Re: [PATCH RFC] btrfs: allow extent buffer helpers to skip cross-page handling

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 16, 2023 at 03:49:06PM +1030, Qu Wenruo wrote:
> Currently btrfs extent buffer helpers are doing all the cross-page
> handling, as there is no guarantee that all those eb pages are
> contiguous.
> 
> However on systems with enough memory, there is a very high chance the
> page cache for btree_inode are allocated with physically contiguous
> pages.
> 
> In that case, we can skip all the complex cross-page handling, thus
> speeding up the code.
> 
> This patch adds a new member, extent_buffer::addr, which is only set to
> non-NULL if all the extent buffer pages are physically contiguous.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Added to misc-next, thanks.

> ---
> Reason for RFC:
> 
> This change would increase the code size for all extent buffer helpers,
> and since there one more branch introduced, it may even slow down the
> system if most ebs do not have physically contiguous pages.

> But I still believe this is worthy trying, as my previous attempt to
> use virtually contiguous pages are rejected due to possible slow down in
> vm_map() call.
> 
> I don't have convincing benchmark yet, but so far no obvious performance
> drop observed either.

I've looked at the assembly code, it's "just" one test/jump in
frequently used functions but the slow path handling cross-page changes
is slow anyway.