Re: [PATCH] ext4: Modify the rec_len helpers to accommodate future cases

[Stephen Zhang]

Theodore Ts'o <tytso@xxxxxxx> 于2023年8月18日周五 01:31写道：
>
> On Mon, Aug 07, 2023 at 09:26:54AM +0800, zhangshida wrote:
> > From: Shida Zhang <zhangshida@xxxxxxxxxx>
> >
> > Following Andreas' suggestion, it is time to adapt these helpers
> > to handle larger records during runtime, especially in preparation
> > for the eventual support of ext4 with a block size greater than
> > PAGE_SIZE.
>
> Is there a reason for landing this now?  We don't have support for
> block_size > PAGE_SIZE yet, and this patch doesn't come for free, at
> least not systems with page_size < 64k.  These inline functions are
> *very* hot and get used in a large number of places.  Have you looked
> to see what it might do to text size of the ext4 code?  And whether
> the expansion to the icache might actually impact performance on CPU
> bound workloads with very large directories?
>
> I will note that there are some opportunities to optimize how often we
> use ext4_rec_len_from_disk.  For example, it gets called from
> ext4_check_dir_entry(), and often the callers of that function will
> need the directory record length.  So having ext4_check_dir_entry()
> optionally fill in the rec_len via a passed-in pointer might be
> worthwhile.

Yep, the best way to do it is to leave it unmerged until it is necessary.
At the same time, I will try to eliminate these regression concerns based
on these suggestions.

Cheers,
Shida

>
> Cheers,
>
>                                                 - Ted