re: [PATCH U-BOOT 2/3] btrfs: btrfs_file_read: allow opportunistic read until the end

[Dominique Martinet <asmadeus@codewreck.org>]

Qu Wenruo wrote on Tue, Apr 18, 2023 at 10:02:00AM +0800:
> >   	/* Read the tailing unaligned part*/
> 
> Can we remove this part completely?
> 
> IIRC if we read until the target end, the unaligned end part can be
> completely removed then.

The "Read the aligned part" loop stops at aligned_end:
> while (cur < aligned_end)

So it should be possible that the last aligned extent we consider does
not contain data until the end, e.g. an offset that ends with the
aligned end:

0            4096    4123
[extent1-----|extent2]

In this case the main loop will only read extent1 and we need the
"trailing unaligned part" if for extent2.

I have a feeling the loop could just be updated to go to the end
`while (cur < end)` as it doesn't seem to care about the end
alignment... Should I update v2 to do that instead?



This made me look at the "Read out the leading unaligned part" initial
part, and its check only looks at the sector size but it probably has
the same problem -- we want to make sure we read any leftover from a
previous extent e.g. this file:
0              4096       8192
[extent1------------------|extent2]
and reading from 4k with a sectorsize of 4k is probably bad will enter
the aligned main loop right away...
And I think that'll fail?...
Actually not quite sure what's expecting what to be aligned in there,
but I just tried some partial reads from non-zero offsets and my board
resets almost all the time so I guess I've found something else to dig
into.
This isn't a priority for me right now but I'll look a bit more when I
have more time if you haven't first.

-- 
Dominique Martinet | Asmadeus